GOAL-BASED NEW SHIP CONSTRUCTION The International Association of Classification Societies (IACS),...

I:\MSC\83\INF-5.DOC

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INTERNATIONAL MARITIME ORGANIZATION

IMO

E

MARITIME SAFETY COMMITTEE 83rd session Agenda item 5

MSC 83/INF.5 3 July 2007 ENGLISH ONLY

GOAL-BASED NEW SHIP CONSTRUCTION STANDARDS

Information submitted by IACS to the Pilot Project on Goal-Based Standards for New Ship

Construction for Bulk Carriers and Oil Tankers

Submitted by IACS

SUMMARY

Executive summary:

This document forwards the IACS documents submitted to the Pilot Project of the Goal-Based Standards for New Ship Construction for Bulk Carriers and Oil Tankers.

Action to be taken:

Paragraph 2

Related document:

MSC 82/24, paragraph 5.29

Introduction 1 The following documents were submitted by IACS to the Pilot Panel in order to assist with the Pilot Panel�s terms of reference. These documents were also copied to the Correspondence Group through the Pilot Panel meeting reports. Annex 1 IACS Documentation Package for the IMO GBS Pilot Project, dated 16 February 2007. Annex 2 IACS Technical Presentation to the IMO GBS Pilot Project, dated 12 March 2007. Annex 3 IMO Pilot Panel Questions to IACS for March 12 meeting. Annex 4 Questions/comments to IACS during presentation on March 12. Annex 5 (Presentation) Goal-Based New Ship Construction Standards, Tier II.2

�Net Scantlings�, dated 16 February 2007. Annex 6 IACS Study Steel Weight Impact from Net Scantling Definition, dated 24 April 2007. Annex 7 IACS Study Impact of Applying the CSR Corrosion Addition on the Hull Girder

Section Modulus, dated 3 June 2007. Action requested of the Committee 2 The Committee is invited to note the information contained in the documents listed above.

***

IACS INTERNATIONAL ASSOCIATION OF CLASSIFICATION SOCIETIES 36 Broadway London, SW1H 0BH, U.K. Tel: +44 (0)20 7976 0660 Email: [email protected]

IACS Documentation Package for the

IMO GBS Pilot Project

16 February 2007

Submitted to:

INTERNATIONAL MARITIME SAFETY ORGANIZATION Maritime Safety Committee

IMO Pilot Project

(MSC 82/24, Paragraph 5.29 and Annex 15)

MSC 83/INF.5

ANNEX 1

IACS - International Association of Classification Societies © All rights reserved. Except as permitted under current legislation no part of this work may be photocopied, stored in a retrieval system, published, performed in public, adapted, broadcast, transmitted, recorded or reproduced in any form or by any means, without prior permission of the copyright owner. Where IACS has granted written permission for any part of this publication to be quoted such quotation must include acknowledgment to IACS. Enquiries should be addressed to The Permanent Secretary, International Association of Classification Societies, 36 Broadway, London, SW1H 0BH Telephone: +44-(0)207 976 0660 Fax: +44-(0)207-808 11007 E-mail: [email protected] TERMS AND CONDITIONS “The International Association of Classification Societies (IACS), its Member Societies and their officers, members, employees and agents (on behalf of whom this notice is issued) shall be under no liability or responsibility in negligence or otherwise to any person in respect of any information or advice expressly or impliedly given in this document, or in respect of any inaccuracy herein or omission herefrom or in respect of any act or omission which has caused or contributed to this document being issued with the information or advice it contains (if any).Without derogating from the generality of the foregoing, neither IACS nor its Member Societies and their officers, members, employees or agents shall be liable in negligence or otherwise howsoever for any indirect or consequential loss to any person caused by or arising from any information, advice, inaccuracy or omission being given or contained herein or any act or omission causing or contributing to any such information, advice, inaccuracy or omission being given or contained herein.” Produced in February 2007 for the International Association of Classification Societies.

MSC 83/INF.5 ANNEX 1 PAGE 2

IMO Pilot Project 16 February 2007 IACS Documentation Package Page i

Contents: 1. General.............................................................................................................................................1 2. Objective ..........................................................................................................................................1 3. Structure of this report...................................................................................................................1 4. Cross reference Table.....................................................................................................................3 5. Commentary ...................................................................................................................................9

Tier II Functional Requirements .......................................................................................................9 DESIGN..............................................................................................................................................11 II.1 Design life .............................................................................................................................11 II.2 Environmental conditions ..................................................................................................11 II.3 Structural Strength ..............................................................................................................14 II.4 Fatigue life ............................................................................................................................30 II.5 Residual strength .................................................................................................................31 II.6 Protection against corrosion...............................................................................................32 II.6.1 Coating life .......................................................................................................................32 II.6.2 Corrosion addition ..........................................................................................................33 II.7 Structural redundancy ........................................................................................................35 II.8 Watertight and weathertight integrity .............................................................................36 II.9 Human element considerations.........................................................................................37 II.10 Design transparency .......................................................................................................38 CONSTRUCTION.............................................................................................................................41 II.11 Construction quality procedures ..................................................................................41 II.12 Survey ...............................................................................................................................42 IN-SERVICE CONSIDERATIONS .................................................................................................43 II.13 Survey and Maintenance................................................................................................43 II.14 Structural accessibility ....................................................................................................43 RECYCLING CONSIDERATIONS ................................................................................................44 II.15 Recycling...........................................................................................................................44

6. Conclusions ...................................................................................................................................44 Appendices A. IMO Goal-based New Ship Construction Standards B. IACS Common Structural for Double Hull Oil Tankers C. Background Documents for the IACS Common Structural for Double Hull Oil Tankers


IMO Pilot Project 16 February 2007 IACS Documentation Package Page ii

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IMO Pilot Project 16 February 2007 IACS Documentation Package Page 1

1. General At the 81st session of the IMO Maritime Safety Committee held in May 2006, IACS agreed to use the recently developed common structural rules as basis for a pilot to conduct a trial application of the IMO Goal-based New Ship Construction Standards (GBS). While IACS has published common rules for both tankers and bulk carriers, in order to limit the scope for the pilot, only the common rules for tankers will be used. Therefore, the IACS 2006 “Common Structural Rules for Double Hull Oil Tankers“(referred to as CSR or Rules in this report), which entered into force on 1 April 2006 have been used.

2. Objective 2.1 Objective of the Pilot Project The objective of the pilot project is to conduct a trial application of Tier III of the GBS for oil tankers and bulk carriers with the intention of validating the Tier III verification framework, identifying shortcomings and making proposals for improvement. Note, the pilot project will test the IMO GBS Tier III Verification Framework and not actually carry out the verification of the IACS CSR at this time. 2.2 Objective of the submission from IACS The objective of the submission from IACS is to provide to the pilot panel a working example of how IACS in the future may provide the background documentation illustrating how classification rules meet the GBS. The intention has been to provide this to the pilot panel in order for them to start their work with an example at hand, and thereby contribute to making the work more concrete.

3. Structure of this report To assist the pilot panel members, a self assessment has been prepared by the IACS team summarising the extent to which IACS CSR meet each of the GBS Tier II functional requirements. This self assessment can be found in the table on the next page. The self assessment indicates where the GBS are covered and where the GBS are not fully covered in the CSR. Possible reasons why the CSR do not fully cover the GBS include;

• the subject area is not normally covered in class newbuilding construction rules, • the subject area is implicitly covered and not explicitly covered, • the subject area is covered by other rules or regulations, • the subject area is only partially covered, • etc.

Wherever an item is indicated as not being fully covered in the CSR an accompanying comment is given. A list of references to CSR for each of the functional requirements is provided in section 4. Further commentary for each of the GBS functional requirements is included in Section 5 of this document.



Self Assessment Summary Table

Item

Fully covered in

CSR

Partially covered in

CSR Not covered

in CSR Comment DESIGN

II.1 Design life

II.2 Environmental conditions

II.3 Structural strength

II.4 Fatigue life

II.5 Residual strength Implicitly addressed in rules.

II.6 Protection against corrosion

II.6.1 Coating life

II.6.2 Corrosion addition

II.7 Structural redundancy Implicitly addressed in rules.

II.8 Watertight and weathertight integrity

II.9 Human element considerations

Partially covered. May be addressed in future SOLAS Reg.

II.10 Design transparency Also addressed by other rules or conventions.

CONSTRUCTION II.11 Construction quality

procedures

II.12 Survey IN-SERVICE CONSIDERATIONS

II.13 Survey and Maintenance

Addressed with respect to design and construction requirements to allow adequate survey of the structure.

II.14 Structural accessibility Addressed in SOLAS Reg II-1/3 on PMA.

RECYCLING CONSIDERATIONS

II.15 Recycling Will be addressed in future IMO Reg. on Recycling of Ships.



4. Cross reference Table The following table includes a quick cross reference to the CSR for each of the GBS Tier II functional requirements.

Cross Reference Table

TIER II (FUNCTIONAL REQUIREMENTS) <from MSC 82/WP.5, 6 Dec 2006>

CSR for Tanker Associated Rule Reference

DESIGN II.1 Design life The specified design life is not to be less than 25 years.

2/3.1.3 – Design life 9/3.2.3.1 – Design fatigue life B/2.4.7.2 – Design life C/1.4.1.3 – Fatigue life C/1.4.1.4 – Fatigue life

II.2 Environmental conditions Ships should be designed in accordance with North Atlantic environmental conditions and relevant long-term sea state scatter diagrams.

2/3.1.7.1 - External environment 2/4.2.6.2(d) – Environmental loads 9/3.2.2.1 – Fatigue loads

II.3 Structural strength Ships should be designed with suitable safety margins:

.1 to withstand, at net scantlings**, in the intact condition, the environmental conditions anticipated for the ship’s design life and the loading conditions appropriate for them, which should include full homogeneous and alternate loads, partial loads, multi-port and ballast voyage, and ballast management condition loads and occasional overruns/overloads during loading/unloading operations, as applicable to the class designation; and

.2 appropriate for all design parameters whose calculation involves a degree of uncertainty, including loads, structural modelling, fatigue, corrosion, material imperfections, construction workmanship errors, buckling and residual strength.

The structural strength should be assessed against excessive deflection and failure modes, including but not limited to buckling, yielding and fatigue. Ultimate strength calculations should include ultimate hull girder capacity and ultimate strength of plates and stiffeners. The ship’s structural members should be of a design that is compatible with the purpose of the space and ensures a degree of structural continuity. The structural members of ships should be designed to facilitate load/discharge for all contemplated cargoes to avoid

Net scantlings: 2/4.3.4 4/2.4 6/3 Intact structure: 2/4.3.5 Environmental loads: 2/3.1.7 2/4.2 7/ Loading conditions: 2/3.1.5 2/3.1.6 2/3.1.8 Tab 2.4.1 2/4.2.5 2/5.4.1.1 thru 5 2/5.4.2 7/2.1, 7/2.2, 8/1.1.2, 8/Tab 8.2.7 thru 9 B/Tab B.2.3 and 4 Accidental loads: 2/4.2.7 Tab 2.4.1 7/2.2.3.2 7/5 Yield acceptance criteria: 2/4.5 2/5.4.1.5 thru 10 2/Tab2.5.1 thru 3 2/5.4.5 and 6 8/Tab 8.1.3 (BM) 8/Tab 8.1.4 (shear) 8/Tab 8.2.4 and 5 (local)





damage by loading/discharging equipment which may compromise the safety of the structure. ** The net scantlings should provide the structural strength required to sustain the design loads, assuming the structure in intact condition and excluding any addition for corrosion.

8/Tab 8.2.10 (PSM) 9/2.2.5 (FEM) 9/Tab 9.2.1 (FEM) Deflection criteria: 2/5.3.1.1(b) 2/5.4.5.1 3/5.3.3.4 8/2.6.1.7 plus individual reqts. 10/2 Buckling criteria: 2/4.5 2/Tab 2.5.2 and 3 8/1.2.1.4 8/1.4 8/2.6.1.6 9/2.2.5.3 10/ D/ Fatigue criteria: 2/4.3.3 Tab 2.5.1 2/5.4.3 2/5.6.5 8/1.5 9/3 B/4 C/ Hull girder ULS: 2/5.6.3 9/1 A/ Compatibility: 2/3.1.7 2/3.1.8 Continuity: 4/3.2 thru 4 8/1.6 8/1.6.5 and 6 8/2.1.4.7 8/2.3.1.3 8/3.1.3 8/4.1.3 8/5.1.3 Loading / Unloading 2/4.2.1 2/Tab 2.5.1 8/1.1.2.2(b)

II.4 Fatigue life The design fatigue life should not be less than the ship’s design life and should be based on the environmental conditions in II.2.

9/3.2.3.1 C/

II.5 Residual strength Ships should be designed to have sufficient strength to

General principle: 2/4.1.2.2(a) and (d) Hull girder ULS:





withstand the wave and internal loads in specified damaged conditions such as collision, grounding or flooding. Residual strength calculations should take into account the ultimate reserve capacity of the hull girder, including permanent deformation and post-buckling behaviour. Actual foreseeable scenarios should be investigated in this regard as far as is reasonably practicable.

A/ which contains post-buckling investigations.

II.6 Protection against corrosion Measures are to be applied to ensure that net scantlings required to meet structural strength provisions are maintained throughout the specified design life. Measures include, but are not limited to, coatings, corrosion additions, cathodic protection, impressed current systems, etc.

(See details below in II.6.1 and 2)

II.6.1 Coating life Coatings should be applied and maintained in accordance with manufacturers’ specifications concerning surface preparation, coating selection, application and maintenance. Where coating is required to be applied, the design coating life is to be specified. The actual coating life may be longer or shorter than the design coating life, depending on the actual conditions and maintenance of the ship. Coatings should be selected as a function of the intended use of the compartment, materials and application of other corrosion prevention systems, e.g. cathodic protection or other alternatives.

6/2 Coatings: 6/2.1.1 11/5.1.8 and 9 Corrosion: 6/3 Cathotic protection: 6/2.1.2 Measurements in service: 12/

II.6.2 Corrosion addition The corrosion addition should be added to the net scantling and should be adequate for the specified design life. The corrosion addition should be determined on the basis of exposure to corrosive agents such as water, cargo or corrosive atmosphere, or mechanical wear, and whether the structure is protected by corrosion prevention systems, e.g. coating, cathodic protection or by alternative means. The design corrosion rates (mm/year) should be evaluated in accordance with statistical information established from service experience and/or accelerated model tests. The actual corrosion rate may be greater or smaller than the design corrosion rate, depending on the actual conditions and maintenance of the ship.

6/3

II.7 Structural redundancy Ships should be of redundant design and construction so that localized damage of any one structural member will not lead to immediate consequential failure of other

General principle: 2/4.1.2.2 (a) and (d)





structural elements leading to loss of structural and watertight integrity of the ship. II.8 Watertight and weathertight integrity Ships should be designed to have adequate watertight and weathertight integrity for the intended service of the ship and adequate strength and redundancy of the associated securing devices of hull openings.

References to other rules: 2/2.1.1, 3/3.1.1.2, 3/3, 5/2.1.2.3 watertight subdivision: 5/2 bulkheads: 8/2.5 watertight boundaries: 8/3.6, 8/4.7, 8/5.6 hull openings and closing arrangements: 11/1

II.9 Human element considerations Ships should be designed and built using ergonomic design principles to ensure safety during operations, inspection and maintenance of ship’s structures. These considerations should include stairs, vertical ladders, ramps, walkways and standing platforms used for permanent means of access, the work environment and inspection and maintenance considerations.

reference to other regulations: 3/3.1.1.2 size of access openings: 5/5.1.1.4 crew protection: 11/2

II.10 Design transparency Ships should be designed under a reliable, controlled and transparent process made accessible to the extent necessary to confirm the safety of the new as-built ship, with due consideration to intellectual property rights. Readily available documentation should include the main goal-based parameters and all relevant design parameters that may limit the operation of the ship.

3/2 Novel designs: 3/4.1.2

CONSTRUCTION

II.11 Construction quality procedures Ships should be built in accordance with controlled and transparent quality production standards with due regard to intellectual property rights. The ship construction quality procedures should include, but not be limited to, specifications for material, manufacturing, alignment, assembling, joining and welding procedures, surface preparation and coating.

2/4.4 Materials: 6/4.1.2 Fabrication: 6/4.1.2 Welding and joint preparation: 6/4.4 6/5

II.12 Survey A survey plan should be developed for the construction phase of the ship, taking into account the ship type and design. The survey plan should contain a set of requirements, including specifying the extent and scope of the construction survey(s) and identifying areas that need special attention during the survey(s), to ensure compliance of construction with mandatory

2/2.1.2.1 2/2.1.3.1(a) and (c) 2/3.1.9 3/2.2.3.1(g) 11/5

MSC 83/INF.5ANNEX 1 PAGE 10




ship construction standards. IN-SERVICE CONSIDERATIONS

II.12 Survey and Maintenance Ships should be designed and constructed to facilitate ease of survey and maintenance, in particular avoiding the creation of spaces too confined to allow for adequate survey and maintenance activities. The survey plan in II.11 should also identify areas that need special attention during surveys throughout the ship’s life and in particular all necessary in-service survey and maintenance that was assumed when selecting ship design parameters.

2/2.1.3.1(d) 2/3.1.9.3 renewal criteria 3/2.2.3.1plans to be on board 5/5 access arrangements 11/2 crew protection 12/1.2 thickness measurements

II.14 Structural accessibility The ship should be designed, constructed and equipped to provide adequate means of access to all internal structures to facilitate overall and close-up inspections and thickness measurements.

3/2.2.2.1(d) 5/5


II.15 Recycling Ships should be designed and constructed of materials for environmentally acceptable recycling without compromising the safety and operational efficiency of the ship.

2/2.1.1 3/3.3



5. Commentary

Tier II Functional Requirements

To demonstrate how the CSR/Tankers address the IMO GBS Tier II Functional Requirements, each of the functional requirements is listed followed by a description of how the CSR/Tanker relates. This report has been organized by sections according to the GBS functional requirements, however the Rules themselves are organized similar to a typical design flow as illustrated in the figure below, which is Figure 2.5.1 from the CSR.



Overview of Structural Design Process

Deck housesSection 11/1

Fore and Aft EndsSection 8/3 & 8/5

Plating and LocalSupport Members

Section 8/2

Longitudinal StrengthSection 8/1

Criteria AssessmentYield, Shear,

Buckling

Primary SupportMembers Inc Bhds

Section 8/2.6

Scantling Requirements

Sloshing and ImpactSection 8/6

Structural designdetails (welding,

brackets)Section 4/3

Machinery SpaceSection 8/4

Strength Assessment(FEM)

Section 9/2

Hull Girder UltimateStrength

Section 9/1

Criteria Assessment

Fatigue StrengthSection 9/3

Nominal StressApproach

Appendix C/1

Global Cargo TankStructural Strength

AnalsisAppendix B/2

Design Verification

Local Fine MeshStructural Strength

AnalysisAppendix B/3

Evaluation of Hot SpotStress for Fatigue

AnalysisAppendix B/4

Criteria AssessmentFatigue Damage

Hot Spot Stress (FE)Approach

Appendix C/2

Criteria AssessmentYield, Shear,

Buckling

MinimumRequirementsMinimum Thickness

Section 8/2

Other MinimumRequirements

Stiffness andProportionsSection 10/2

Hull Girder Inertiaand Section Modulus

Section 8/1

GeneralRule Requirements

Basic InformationSection 4

StructuralArrangements

Section 5

MaterialsSection 6

General DetailedRequirements

Section 11

Design Requirements

OperationalRequirements

Section 2/2

Design BasisSection 2/3

Loading ManualSection 8/1

Loads

Static LoadsSection 7/2

Dynamic LoadsSection 7/3

Impact and SloshingLoads

Section 7/4

Load CombinationsSection 7/6

Static LoadCombinations

Section 7/6

Static Plus DynamicLoad Combinations

Section 7/6

Dynamic LoadCombination FactorsDLCFs Section 7/6.5

Accidental LoadsSection 7/5



DESIGN

II.1 Design life

Rating: The functional requirement is covered by CSR. Comment: In Tier II.1 of the Goal based standards the design life, defined in Goal 5 of Tier I, is to be 25 years. CSR definitions of design life are given in Section 2/3.1.3. These definitions are essentially the same as the one provided in Tier I. The design life of 25 years is an input parameter for the determination of the values of the scantling loads, fatigue loads, expected fatigue life and corrosion wastage allowances: .1 In CSR, the characteristic value of loads used in ultimate strength analysis is the

expected maximum load likely to be encountered during the design life, i.e. 25 years. With a mean wave period of about 9 seconds, 25 years corresponds to 108 cycles. Influence of design life variations on characteristic loads is negligible: less than 1% variation for a life increase of five years from 20 to 25 years as compared to typical pre-CSR requirements.

.2 The increase of design fatigue life from the past practice of 20 years to 25 years has an important influence on the fatigue checking of the structure, see section 5.II.4 Fatigue of this report.

.3 To take into account general uniform corrosion of the structure of the ships, values of wastage allowances are given in CSR Rules. The wastage allowances were determined such that 95% of the measured thicknesses present in the IACS statistics are larger than the renewal thickness given in the rules at the end of the design life (25 years of service).

CSR Reference: CSR-reference content comment Sec 2/3.1.3 Design life Sec 2/5.4.2.4 Description regarding the 108

cycles


Rating: The functional requirement is covered by CSR. Comment: The functional requirement II.2 is covered by CSR for Tankers. The rule text explicitly specifies that the rule requirements are based on a ship trading in the North Atlantic wave environment for its entire design life. .1 Sea state data It is specified in the rule text that wave loads are derived using the sea state data given in IACS Recommendation No. 34. This recommendation gives the wave data using a



scatter diagram where the probability of sea-states is described as occurrences per 100000 observations. The area covered by the scatter diagram is also specified. The scatter diagram given in Rec. 34 is developed based on wave data obtained from British Marine Technology. The sea-state data that the rule requirements are based on, and background documentation of the scatter diagram used, can be found in the following publications:

- IACS Recommendation No. 34, “Standard Wave Data” - British Marine Technology (Primary contributors Hogben N., Da Cunha, L.F. and

Oliver, H.N.). “Global Wave Statistics”, Unwin Brothers Limited, London 1986. - Bitner-Gregersen, E.M., Cramer, E.H., Korbijin, F., “Environmental Description for

Long-term Load Response of Ship Structures”, ISOPE June 1995, The Hague, The Netherlands.

CSR reference: CSR-reference content comment Sec 2/3.1.7.1 External environment

.2 Environmental loads The basis for the development of load formulations using the specified wave environment is explained in the following. The Rule formulations for the wave loads are based on envelope values calculated by numerical wave load analysis and regression analysis, and calibrated with feedback from service experience and model tests. The envelope value is the long term value, at a given probability level, taking into consideration the effect of all wave headings. The general principles for the derivation of the wave load values are: (a) the application of load values is consistent for all similar load scenarios (b) the characteristic load value is selected to suit the purpose of the application of the load and the selected structural assessment method, e.g. for strength assessment the expected lifetime maximum load is applied while for fatigue assessment an average value representing the expected load history is applied (c) load calculations are performed using 3-D linear hydrodynamic computational tools. The effects of speed are considered (d) the derivation of characteristic wave loads is based on a long term statistical approach which includes representation of the wave environment (North Atlantic scatter diagram), probability of ship/wave heading and probability of load value exceedance based on IACS Rec. 34. All of which result in envelope values (e) non-linear effects are considered for the expected lifetime maximum loads. The hydrodynamic calculations are based on: (a) the Pierson-Moskowitz wave spectrum (b) a wave energy-spreading of cos2 (c) an equal probability on all wave headings (d) a 30 degree step of ship/wave heading

The speed and loading condition are chosen based on the corresponding application of load and the structural assessment method. Thus, for: (a) strength evaluation; a heavy ballast condition and a full load condition at scantling draught have been used for the assessment, applying no forward speed, as tankers are



full-form ships with negligible manoeuvring speed in extreme heavy weather due to voluntary and involuntary reasons; (b) fatigue assessment; normal ballast and full load condition at design draught have been evaluated as the two most common sailing conditions. A speed of 75% of service speed has been taken as the average speed over the lifetime, taking into account effects of slamming, bow submergence, added wave resistance and voluntary speed reduction. The considered wave-induced loads include: (a) hull girder loads (i.e., vertical and horizontal bending moments) (b) dynamic wave pressures (c) dynamic tank pressures. The probability of occurrence is selected based on the purpose of application of the load and the selected structural assessment method to be as follows: (a) the loads for fatigue assessment are based on a probability of exceedance of 10-4, which means loads which occur frequently. The 10-4 is the reference probability level that together with a Weibull shape parameter and average zero-crossing period define the expected load history. (b) the loads for strength evaluation are based on a probability of exceedance of 10-8. The probability level represents the expected maximum load during the design life. The exception is the sloshing loads, where a probability level of 10-4 is used, which is a load that occurs frequently. General formulae for linear wave induced ship motion, acceleration, hull girder loads and wave pressures are given at both 10-8 and 10-4 probability levels. The design load combinations corresponding to the identified load scenarios produce realistic design load sets suitable for the design and verification of the structural capability. Design load sets apply all the applicable simultaneously acting static and dynamic local load components and static and dynamic global load components for the design of a particular or group of structural members. The combination of dynamic loads considers all simultaneously occurring dynamic load components. In deriving the simultaneously occurring loads, one particular load component is maximised or minimised and the relative magnitude of all simultaneously occurring dynamic load components is specified by the application of dynamic load combination factors (DLCF) based on the envelope load value. These dynamic load combination factors are based on the application of the equivalent design wave approach and are given as tabulated values. For scantling requirements and strength assessments, correction factors to account for non-linear wave effects and operational considerations in heavy weather are applied to the linear loads. In beam sea condition a heading correction factor of 0.8 to account for operational considerations are applied to the linear loads. This is done because the assumption of equal probability of all wave headings is not considered to be correct for extreme conditions, since the ship in such weather will be steered up against the waves. For the fatigue requirements given, the load assessment is based on the expected load history and an average approach is applied. The expected load history for the design life is characterised by the 10-4 probability level of the dynamic load value, the load history for each structural member is represented by Weibull probability distributions of the corresponding stresses.



The fatigue analysis is calculated for two representative loading conditions covering the ship’s intended operation. These two conditions are: (a) full load homogeneous conditions at design draught (b) normal ballast condition. The ships life is divided into three operational phases with 42.5% in full load at sea, 42.5% in ballast at sea and the remaining 15% in harbour or sheltered waters. Correction factors to account for speed effects are applied to the linear loads for fatigue assessment. Also factors to calculate the loads at probability levels 10-8 and 10-4 are applied. CSR references: CSR-reference content comment Sec 2/4.2.6 Environmental loads Sec 2/5.4.2 Design loads for scantling

requirements and strength assessment

Sec 2/5.4.3 Design loads for fatigue assessment

Sec 7/3 Dynamic load components Sec 7/6 Combination of loads

II.3 Structural Strength

Rating: The functional requirement is covered by CSR. Comment: The GBS Tier II.3 criteria calls for the documentation of the structural requirements included in the class rules. .1 Safety Margins The GBS lists various items which should be taken into account when establishing suitable safety margins in the rules. The items mentioned are each discussed as follows: a) Environmental conditions: The environmental loads included in the CSR, which are used during the assessment of structural strength, have been based on a 25 year exposure to the North Atlantic environment. The probability of exceedance levels for the various individual design environmental loads are included in Section 5.II.2 of this report. While the design loads of the North Atlantic have been used to formulate the design loads, most vessels do not typically trade exclusively in the North Atlantic. Therefore there is a safety factor associated with relating the actual environment under which the vessel trades versus the North Atlantic environment, as the CSR have not included reductions to the design loads to account for actual benign environments. The safety margin varies based on the future trading patterns of the vessels.



These environmental conditions are used to develop the dynamic wave-induced components of the design loads for longitudinal hull girder strength and the strength evaluation of local structural members. CSR references: CSR-reference content comment Sec 2/3.1.7 External environment Sec 7/3 Dynamic loads

b) Loading conditions: Representative design cargo and ballasting loading conditions are specified to envelope the actual vessel loading conditions. The design loading conditions include various combinations of full and empty tanks to represent homogeneous, alternate, partial, multi-port, ballast, and ballast management conditions. If actual vessel loading conditions include non-typical conditions such as asymmetric loading or simultaneously emptying all cargo tanks across a section, the Rules state that they also have to be used in the structural evaluation. While the Rule specified loading conditions which include checkerboard or alternate tank loading have been used to formulate the design loads, most vessels typically trade in homogeneous full load or ballast load conditions. Therefore there is a safety factor associated with relating the actual loading conditions under which the vessel trades versus the Rule conditions. As this depends on the unknown future loading patterns of the vessels, there is no way of actually quantifying the safety margin attributed to this. These vessel loading conditions are used to develop the static components of the design loads for longitudinal hull girder strength and the strength evaluation of structural members. Additional information on the loading conditions is included in Section 5.II.2 of this report. The Rules relate the design loading conditions to the actual operation of the vessel by specifying that loading conditions and operation instructions be included in the vessel Loading Manual and/or Loading Instrument which will be used by the vessels’ operating personnel. The Rules require that the Loading Manual include design parameters and operational limitations upon which approval of the hull scantlings have been based. Limitations on permissible still water bending moment and shear forces, scantling draft, minimum draft, minimum forward draft, allowable cargo density, ballast water exchange operations, and the design speed are to be included. The following table, which is a partial copy of Table B.2.3 from the CSR, illustrates representative loading conditions to be evaluated in the FEM analysis which are included in the Rules.



FE Load Cases for Tankers with Two Oil-tight Longitudinal Bulkheads Still Water Loads Dynamic load cases

Strength assessmen

t (1a)

Strength assessment against hull girder

shear loads (1b) Loadin

g Pattern

Figure Draught % of

Perm. SWBM(2)

% of Perm.

SWSF(2) Midship region

Forward region

Midship and aft regions

Design load combination S + D (Sea-going load cases) 100% (sag) See note 3 1 \ \

A1

P

S

0.9 Tsc 100% (hog)

100% (-ve fwd) See note 4

2, 5a \ \

100% (sag) See note 3 1 \ \

A2

P

S

0.9 Tsc 100% (hog)


2, 5a \ \


2 4 2

A3(6)

P

S

0.55 Tsc see note 5

100% (hog) 100%

(-ve fwd) See note 5

5a \ \

A4

P

S

0.6 Tsc 100% (sag)

100% (+ve fwd)

See note 4

1, 5a \ \

100% (+ve fwd)

See note 5

1 3 1

A5(7)

P

S

0.8 Tsc See note 6

100% (sag) 100%

(+ve fwd)

See note 4

5a \ \

A6

P

S

0.6 Tsc 100% (hog)


5a \ \

CSR references: CSR-reference content comment Sec 2/3.1.5 Operating conditions Sec 2/3.1.6 Operating draughts Sec 7/2.1 Static hull girder loads Sec 8/1.1 Loading guidance Sec 8/1.1.2 Loading manual Sec 8/1.1.3 Loading computer program

c) Local loads:



The above mentioned wave-induced dynamic (D) and loading condition static (S) load components are combined in order to calculate the maximum local loads (S + D) used to evaluate structural members. Design loads included in the Rules also contain margins to cover accidental (A) loads such as occasional overruns or overloads during loading or unloading operations. This includes the height of air pipes and pressure relief vale settings. Details of the determination of the local loads are included in Section 5.II.2 of this report. The following table, which is a copy of Table 2.4.1 from the CSR, indicates load categories included in the Rules.

Load Categorisations

Lightship weight Steel weight and outfit Machinery and permanent equipment

Buoyancy loads Buoyancy of the ship Variable loads Cargo

Ballast water Stores and consumables Personnel Temporary equipment

Operational Loads

Other loads Tug and berthing loads Towing loads Anchor and mooring loads Lifting appliance loads Dynamic wave pressures Cyclic loading due to wave action

including inertia loads Dynamic loads and dynamic tank pressures due to ship accelerations

Environmental loads

Impact loads or resonant loads Wave impacts Bottom slamming Liquid sloshing in tanks Green sea loads

Accidental loads Flooding of compartments Deformation loads Thermal loads

Deformations due to construction CSR references: CSR-reference content comment Sec 2/3.1.8 Internal environment (cargo and

water ballast tanks)

Sec 2/4.2.3 Load categorisation Tab 2.4.1 Load categorisation Sec 2/4.2.5 Operational loads Sec 2/4.2.7 Accidental loads Sec 7/2.2 Local static loads Sec 7/5 Accidental loads Tab 8.2.7 Design load sets for plating and

local support members

Tab 8.2.8 Specification of design load combination, acceptance criteria and other load parameters for each design load set

Tab 8.2.9 Design load sets for primary support members



d) Load combination: Design load combinations combine local and hull girder load components to represent design load scenarios. The effects of combining the dynamic (D) and the static (S) loads are also included in the combined design loads. The design scenarios are selected to encompass all scenarios that can reasonably occur during operation. The loading scenarios include the assessment of tank boundaries, e.g. bulkheads, based on the most severe combination of loading hence conditions are assessed with a full tank content on one side and an empty tank on the other side. The situation with the tank contents reverse are also considered. Similarly the shell envelope is assessed for conditions at the deepest draught without internal filing and at the lowest draught with maximum internal filling. The loads are combined for evaluation of the hull girder and structural members in order to consider the most unfavourable combination of load effects. A variety of different load cases are applied in order to provide maximum loads applied to individual areas of the structure rather than one load case which attempts to envelope all maximum loads simultaneously, since maximum loads acting simultaneously do not actually occur in operation. These combined load effects are used to develop the longitudinal hull girder strength and the strength evaluation of structural members. The following table, which is Table 2.5.1 from the CSR, illustrates the combination of loads.



Load Scenarios and Corresponding Rule Requirements

Load Scenarios Rule Requirements

Design Load Combination (specified in Section 7/6) Operation

Loads

(that the vessel is exposed to and is to withstand)

Ref. no Notation

Design Format

(specified in

Sections 8 and 9)

see Note 1

Acceptance Criteria Set

(specified in Sections 8 and 9)

Seagoing operations

1. SG + SL + DG + DL ≤ η2 R1 AC2 Static and dynamic loads in heavy weather 1 S + D

2. γS SG + γD DG ≤ R2/ γR2 AC2

Impact loads in heavy weather 2 Impact SL + Dimp ≤ η3 Rp AC3

Internal sloshing loads 3 Sloshing SG + Dslh ≤ η1 R1 AC1

Transit

Cyclic wave loads 4 Fatigue DM ≤ ∑ηi / Ni -

BWE by flow through or sequential methods

Static and dynamic loads in heavy weather 5 S + D SG+SL+ DG + DL ≤ η2R1 AC2

Harbour and sheltered operations

Loading, unloading and ballasting

Typical maximum loads during loading, unloading and ballasting operations

6 S SG + SL ≤ η1 R1 AC1

Tank testing Typical maximum loads during tank testing operations

7 S SG+ SL1≤ η1 R1 AC1

Special conditions in harbour

Typical maximum loads during special operations in harbour, e.g. propeller inspection afloat or dry-docking loading conditions

8 S SG+ SL ≤ η1 R1 AC1

Accidental condition

for water tight boundaries

1. SL ≤ η2 R1 AC2

Accidental flooding

Typically maximum loads on internal watertight subdivision structure due to accidental flooding

9 A for collision bulkhead

2. SL ≤ η1 R1 AC1

Note

1. The symbols defined in this column are defined in the text of 5.4

Where:

DG

DL

DM

SG

SL

Ri

dynamic global load

dynamic local load

cumulative fatigue damage ratio

static global load

static local load

structural capacity



CSR references: CSR-reference content comment Sec 2/4.2.2 Design load combinations Sec 2/5.4.1.1 to 5 Load-capacity based requirements Tab 2.5.1 Load scenarios and corresponding

rule requirements

Sec 2/5.4.2 Design loads for scantling requirements and strength assessment (FEM)

Sec 7/6 Combination of loads Tab 7.6.1 Design load combinations Tab 8.2.7 Design load sets for plating and

local support members

Tab 8.2.8 Specification of design load combination, acceptance criteria and other load parameters for each design load set

Tab 8.2.9 Design load sets for primary support members

Tab B.2.3 FE load cases Tab B.2.4 FE load cases

e) Structural modelling: There are two general forms for structural modelling included in the Rules. The first applies beam and plate theory and prescriptive buckling formulations. The second involves application of finite element modelling. The first form of structural modelling consists of using engineering principles to calculate section cross area, inertia, section modulus, web area and plate or shell membrane properties, and is associated with the prescriptive rules covering such items as bending, shear and buckling. This type of modelling is used to assess the structural properties of the vessel during the initial stages typically employing a working stress design (WSD) format. The working stress level is determined by applying the design loads using beam and plate theory and buckling formulae. This working stress level is then compared against an allowable stress. In many cases the formula is rearranged mathematically to include the allowable stress and the result is the required structural property such as thickness, section modulus, etc. The Rules contain details on the section properties to be used with the Rule requirements. The second form of structural modelling using a finite element (FE) model also employs a working stress design (WSD) format. The Rules include detail specification of the FE model such as; model extent, structure to be modelled, openings to be modelled, properties, element size, element type, aspect ratio, and boundary conditions. The FE analysis employs a series of models using a global model to represent the overall hull girder structure and then using local fine mesh models to review high stress gradient areas and stress concentrations. Finally, very fine mesh FE models are used to zoom in and assess the hopper knuckle connection between the inner-bottom and the hopper plate. The Rules include detail specifications for the fine mesh models similar in content to the global model mentioned above.



It should be noted that all structural models employ the net thickness concept in which the actual as-built thickness is reduced to represent in service diminution due to corrosion. The net thickness concept is described in section 5.II.3.5 of this report. CSR references: CSR-reference content comment Sec 2/4.3 Structural capacity assessment Sec 2/5.4.4.1 Structural response analysis Sec 3/5 Calculation and evaluation of

scantling requirements

Sec 4/2 Structural idealization Sec 9/1.3 Hull girder bending moment

capacity Hull girder ultimate strength

Sec 9/2.2.2 Structural modelling Global FEM Sec 9/2.3.2 Structural modelling Fine mesh FEM App A/2.2.2 Assumptions and modelling of the

hull girder cross-section Hull girder ultimate strength

App B/2.2 Structural modelling Global FEM App B/3.2 Structural modelling Fine mesh FEM App B/3.4 Application of loads and boundary

conditions Fine mesh FEM

App B/4.2 Structural modelling Fatigue App B4.4 Boundary conditions Fatigue

f) Fatigue: For fatigue considerations, please refer to section 5.II.4 of this report. g) Corrosion: For corrosion considerations, please refer to section 5.II.6.2 of this report. h) Material imperfections: The CSR include the IACS requirements for materials covering strength properties, material grades and required application. The remainder of the detail requirements for materials such as the chemical makeup, through thickness properties, testing, etc. are referenced to be in accordance with the individual Classification Society rules. While the minimum strength properties of yield and ultimate tensile strength are specified in the CSR, the actual physical properties of materials fitted in the ships are usually greater. However these margins are not accounted for and no safety margin is attributed to this. The strength requirements in the CSR are based on the assumption that the material is manufactured in accordance with minimum strength properties and the allowable under thickness rolling tolerances specified in IACS UR W13. Please also refer to section 5.II.11 of this report. CSR references: CSR-reference content comment Sec 2/4.4.1 Materials Sec 2/5.5 Materials Sec 6/1 Steel grades

i) Construction workmanship errors:



For construction and workmanship considerations, please refer to section 5.II.11 of this report. j) Buckling: The buckling criteria in the CSR include various levels of complexity that build upon one another. The simplest buckling check is in the form of stiffness and proportion ratios that relate simplified buckling and deflections to the most basic structural property such as panel spacing, unsupported flange breadth or pillar length. Using the spacing, flange length or pillar lengths, ratios are used to determine related permissible thicknesses. The next level of buckling check is performed using prescriptive buckling based on classic Euler buckling of plates, shells, columns and torsional buckling modes. Finally an advanced buckling analysis un-stiffened and stiffened plate panels is based on nonlinear analysis techniques. The most advanced buckling analysis includes an allowance for redistribution of loads such that the ultimate capacity of the panel is calculated. CSR references: CSR-reference content comment Sec 2/5.4.5.2 Structural capacity assessment Sec 8/1.4 Hull girder buckling strength Sec 8/2.6.1.6 Primary support members Web buckling, ref. to 10/2.3 Sec 9/2.2.5.3 Acceptance criteria FEM Sec 10 Buckling and ultimate strength App D Buckling strength assessment

k) Residual strength: For residual strength considerations, please refer to section 5.II.5 of this report. .2 Strength Assessments The GBS lists various items which should be assessed in the rules. The items mentioned are each discusses as follows: a) Members to be evaluated: The CSR include requirements for the structural evaluation of all strength components of the vessel. The evaluations of the cargo block region of the vessel is based on both prescriptive and a finite element analysis. Prescriptive requirements are included for the forward and aft regions and the deckhouse structure . See the following figure, which is Figure 1.1.1 from CSR, for a map of references to the applicable CSR section.



Schematic Layout of the Rules

Aft end& Machinery Room

Fore endCargo Area

Ship in operation renewal criteria 12

Testing procedures 11/5

Equipment 11/4

Support structure and structural appendages 11/3

Crew protection 11/2

Hull openings and closing arrangements 11/1

Topic Sections

Sections

8/5.4

8/5.3

8/5.2

8/5.1

8/4.5-4.8

8/4.4

8/4.3

8/4.2

8/4.1

Aft end deck structure

Aft end shell structure

Aft end bottom structure

Aft end general structure

Machinery internal structure

Machinery deck structure

Machinery side structure

Machinery bottom structure

Machinery general structure

Topic

8/5.5-5.7Aft end internal structure

Sections

9/2

9/1

8/6.2

8/2.6

8/2.5

8/2.4

8/2.3

8/2.2

8/1Hull girder strength

9/3

Strength assessment (FEM)

Hull girder ultimate strength

Sloshing

Primary support members

Bulkheads

Inner bottom

Hull envelope framing

Hull envelope plating

Topic

Fatigue strength

Sections

8/6.4

8/6.3

8/3.5-3.9

8/3.4

8/3.3

8/3.2

8/3.1

Topic

Bow impact

Bottom slamming

Internal structure

Deck structure

Side structure

Bottom structure

General structure

Loads 7

Materials and Welding 6

Structural Arrangement 5

Basic Information 4

Rule Application 3

Rule Principles 2

Introduction 1

Topic Sections



b) Failure modes: The criteria for the assessment of scantlings are based on a working stress design (WSD) method. The failure modes include yielding, buckling and fatigue. Deflection criteria is also included and covered in the next section of this report. The acceptance criteria included in the CSR have been related to the loading scenario as shown in Table 2.5.1 as copied in this report Section 5.II.3.1.d. The failure modes associated with the scenario are indicated in the following tables, which are Tables 2.5.2 and2.5.3 from the CSR.

Principal Acceptance Criteria - Rule Requirements

Plate panels and Local Support Members

Primary Support Members Hull girder members

Acceptance criteria set Yield Buckling Yield Buckling Yield Buckling

AC1: 70-80% of

yield stress

Control of stiffness and proportions.

Usage factor typically 0.8

70-75% of yield stress


Pillar buckling

75% of yield stress

NA

AC2: 90-100% of yield stress



85% of yield stress


Pillar buckling


Usage factor

typically 0.9

AC3: Plastic criteria

Control of stiffness and proportions

Plastic criteria


NA NA

Principal Acceptance Criteria - Design Verification - FE Analysis

Global cargo tank analysis Local fine mesh analysis

Acceptance criteria set Yield Buckling Yield




local mesh as 136% of yield stress

averaged stresses as global analysis






CSR references: Sections 2/4.5, 2/5.4.1.5 to 10, Table 2.5.1 to 3, 2/5.4.5 and 2/5.4.6.



Yielding: the yielding allowable stresses for bending and shear modes specified for hull girder, primary support members and local members are generally shown in the above tables. More detailed information on the allowable stresses for each individual component is included in the CSR references listed below. CSR references: Table 2.5.2, Table 2.5.3, Sections 8/1.2, Table 8.1.3, 8/1.3, Table 8.1.4, Table 8.2.4, Table 8.2.5, Table 8.2.10, 9/2.2.5 and Table 9.2.1. Buckling: the buckling allowable limits specified for hull girder, primary support members and local members are generally shown in the above tables. More detailed information on buckling criteria for each individual component is included in the CSR references listed below. CSR references: Table 2.5.2, Table 2.5.3, 8/1.4.2.6 to 8/1.4.2.8, Table 9.2.2, 10/2.3, 10/3.2.1.3, 10/3.3.2.1, 10/3.3.3.1 and D/4. Fatigue: the fatigue criteria is associated with the design life of 25 years and exposure to the North Atlantic environment. See Section 5.II.4 of this report and the CSR references below for additional details. CSR references: Sections 2/4.3.3, Tab 2.5.1, 2/5.4.3, 2/5.6.5, 8/1.5, 9/3, B/4, C/ c) Deflection: Hull girder deflection requirement is covered by a minimum vertical hull girder moment of inertia. Local structural deflection is generally covered in the CSR by inclusion of minimum thicknesses, minimum depth-to-thickness ratios and buckling control measures. The establishment of the deflection criteria was based on the existing satisfactory service associated with the existing class rules. CSR references: Sections 2/5.3.1.1(b), 2/5.4.5.1, 3/5.3.3.4, 8/1.2.2, 8/2.6.1.7 plus individual requirements, and 10/2. .3 Ultimate Strength The ultimate strength evaluations cover hull girder properties as well as individual stiffened plate panels. a) Ultimate strength of the hull girder The evaluation of the hull girder is the most important component of the strength assessment. The CSR include hull girder longitudinal strength evaluations controlling yielding and buckling based on working stress design (WSD) levels associated with the static and dynamic load components. The in-service operational limits are also closely controlled in order to remain within the WSD limits. In addition, to provide an additional check for the hull girder, an ultimate limit evaluation is performed to check the condition of the vessel in extreme at-sea conditions using the following general expression.



R

UsagwvWswS γ

MMγMγ ≤+ −

Where:

Msw sagging still water bending moment.

Mwv-sag sagging vertical wave bending moment.

MU sagging vertical hull girder ultimate bending capacity.

γ S, γ W, γ R are the partial safety factors for the design load combinations.

Partial Safety Factors

Design load combination

Definition of Still Water Bending Moment, Msw γ S γ W γ R

a) Permissible sagging still water bending moment 1.0 1.2 1.1

b) Maximum sagging still water bending

moment for homogenous full load condition

1.0 1.3 1.1

Where:

γ S partial safety factor for the sagging still water bending moment

γ W partial safety factor for the sagging vertical wave bending moment covering environmental and wave load prediction uncertainties

γ R partial safety factor for the sagging vertical hull girder bending capacity covering material, geometric and strength prediction uncertainties

Partial safety factors increasing the magnitude of the wave-induced bending moment by 20 and 30 percent are applied in conjunction with the permissible and most probable still water bending moment respectively. The calculation procedure for the determination of the hull girder bending capacity, is included in Appendix A of the CSR. CSR references: CSR-reference content comment Sec 2/5.6.3 Design verification - hull girder

ultimate strength

Sec 9/1 Hull girder ultimate strength Requirements App A Hull girder ultimate strength Procedure

b) Ultimate strength of plates and stiffeners In general the CSR includes local plate criteria that employs working stress design (WSD) format, however, some conditions and locations are permitted to approach the



ultimate strength of a plate panel. The modes are defined in the advanced buckling section 10/4 and Appendix D of the CSR as follows: Method 1 – buckling capacity with allowance for redistribution of load. This defines the upper bound value of the buckling capacity and represents the maximum load the panel can carry without suffering major permanent set and is effectively the ultimate load carrying capacity of a panel. The buckling capacity is taken as the load that results in the first occurrence of membrane yield stress anywhere in the stiffened panel. In calculating this, load redistribution within the structure is taken into account. This redistribution of load is a result of elastic buckling of component plates, such as the plating between the stiffeners. Method 2 - buckling capacity with no allowance for redistribution of load. This defines the lower bound value of the buckling capacity. In calculating the buckling strength, no internal redistribution of load is to be taken into account. Hence this is more conservative than the upper bound value given by Method 1 and checks that the panel does not suffer large elastic deflections with consequent reduced in-plane stiffness. CSR references: CSR-reference content comment Sec 10/4 Advanced buckling analysis Requirements App D Buckling strength assessment Procedure

.3 Structure compatibility a) purpose of the space The structural requirements of the CSR include consideration of the purpose and associated environment of the space to which the structure is exposed. This can be either the external environment such as temperature exposure, marine corrosive environment. or the internal environments of cargo, ballast and dry spaces such as liquid density, temperature and corrosive nature. These environments which relate to the purpose of the space influence the material grade requirements, corrosion additions. CSR references: Sections 2/3.1.7 and 2/3.1.8. b) structural continuity Structural continuity, termination of members and alignment with backup structure is covered in the CSR. The objective of the structural continuity requirements is to effectively avoid hard spots, notches and stress concentrations. The CSR has requirements for large hull girder longitudinal members as well as for the end termination of primary and local members. Another important reason for including this in the rules is to clarify the end connection continuity associated with the rule formulations. For instance the continuity of the ends dictate the end connection of a beam which in-turn dictate the bending moment, e.g. fixed-fixed or pinned-pinned, and then influence the associated structural requirement. Therefore the rules contain quite extensive coverage of this subject as listed below. CSR references: CSR-reference content comment Sec 4/3.2 to 4 Structure design details Local and primary support

member end connections



Sec 8/1.6 Tapering and structural continuity of longitudinal hull girder elements

Sec 8/1.6.5 and 6 Structural continuity Longitudinal bulkheads and longitudinal stiffeners

Sec 8/2.1.4.7 General scanting requirements End connections Sec 8/2.3.1.3 Hull envelope framing End connections Sec 8/3.1.3 Structural continuity Forward of the forward

cargo tank Sec 8/4.1.3 Structural continuity Machinery space Sec 8/5.1.3 Structural continuity Aft end

.4 Facilitate loading/unloading In addition to the operating loads that most designers consider, the CSR also include loading and unloading conditions in the matrix of design loads to be considered. See CSR Table 2.5.1 as copied in this report Section 5.II.3.1.d. Loading conditions upon which the vessel is approved, which include loading and unloading operations are required to be included in the vessel Loading Manual as indicated in Section 8/1.1.2.2(b) of the CSR. CSR references: CSR-reference content comment Sec 2/4.2.1 Load scenarios Tab 2.5.1 Load scenarios and corresponding

rule requirements

Sec 8/1.1.2.2(b) Loading manual Harbour/sheltered water conditions

.5 Net scantlings The net scantling approach is used to perform the ship design and verification calculations using scantlings in an assumed future corroded condition. Therefore the design is assessed for the critical load cases for the different assessment criteria such as strength (e.g. yielding, ultimate strength and buckling) and fatigue, while in an expected corroded condition. This expected corroded condition is typically defined in association with the assessment criteria type and the structural arrangement of the vessel being investigated. While the expected corrosion additions which are to be used in design calculations can be accurately defined in a design code or classification society rule, the actual corrosion experienced in-service can vary depending on maintenance performed, coatings provided, coating maintenance, cargo carried, ballast carried, operating environments, loading/unloading processes, etc. Therefore the actual corrosion experienced by a particular ship may be larger or smaller depending on the actual operating conditions and maintenance of the ship throughout its life cycle. Since the actual corrosion in-service depends on a wide variety of factors that can not be fully anticipated and controlled, the Rules use a design net thickness approach that is aligned and compatible with the associated thickness gauging and renewal requirements that are applied to the vessel. Ships are subjected to thickness measurement requirements during their lifetime. When local thicknesses measured do not comply with the requirements, renewals are required to replace the local plating or



stiffening members to their original condition, thereby keeping the individual structural elements in a state that is generally thicker than the net scantlings used in the original design calculations. In-service diminution allowances for hull girder section modulus and the thickness of individual structural elements are generally set by classification society rules. However, it should be noted that resolution A.744(18), as amended, specifies allowable diminution of the hull girder section modulus for oil tankers 130m in length and upwards and over 10 years of age (ref. resolution MSC.105(73)). Additionally, recommended criteria for specific structural members of single side skin bulk carriers are provided in the IACS Unified Requirements which are referenced by resolution MSC.145(77). The in-service minimum thickness requirements contained in classification society rule requirements (e.g., IACS UR S7) generally indicate stringent measurement criteria to be used for the assessment of members contributing to hull girder strength and less stringent localized measurement criteria to be used for the assessment of individual local members. The following summary may be made: .1 Hull Girder Longitudinal Strength Members – the global corrosion or average

corrosion of the members contributing to the hull girder longitudinal strength are permitted to waste to the degree whereby the hull girder section modulus is reduced by no more than 10 percent. This in effect limits the corrosion of the deck and bottom members to an average of about 10 percent of the original required thickness. This is consistent with resolution MSC.105(73).

.2 Individual Structural Elements – the local thickness diminution allowance for

individual plating and stiffening elements is typically in the range of 2.5 to 4.0 mm. These local individual allowances are generally greater than the 10 percent average which are also applicable for the structural members contributing to hull girder section modulus referred to in .1 above.

.3 Local Pitting, Grooving and Edge Corrosion – for completeness of the rules the

thickness diminution allowance for pitting, grooving and edge corrosion of plating and stiffening elements, typically in the range of 25 to 30 percent of required gross thickness, is included in the CSR. These localized items are checked in service and renewed when necessary, but specific accounting is not included in the strength criteria other than via calibration with actual vessel service.

In the CSR, the overall average corrosion for hull girder cross-section and primary support members is given by simultaneously deducting half the local corrosion addition from all structural members comprising the respective cross-sections. This replicates a 10 percent reduction of global strength which will later be monitored in-service. The assessment of local scantlings is performed based on the superposition of stresses associated with the reduced hull girder properties and the local stresses associated with the local full deduction of the corrosion additions. In other words, the CSR assumes that the structure is corroded locally to the maximum allowed and the hull girder is reduced to the maximum allowed overall hull girder corrosion. Since fatigue is a time-dependant phenomenon that takes place over long periods of the ship’s life, stress calculations associated with fatigue should reflect variations in thicknesses due to corrosion through the design life (e.g. consider full “as-built” scantlings for the vessel in the initial stage of its operational life and expected design net scantlings at the end of the assumed design life). However the CSR contains a



simplification which uses the average scantling properties between the initial as-built stage and the expected corroded state at the end of the assumed design life. CSR references: CSR-reference content comment Sec 2/4.3.4 Net thickness approach Sec 4/2.4 Geometrical properties of local

support members

Sec 6/3 Corrosion additions

II.4 Fatigue life

Rating: The functional requirement is covered by CSR. Comment: In the goal based standards, the design fatigue life should be not less than the design life and should be based on North Atlantic Environmental conditions. The fatigue life calculation procedures of CSR are based on three common major hypotheses: .1 The long term distribution of stresses in the structure of the ship sailing in North-

Atlantic environment may be represented by a two-parameter Weibull law. The best fit of the Weibull distribution to the North-Atlantic scatter diagram is obtained by selecting a probability of occurrence (10-4) for the scale parameter of the Weibull law.

.2 The linear damage accumulation rule of Miner’s sum is valid and a unit value of the damage ratio D corresponds to fatigue cracking.

.3 The expected fatigue life is to be greater or equal to the design life (i.e. 25 years). The Weibull law is defined as follows:

])(exp[1)()(Pr ξ

wxxFxeStressRangobability −−==<

With ξ the shape parameter and w = Sr/ln(Nr)1/ξ the scale parameter . In the expression of the scale parameter, Sr is the stress range computed at 1/Nr probability level. The best fit with the scatter diagram is obtained by taking Nr = 104 cycles. The value of ξ is obtained by a fitting procedure and lead to a value around 1.0: 0.85 to 1.05 according to the rule set and the length of the ship. The fatigue cracking appears when the damage ratio is greater than 1, therefore the

damage ratio ∑=

=

=ntoti

i i

i

Nn

D1

is to be less than 1 where the number of cycles is summed on

the whole fatigue life of the vessel of 25 years. In the damage ratio expression, ni is the number of cycles of stress range Si and Ni the number of cycles leading to failure according to the S-N curve, at the stress range Si.



CSR references: CSR-reference content comment Sec 9/3 Fatigue strength Requirement, not less than 25

years App C Fatigue strength assessment Procedure

II.5 Residual strength

Rating: The functional requirement is partially covered by CSR. Comment: The rules explicitly states that only intact structure is considered: 2/4.3.5.1 All strength calculations are based on the assumption that the structure is intact. The residual strength of the ship in a structurally damaged condition is not assessed. Hence, requirements to residual strength as formulated in Tier II.5 are not explicitly covered by the rules. However, it is stated as a general principle in the rules that the ship’s structure is designed such that it has adequate structural redundancy to survive in the event that the structure is accidentally damaged: 2/4.1.2.2(d) it has adequate structural redundancy to survive in the event that the structure is accidentally damaged; for example, minor impact leading to flooding of any compartment. This statement indicates that the rule development implicitly covered residual strength. This was based on typical inherent residual strength exhibited by existing vessels upon which the rules were calibrated. Flooding is included in the rules as an accidental load: 4.2.7.1 The accidental load scenarios cover loads acting on local structure as a consequence of flooding in accordance with the assumptions made in IMO regulations. This relates to the assessment of the watertight subdivision boundaries. Only the local scantlings due to flooding pressure is checked. The effect of the flooding pressure on the hull girder loads is not accounted for in the hull girder strength assessment. The effect of structural damage on the hull girder capacity resulting from collision or grounding is not assessed in CSR. The effect of collision damage in the upper part of the side was assessed using probabilistic methods in the SAFEDOR project. The conclusion from this study was that the intact condition is dimensioning for the hull girder strength, and that requirements for the damaged case therefore could be omitted. This study is documented in the following reference:



Hørte, T. et al., Probabilistic methods applied to structural design and rule development, RINA Conference, January 2007 Post-buckling behavior is included in the hull girder ultimate strength calculations, but the calculations are only carried out for intact structure. CSR references: CSR-reference content comment Sec 2/4.3.5.1 Intact structure Sec 2/4.1.2.2 Design principles Sec 2/4.2.7.1 Accidental loads Sec 7/2.2.3.4 Flooding pressure Sec 7/5 Accidental loads App A/2.3 Hull girder ultimate strength


Rating: The functional requirement is covered by CSR. Comment: The following two sub-sections pertain to providing protection against corrosion or anticipating corrosion in the strength calculations. The overall goal being that the required scantlings meet the intended strength provisions thoughout the specified design life.

II.6.1 Coating life

With regard to the mandatory use of coatings, the CSR includes it in Section 6/2 Corrosion. The purpose and intention of this section is to ensure that the Rules are inline with the SOLAS requirement with respect to corrosion prevention of ballast tanks. The text provides reference to the requirements of SOLAS Reg. II-1/3-2, IMO Resolution A.798(19) and IACS UI SC 122. The requirements are open with respect to application date, which at the time of publishing the rules was yet to be finalized by IMO. It has now been determined that the application date for vessels to which the CSR apply is 8 December 2006, which is based on the building contract date. As described in the section 6/1.1.1.2, for ships contracted for construction on or after 8 December 2006 which is the date of IMO adoption of the amended SOLAS Regulation II-1/3-2, the coatings of internal spaces subject to the amended SOLAS regulation are to satisfy the requirements of the IMO performance standard. The IMO performance standard means IMO Resolution MSC.215(82) – “Performance standard for protective coatings for dedicated seawater ballast tanks in all types of ships and double-side skin spaces of bulk carriers”. The referenced requirements cover the following items related to information and documentation for II.6.

.1 Locations and/or spaces where coatings are required to be used



.2 Types of coating to be used for the various spaces

.3 Reference coating performance standards Regarding allowances when other corrosion prevention systems are used, the sections “6/2.1.2 Internal cathodic protection systems” and “6/2.1.3 Paint containing aluminium” cover allowances when other corrosion prevention systems are used. CSR references: CSR-reference content comment Sec 6/2 Corrosion Protection Including

Coatings


The CSR corrosion additions are located in Section 6/3. Firstly, it should be noted that CSR for tankers does not employ a corrosion rate approach but a more advanced approach using a stochastic corrosion propagation model. The CSR complies with the functional requirements of Tier II.6.2 Corrosion addition by following the latter approach. Local corrosion additions for typical structural elements within the cargo tank region are shown in Table 6.3.1 and Fig. 6.3.1. In addition, the relation between corrosion addition and wastage allowance is described in “Section 6/3.2”. The local corrosion additions are derived by adding 0.5mm to wastage allowances for the particular local structural element. The background on the relationship of corrosion additions and wastage allowances is explained in Section 2/4.3.4 (Net thickness approach) and the details on local wastage allowances, are explained in Section 12/1.4 (Renewal criteria of local structure for general corrosion) of the CSR. Structures considered and the appropriate wastage allowance values for each side of structural elements are as given in Table 12.1.2 of the CSR. The 0.5mm is added in reserve for the wastage occurring between the inspection intervals of approximately 2.5 years. The verification of the local strength of the vessel is performed on the local net thickness (gross minus corrosion addition tcorr) and the global strength of the vessel is performed at global net thickness (gross minus 50 percent of the corrosion addition tcorr). As the wastage allowance is assessed based on thickness measurements performed in connection with the renewal survey some margin is needed on the wastage allowance as the vessel will operate for approximately another 2.5 years before being re-assessed. During this 2.5 year interval the thicknesses should not reduce below the net thickness. In this context, as corrosion additions are completely consistent with wastage allowances. The total “corrosion addition” or ”wastage allowance” values used in the CSR were based on the stochastic corrosion propagation model and information that were being



used by IACS ex-WP/S (Working Party/Strength) to arrive at wastage allowance values based on historical data on record of gaugings. In some areas of the structure a extra margin was added to account for the variability of corrosion based on service experience. The general philosophy for establishing “corrosion additions” or ”wastage allowances” was that they are to be: (a) based, in general, on the premise that today’s practice is a reference point, and

departures from today’s practice will need to be backed-up with technical justification;

(b) established based on the basic assumption of coatings provided (where required) at time of newbuilding, however, there should not be provisions to reduce wastage allowance values based on “superior” coating systems or extra-ordinary maintenance of coating systems or another type of corrosion protection system;

(c) appropriate for a 25-year service life; (d) based on absolute numbers, i.e., 4.0mm (not 25%); (e) independent of type of local failure mode employed, i.e., yielding, buckling, or

fatigue; (f) based on published data and recent experience of IACS member societies; The following basic assumptions were made: (a) with respect to stiffener and web members, wastage should be based on thickness

loss, not section modulus loss; (b) wastage values, though linked to net thickness deductions, should first be

developed independently of the net thickness deductions, and based on the philosophy outlined above;

(c) the wastage values should be based on typical wastage values experienced in service for crude oil carriers;

(d) dependencies on cargo type and vessel size should be considered, but should not be variables used for determining the actual value of the permitted wastage on a ship-by-ship basis;

(e) structural elements within the same area, environment and orientation should as far as possible have the same wastage allowance; and

(f) safety margins should not be included in wastage allowances (i.e., criticality issues should be dealt with in “net” requirements, and not with an increase in the wastage allowance).

Based on the above and following IMO discussion regarding GBS, IACS carried out statistical analysis of collected corrosion data and evaluated “corrosion addition” or ”wastage allowance” values by using the 95 percent probability level corrosion measurement values for a 25-year life. Furthermore, each of the individual societies took into consideration data that they had on hand regarding their own in-house reports and studies in addition to published corrosion data when finally determining “corrosion addition” or ”wastage allowance” values appropriate for a 25-year service life. CSR references: CSR-reference content comment Sec 6/3 Corrosion additions Sec 12/1/4 Renewal criteria of local structure

for general corrosion



References and Background Documents [1] IMO Resolution A.798(19), Guidelines for the selection, application and

maintenance of corrosion prevention systems of dedicated seawater ballast tanks [2] IACS UI SC 122, Corrosion Prevention in Seawater Ballast Tanks [3] IMO Resolution MSC.215(82) – Performance standard for protective coatings for

dedicated seawater ballast tanks in all types of ships and double-side skin spaces of bulk carriers

[4] IMO Resolution MSC.216(82) – Adoption of amendments to the international convention for the safety of life at sea, 1974, as amended

[5] Sone, H. et al., Evaluation of Thickness Diminution in Steel Plates for the Assessment of Structural Condition of Ships in Service、ClassNK Technical Bulletin Vol.21, 2003.

II.7 Structural redundancy

Rating: The functional requirement is partially covered by CSR. Comment: Requirements to structural redundancy are not covered explicitly by the rules. However, it is stated as a general principle in the rules that the ship’s structure is designed such that it has inherent redundancy See CSR 2/4.1.2.2(a): The ship’s structure works in a hierarchical manner and, as such, failure of structural elements lower down in the hierarchy should not result in immediate consequential failure of elements higher up in the hierarchy. This statement indicates that the rule development implicitly covered structural redundancy. This was based on typical inherent redundancy exhibited by existing vessels upon which the rules were calibrated. It is worth noting that a double hull by its very nature is a very redundant structure. It offers structural redundancies against collisions and groundings, including damages or failures of structural members in either the inner hull or outer hull. The risk of a major structural failure or casualty is much less in a double hull tanker than a single hull tanker because of its structural redundancy. The use of “criticality class” during the rule development can be considered as contributing to the redundancy of the structure. During the rule development, each structural component was classified according to the criticality with respect to the consequences of failure. At the top level of the hierarchy is the hull girder, while the local plate element is at the bottom. This hierarchical structure was used for setting the acceptance criteria and selecting the capacity models. As a consequence, stricter requirements are applied to the elements high up in the hierarchy. This means that less critical local elements will collapse first, without leading to collapse of higher-level elements. The use of advanced buckling methods for buckling assessment ensures redundancy of stiffened panels, by allowing local plates to buckle and require that the stiffeners are able to carry the redistributed forces. This principle gives strong stiffeners and weaker plates, and thereby redundant panels. In contrast to stiffened panels, corrugated bulkheads are generally not redundant, since collapse of the plate flange leads to collapse of the entire bulkhead. The CSR does not have special requirements for redundancy related to corrugated bulkheads however,



additional and more complex acceptance criteria are provided and the buckling criteria is lowered to account for this. Especially longitudinal horizontally corrugated bulkheads are critical, due to their contribution to the longitudinal strength. CSR references: CSR-reference content comment Sec 2/4.1.2.2 Design principles App D/1 Advanced buckling analusis

CSR External background documentation, available on IACS Web Site: Section 2/4.5.1 Criticality class of structural elements


Rating: the functional requirement is covered by CSR. Comment: The main principles of watertight and weathertight integrity with respect to the subdivision of the ship hull (Sec 5/2 of CSR) are given by the SOLAS Convention of IMO, referenced by Sec3/3.3 and Sec2/2.1.1 of CSR. The position of bulkheads in the cargo area and therefore the number of bulkheads is, in case of the type of ship considered, determined by the limits of cargo tank size with respect to the possible oil outflow and the damage stability (Sec5/2.1.2). These limits are given in the current MARPOL and SOLAS requirements, which are referenced by Sec5/2.1.2, Sec2/2.1.1 and Sec3/3.3. Particular requirements with respect to bulkhead construction and scantlings of watertight boundaries in different areas of the ship are given in Sec8/2.5, Sec8/3.6, Sec8/4.7 and Sec8/5.6. General requirements related to the securing devices for hull openings are prescribed by requirements of the International Load Line convention and the SOLAS convention of IMO. Particular, ship type specific items are sufficiently described in Sec11/1 of CSR. In particular requirements regarding shell and deck openings are covered by Sec11/1.1, requirements related to air and sounding pipes are covered by Sec11/1.3, requirements for openings in superstructures and deck house sides are included in Sec11/1.4 and requirements to overflows and vents etc. are included in Sec11/1.5. CSR-reference content comment Sec2/2.1.1 Reference is made to IMO

regulations

Sec3/3.1.1.2 Reference is made to regulations of international, national, canal and other authorities

Sec3/3 Reference is made to requirements of national and international regulations

Statement that compliance with national and international regulations is not necessary scope of class approval but scope of review by flag state administration

Sec5/2 Watertight subdivision Sec5/2.1.2.3 Reference is made to requirements

of national regulations

Sec8/2.5 Scantlings of Bulkheads



Sec8/3.6 Watertight boundaries in fore-ship area

Sec8/4.7 Watertight boundaries in machinery space

Sec8/5.6 Watertight boundaries at aft end of the ship

Sec11/1 Hull openings and closing arrangements

Sec11/1.1 Shell and deck openings Sec11/1.2 Ventilators Sec11/1.3 Air and sounding pipes Sec11/1.4 Deck houses, companionways Sec11/1.5 Scuppers, inlets, discharges


Rating: The functional requirement is partially covered by CSR. Comment: Human element considerations with respect to the ship’s structure are mainly related to sufficient opening-space for inspection, maintenance, repair and rescue operations, guard rails, ladders, flush decks, covers etc. They are only in scope of classification rules with respect to class surveys (sufficient opening spaces, breadth of access ways etc.). In general this functional requirement is subject of national requirements of flag state authorities and accidental prevention regulations of employer’s liability insurance associations and similar organisations. Furthermore there does exist regulations of Tier V like ISO and other industry-standards e.g. ISO 799 “pilot ladders” and DIN 81705 “removable guard rails for seagoing ships”. The requirements are included relative to a number of different sections of the CSR. Special requirements to the protection of the crew members by means of bulwarks and guard rails are given in Sec 11/2.1. Sizes of openings and details of portable plates are included in Sec 11/1.1 Sizes of access openings are described in Sec 5/5.1 Only the ship-type special requirements are introduced in detail in the CSR. For more general requirements cross-reference is made to effective rules and regulations of the flag state authorities such as SOLAS with respect to accidental prevention and ergonomics. CSR-reference content Comment Sec3/3.1.1.2 Reference is made to regulations of

international, national, canal and other authorities

Sec5/5.1.1.4 Size of access openings Sec11/1.1.11 Portable plates Sec11/2 Crew protection Sec11/2.1 Bulwarks and Guardrails Sec11/2.2 Tank Access see also table 11.2.2 Sec11/2.3 Bow Access see also table 11.2.2



II.10 Design transparency

Rating: The functional requirement is partially covered by CSR. Comment: The functional requirement as written is partially covered by CSR. The “design process” itself is not addressed by classification rules. Elements of the functional requirements of section II.10 are addressed in the sections of CSR as far as the compliance with the classification requirement is to be assured; these are provided below. Neither of the documents, nor any of the classification requirements, address the matter of intellectual property rights. This issue is considered to be outside of classification matters and a contractual matter between the owner, the builder and the manufacturer, as appropriate. Section 3 contains requirements pertaining to documentation, plans and data that are required to be submitted to the classification society. These documents cover the loading information, calculation data. The plans and supporting calculations which need to be submitted and/or supplied on board are listed.

Section 2/ 2.1.3 Responsibilities of Classification Societies, builders and owners; 2.1.3.1 These Rules address the hull structural aspects of classification and do not include requirements related to the verification of compliance with the Rules during construction and operation. The verification of compliance with these Rules is the responsibility of all parties and requires that proper care and conduct is shown by all parties involved in its implementation. These responsibilities include: (a) general aspects: • relevant information and documentation involved in the design, construction and

operation is to be communicated between all parties in a clear and efficient manner. The builder is responsible for providing design documentation according to requirements specified in the Rules. Other requirements for information and documentation are specified by the requirements and approval procedures of the individual Classification Societies

• quality systems are applied to the design, construction, operation and maintenance activities to assist compliance with the requirements of the Rules.

(b) design aspects: • it is the responsibility of the owner to specify the intended use of the ship, and the

responsibility of the builder to ensure that the operational capability of the design fulfils the owner’s requirements as well as the structural requirements given in the Rules

• the builder shall identify and document the operational limits for the ship so that the ship can be safely and efficiently operated within these limits

• verification of the design is performed by the builder to check compliance with provisions contained in the Rules in addition to national and international regulations

• the design is performed by appropriately qualified, competent and experienced personnel

• the classification society is responsible for a technical review and audit of the design plans and related documents for a ship to verify compliance with the appropriate classification rules.



Section 2/3.1.1.3 The design basis used for the design of each ship is to be documented and submitted to the Classification Society as part of the design review and approval. All deviations from the design basis are to be formally advised to the Classification Society. Section 4/3.1.1.1: A booklet of standard construction details is to be submitted for review. Section 9/2.1.2.1 A detailed report of the structural analysis is to be submitted to demonstrate compliance with the specified structural design criteria. This report shall include the following information: (a) list of plans used including dates and versions (b) detailed description of structural modelling including all modelling assumptions and

any deviations in geometry and arrangement of structure compared with plans (c) plots to demonstrate correct structural modelling and assigned properties (d) details of material properties, plate thickness, beam properties used in the model (e) details of boundary conditions (f) details of all loading conditions reviewed with calculated hull girder shear force and

bending moment distributions (g) details of applied loads and confirmation that individual and total applied loads are

correct (h) plots and results that demonstrate the correct behaviour of the structural model

under the applied loads (i) summaries and plots of global and local deflections (j) summaries and sufficient plots of stresses to demonstrate that the design criteria are

not exceeded in any member (k) plate and stiffened panel buckling analysis and results (l) tabulated results showing compliance, or otherwise, with the design criteria (m) proposed amendments to structure where necessary, including revised assessment

of stresses, buckling and fatigue properties showing compliance with design criteria.

Section 9/2.1.3.3 A computer program that has been demonstrated to produce reliable results to the satisfaction of the Classification Society is regarded as a recognised program. Section 9/2.2.3.2 The standard load cases to be used in the structural analysis are given in Appendix B/2.3.1. These load cases cover seagoing conditions (design load combination S + D) and harbour/tank testing conditions (design load combination S). Section 9/2.2.3.3 Where the loading conditions specified by the designer are not covered by the standard load cases then these additional loading conditions are to be examined, see also Appendix B/2.3.1. Section 9/2.2.5.1 – Cargo tank structural strength analysis. Verification of results against the acceptance criteria is to be carried out in accordance with Appendix B/2.7. Section 9/2.3.5.1 – Local fine mesh structural strength analysis. Verification of stress results against the acceptance criteria is to be carried out in accordance with Appendix B/3.5.



Section 9/3.1.1.3 The fatigue analysis is to be carried out using either a ‘nominal stress approach’ or a ‘hot spot stress approach’ depending on the structural details, as specified in 3.4. The procedure is illustrated in Figure 9.3.1. Section 2/3.1.5 Operating conditions 3.1.5.1 The ship is to be capable of carrying the intended cargo with the necessary flexibility in operation to fulfil its design role. Specification of cargo loading conditions as required by the Rules and any additional cargo loading conditions required by the owner are the responsibility of the designer. 3.1.5.2 The Rules assume the following: (a) a minimum set of specified loading conditions as defined in the Rules are examined. These are to include both seagoing and harbour loading conditions (b) in addition to the minimum set of specified loading conditions, all relevant additional loading conditions covering the intended ship’s service which result in increased still water shear force, bending moments or increased local static loadings are to be submitted for review (c) the Trim and Stability Booklet, Loading Manual and loading computer systems specify the operational limitations to the ship and these comply with the appropriate statutory and classification requirements (d) all cargo tanks are from a local strength point of view including sloshing designed for unrestricted filling for a cargo density as specified in 3.1.8. Limitations to loading patterns resulting in full or empty adjacent tanks as specified in the Rules and the Loading Manual do however apply for primary support members and hull girder shear force and bending moments. The Rules refer to the loading conditions and design loading and ballast conditions upon which the approval of the hull scantlings is based are. The conditions which, as a minimum, should be included in the Loading Manual are listed (section 8, 1.1). The Loading Manual is to include the design basis and operational limitations upon which the approval of the hull scantlings are based. The information listed in Table 8.1.1- Design Parameters is to be included in the Loading Manual. Section 2/4.6 Principle of Safety Equivalence 4.6.1 General 4.6.1.1 Novel designs deviating from the design basis or structural arrangements covered by the Rules will be subject to special consideration. The principle of equivalence is to be applied to the novel design, hence it must be demonstrated that the structural safety of the novel design is at least equivalent to that intended by the Rules. 4.6.1.2 The principle of equivalence may be applied to alternative calculation methods. 4.6.1.3 A systematic review process was undertaken in developing these Rules. This identified and evaluated the likely consequences of hazards due to operational and environmental influences on tanker structural configurations and arrangements covered by these Rules. For novel designs, dependent on the nature of the deviation, it may be necessary to conduct an independent systematic review to document equivalence with the Rules. The equivalence procedure is also addressed in section 3.4. ________ The information to be required for inclusion in the Ship Construction File is currently defined in UR Z23, section 10.



CONSTRUCTION

II.11 Construction quality procedures

Rating: The functional requirement is covered by CSR. Comment: The functional requirements of section II.11 are addressed in the sections of CSR and in IACS Unified Requirement Z23 as provided below. Neither of the documents, nor any of the classification requirements, address the matter of intellectual property rights. This issue is considered to be the contractual matter between the owner, the builder and the manufacturer, as appropriate. References: CSR Tanker: Section 2/2.1.3 Responsibilities of Classification Societies, builders and owners Section 2/2.1.3.1(c) (c) construction aspects: • the builder is responsible for ensuring that adequate supervision and quality control is

provided during the construction • construction is to be carried out by qualified and experienced personnel • workmanship, including alignment and tolerances, is to be in accordance with

acceptable shipbuilding standards • the Classification Society is responsible for auditing to verify that the construction and

quality control are in accordance with the plans and procedures. The Rules address design and dimensions of welds as well as requirements for welding sequence, qualification of welders, welding procedures and welding consumables (section 6/4.4 and 5). In addition to below reference in UR Z23, CSR section 6 requires that the structural fabrication is to be carried out, in accordance with ‘IACS Recommendation 47, Shipbuilding and Repair Quality Standard for New Construction’ or a recognised fabrication standard which has been accepted by the Classification Society prior to the commencement of fabrication/construction, and lists what is required in the fabrication standard. Section 2/3.1.9 The structural requirements included in the Rules were developed with the assumption that construction and repair will follow acceptable shipbuilding and repair standards and tolerances. The Rules may require that additional attention is paid during construction and repair of critical areas of the structure. The Rules define the renewal criteria for the individual structural items. The structural requirements included are developed on the assumption that the structure will be subject to periodical survey in accordance with individual Classification Society Rules and Regulations. UR Z23 will be implemented into individual IACS Member’s Rules and Regulations. UR Z23, 7.4



Shipbuilding quality standards for the hull structure during new construction are to be reviewed and agreed during the kick-off meeting. Structural fabrication is to be carried out in accordance with IACS Recommendation 47, “Shipbuilding and Repair Quality Standard for New Construction”, or a recognized fabrication standard which has been accepted by the Classification Society prior to the commencement of fabrication/construction. The work is to be carried out in accordance with the Rules and under survey of the classification society. Table 1 provides a list of surveyable items for the hull structure covered by this UR and address welding consumables, welder qualification, welding – mechanical properties (welding procedures), welding equipment, welding environment, welding supervision, welding- surface discontinuities, welding – embedded discontinuities, steel preparation and fit up, surface preparation, marking and cutting, straightening, forming, conformity with alignment/fit up/gap criteria, conformity for critical areas with alignment/fit up or weld configuration, steelwork process, e.g. sub-assembly, block, grand and mega block assembly, pre-erection and erection, closing plates, remedial work and alteration, tightness testing, including leak and hose testing, hydropneumatic testing, structural testing, corrosion protection systems, e.g. coatings, cathodic protection, installation, welding and testing of: hatch covers, doors and ramps integral with the shell and bulkheads, rudders, forgings and castings, appendages, equipment forming the watertight and weathertight integrity of the ship, e.g. overboard discharges, air pipes, ventilators, freeboard marks and draft marks, principal dimensions.

II.12 Survey

Rating: The functional requirement is covered by CSR. Comment: This functional requirement is addressed in IACS Unified Requirement Z23, in particular paragraphs 7.1, 7.2 and 7.3 and Table 1 focusing on the specific activities that need to be planned for and addressed. Prior to commencing any newbuilding project, the society is to discuss with the shipbuilder at a kick off meeting the items listed in Table 1. The purpose of the meeting is to agree how the list of specific activities shown in Table 1 is to be addressed. The meeting is to take into account the shipbuilders construction facilities and ship type and deal with sub-contractors if it is known that the builder proposes to use them. The shipyard is to be informed of likely intervals for sampling and patrol activities. A record of the meeting is to be made, based upon the contents of the Table – the Table can be used as the record with comments made into the appropriate column. If the society has nominated a surveyor for a specific newbuilding project then the surveyor is to attend the kick off meeting. The builder is to be asked to agree to undertake ad hoc for the builder to agree to keep the classification society advised of the progress of any investigation. Whenever an investigation is undertaken, the builder is to be requested, in principle, to agree to suspend relevant construction activities if warranted by the severity of the problem. The records are to take note of specific published Administration requirements and interpretations of statutory requirements. The record of the meeting is to be updated as the construction process progresses in the light of changing circumstances, e.g. if the shipbuilder chooses to use or change sub-



contractors, or to incorporate any modifications necessitated by changes in production or inspection methods, rules and regulations, structural modifications, or in the event where increased inspection requirements are deemed necessary as a result of a substantial non-conformance or otherwise.

IN-SERVICE CONSIDERATIONS


Rating: The functional requirement is partially covered by CSR. Comment: The functional requirement is fulfilled with respect to design and construction requirements to allow adequate survey of the structure. This includes the avoidance of closed spaces and the size of access openings (Sec 5/5 and Sec11/2.2, 2.3 and table 11.2.2). Criteria for planning survey and maintenance are not explicitly included in the CSR. A reference is made to the Unified Requirement Z 10.4 of IACS with respect to the assessment and the related inspections and surveys for thickness measurements in section 12/1.2.1. The hull survey for new constructions is regulated by the Unified Requirement Z 23. It is stated, that the CSR do not include requirements related to the verification of compliance with the rules during construction and operation in section 2/2.1.3. The owner and the individual Classification Society are responsible for maintaining the ship and verify the compliance with the class requirements in accordance with the Classification Society survey scheme as stated in Sec 2/2.1.3.1(d). CSR-reference content comment Sec2/2.1.3 Responsibilities of Classification

Societies, builders and owners

Sec5/5 Access Arrangements Sec5/5.1.1.4 Size of access openings Sec11/2.2 Tank Access see also table 11.2.2 Sec11/2.3 Bow Access see also table 11.2.2 Sec12/1.2 Assessment of thickness

measurements Reference to UR Z 10.4 and requirements of individual Classification Society

II.14 Structural accessibility

Rating: The functional requirement is not covered by CSR. Comment: In the goal based standards, means of access to the ship’s structure for inspection and thickness measurements are required according to Tier II.14. The CSR refers to SOLAS Ch II-1, Part A-1, regulation 3-6, see CSR Section 5/5.



Both sets of rules add requirements for access to specific areas: duct keel and pipe tunnel in CSR for oil tankers, shaft tunnels and steering gear compartment in CSR for bulk carriers. Reference documents Reference documents are the SOLAS requirements Ch II-1 regulation 3-6, resolution MSC 158(78) and IACS UI SC 191.


II.15 Recycling

Rating: The functional requirement is not covered by CSR. Comment: Recycling matters are not scope of today’s classification rules. Therefore requirements regarding recycling of the ship structure are not explicitly included in CSR. Reference is made, that other national or international rules and regulations may exist, which are relevant for the particular ship. It is noted that the MEPC plans to address this topic in a future IMO mandatory instrument on Recycling of Ships.

6. Conclusions

This report was prepared by IACS to provide a working example of how IACS in the future may provide background documentation illustrating how classification rules meet the GBS. This was done to assist IMO conduct a pilot trial application of Tier III of the GBS for oil tankers and bulk carriers. The intention of the pilot is to validate the Tier III verification framework, identifying shortcomings and making proposals for improvement. The pilot project will test the IMO GBS Tier III Verification Framework and not actually be the verification of the IACS CSR at this time.


IMO Pilot Project 16 February 2007 IACS Documentation Package Appendix A - Page 1

Appendix A

IMO Goal-based New Ship Construction Standards

To assist the Pilot Project members, the following is a copy of the GBS Tier I and II.

TIER I 1

Ships are to be designed and constructed for a specified design life to be safe and environmentally friendly, when properly operated and maintained under the specified operating and environmental conditions, in intact and specified damage conditions, throughout their life.

.1 Safe and environmentally friendly means the ship shall have adequate strength, integrity and stability to minimize the risk of loss of the ship or pollution to the marine environment due to structural failure, including collapse, resulting in flooding or loss of watertight integrity.

.2 Environmentally friendly also includes the ship being constructed of materials for environmentally acceptable dismantling and recycling.

.3 Safety also includes the ship’s structure being arranged to provide for safe access, escape, inspection and proper maintenance.

.4 Specified operating and environmental conditions are defined by the operating area for the ship throughout its life and cover the conditions, including intermediate conditions, arising from cargo and ballast operations in port, waterways and at sea.

.5 Specified design life is the nominal period that the ship is assumed to be exposed to operating and/or environmental conditions and/or the corrosive environment and is used for selecting appropriate ship design parameters. However, the ship’s actual service life may be longer or shorter depending on the actual operating conditions and maintenance of the ship throughout its life cycle.

1 Report of MSC 80, MSC 80/24, paragraph 6.39



TIER II FUNCTIONAL REQUIREMENTS 2

(Applicable to new oil tankers and bulk carriers in unrestricted navigation*) DESIGN II.1 Design life The specified design life is not to be less than 25 years. II.2 Environmental conditions Ships should be designed in accordance with North Atlantic environmental conditions and relevant long-term sea state scatter diagrams. II.3 Structural strength Ships should be designed with suitable safety margins:

.1 to withstand, at net scantlings**, in the intact condition, the environmental conditions anticipated for the ship’s design life and the loading conditions appropriate for them, which should include full homogeneous and alternate loads, partial loads, multi-port and ballast voyage, and ballast management condition loads and occasional overruns/overloads during loading/unloading operations, as applicable to the class designation; and

.2 appropriate for all design parameters whose calculation involves a degree of uncertainty, including loads, structural modelling, fatigue, corrosion, material imperfections, construction workmanship errors, buckling and residual strength.

The structural strength should be assessed against excessive deflection and failure modes, including but not limited to buckling, yielding and fatigue. Ultimate strength calculations should include ultimate hull girder capacity and ultimate strength of plates and stiffeners. The ship’s structural members should be of a design that is compatible with the purpose of the space and ensures a degree of structural continuity. The structural members of ships should be designed to facilitate load/discharge for all contemplated cargoes to avoid damage by loading/discharging equipment which may compromise the safety of the structure. II.4 Fatigue life The design fatigue life should not be less than the ship’s design life and should be based on the environmental conditions in II.2. II.5 Residual strength

2 Report of MSC 82, MSC 82/WP.5, ANNEX I * Unrestricted navigation means that the ship is not subject to any geographical restrictions (i.e. any oceans, any seasons) except as limited by the ship’s capability for operation in ice. ** The net scantlings should provide the structural strength required to sustain the design loads, assuming the structure in intact condition and excluding any addition for corrosion.



Ships should be designed to have sufficient strength to withstand the wave and internal loads in specified damaged conditions such as collision, grounding or flooding. Residual strength calculations should take into account the ultimate reserve capacity of the hull girder, including permanent deformation and post-buckling behaviour. Actual foreseeable scenarios should be investigated in this regard as far as is reasonably practicable. II.6 Protection against corrosion Measures are to be applied to ensure that net scantlings required to meet structural strength provisions are maintained throughout the specified design life. Measures include, but are not limited to, coatings, corrosion additions, cathodic protection, impressed current systems, etc. II.6.1 Coating life Coatings should be applied and maintained in accordance with manufacturers’ specifications concerning surface preparation, coating selection, application and maintenance. Where coating is required to be applied, the design coating life is to be specified. The actual coating life may be longer or shorter than the design coating life, depending on the actual conditions and maintenance of the ship. Coatings should be selected as a function of the intended use of the compartment, materials and application of other corrosion prevention systems, e.g. cathodic protection or other alternatives. II.6.2 Corrosion addition The corrosion addition should be added to the net scantling and should be adequate for the specified design life. The corrosion addition should be determined on the basis of exposure to corrosive agents such as water, cargo or corrosive atmosphere, or mechanical wear, and whether the structure is protected by corrosion prevention systems, e.g. coating, cathodic protection or by alternative means. The design corrosion rates (mm/year) should be evaluated in accordance with statistical information established from service experience and/or accelerated model tests. The actual corrosion rate may be greater or smaller than the design corrosion rate, depending on the actual conditions and maintenance of the ship. II.7 Structural redundancy Ships should be of redundant design and construction so that localized damage of any one structural member will not lead to immediate consequential failure of other structural elements leading to loss of structural and watertight integrity of the ship. II.8 Watertight and weathertight integrity Ships should be designed to have adequate watertight and weathertight integrity for the intended service of the ship and adequate strength and redundancy of the associated securing devices of hull openings. II.9 Human element considerations Ships should be designed and built using ergonomic design principles to ensure safety during operations, inspection and maintenance of ship’s structures. These considerations should include stairs, vertical ladders, ramps, walkways and standing platforms used for permanent means of access, the work environment and inspection and maintenance considerations.



II.10 Design transparency Ships should be designed under a reliable, controlled and transparent process made accessible to the extent necessary to confirm the safety of the new as-built ship, with due consideration to intellectual property rights. Readily available documentation should include the main goal-based parameters and all relevant design parameters that may limit the operation of the ship. CONSTRUCTION II.11 Construction quality procedures Ships should be built in accordance with controlled and transparent quality production standards with due regard to intellectual property rights. The ship construction quality procedures should include, but not be limited to, specifications for material, manufacturing, alignment, assembling, joining and welding procedures, surface preparation and coating. II.12 Survey A survey plan should be developed for the construction phase of the ship, taking into account the ship type and design. The survey plan should contain a set of requirements, including specifying the extent and scope of the construction survey(s) and identifying areas that need special attention during the survey(s), to ensure compliance of construction with mandatory ship construction standards. IN-SERVICE CONSIDERATIONS II.13 Survey and Maintenance Ships should be designed and constructed to facilitate ease of survey and maintenance, in particular avoiding the creation of spaces too confined to allow for adequate survey and maintenance activities. The survey plan in II.12 should also identify areas that need special attention during surveys throughout the ship’s life and in particular all necessary in-service survey and maintenance that was assumed when selecting ship design parameters. II.14 Structural accessibility The ship should be designed, constructed and equipped to provide adequate means of access to all internal structures to facilitate overall and close-up inspections and thickness measurements. RECYCLING CONSIDERATIONS II.15 Recycling Ships should be designed and constructed of materials for environmentally acceptable recycling without compromising the safety and operational efficiency of the ship.


IMO Pilot Project 16 February 2007 IACS Documentation Package Appendix B - Page 1

Appendix B

IACS Common Structural for Double Hull Oil Tankers

This report was prepared in association with the IACS 2006 “Common Structural Rules for Double Hull Oil Tankers“(referred to as CSR or Rules in this report), which entered into force on 1 April 2006. A copy of these Rules is available from any IACS member or may be downloaded from the IACS web site free of charge at the following: www.iacs.org.uk The CSR and this report refer to IACS Unified Requirements, which may also be obtained from the above web site.


IMO Pilot Project 16 February 2007 IACS Documentation Package Appendix B - Page 2



IMO Pilot Project 16 February 2007 IACS Documentation Package Appendix C - Page 1

Appendix C

Background Documents for the IACS Common Structural for Double Hull Oil Tankers

This report was prepared to assist IMO conduct a pilot trial application of Tier III of the GBS for oil tankers and bulk carriers is not intended to actually be the verification of the IACS CSR themselves. The Section 5 commentary of this report was generally prepared in order to summarize and illustrate how the CSR relates to the GBS. It is noted that some members of the Pilot Project may wish to delve deeper into the background of the IACS CSR. At the time of writing this report, IACS is in the process of placing a copy of the background documents for the CSR for Tankers on the IACS web site. Once posted, a copy of the background documents may be downloaded from the IACS web site free of charge at the following: www.iacs.org.uk


IMO Pilot Project 16 February 2007 IACS Documentation Package Appendix C - Page 2



***

IACS Technical Presentationto the

IMO GBS Pilot Project

12 March 2007

INTERNATIONAL MARITIME ORGANIZATIONMaritime Safety Committee

2IMO Pilot Project Meeting - 12 March 2007

Objectives

1. Pilot Project

• Trial application of Tier III

• Validation of Tier III

• ID shortcomings and propose improvements

• Not actual verification of the IACS CSR at this time

2. Submission from IACS

• Provide working example of how IACS may provide

documentation to illustrate how rules meet GBS

• Concrete example to assist Pilot Panel

• CSR Tanker used for example

MSC 83/INF.5

ANNEX 2


Self Assessment Table

Item

Fully covered in

CSR


CSR Not covered

in CSR Comment

DESIGN

II.1 Design life



II.4 Fatigue life

II.5 Residual strength Implicitly addressed in rules.


II.6.1 Coating life


II.7 Structural redundancy Implicitly addressed in rules.

II.8 Watertight and


Self Assessment Table

Item

Fully covered in

CSR


CSR Not covered

in CSR Comment



Partially covered. May be addressed in future SOLAS Reg.

II.10 Design transparency Also addressed by other rules or conventions.

CONSTRUCTION


II.12 Survey



Addressed with respect to design and construction requirements to allow adequate survey of the structure.

II.14 Structural accessibility Addressed in SOLAS Reg II-1/3 on PMA.


II.15 Recycling Will be addressed in future IMO Reg. on Recycling of Ships.



IACS views so far…

1. For clarity, ease of understanding, and ability to modify or adapt in the future; separate documents or sections to be developed for the Tier III Procedure, Information / Documentation, and Evaluation Criteria.

2. The information/Documentation and Evaluation Criteria should be practical for the GoE and sufficiently flexible to account for future technical development. (Previous IACS comments included in Coordinators Consolidated Text)

3. The Evaluation Criteria in Tier III should augment or clarify Tier II, should not contain “additional” functional requirements

4. Will need clarification on how to address GBS topics that are covered by IMO regulations or industry standard and not in the Class rules.


IACS views so far…

Procedure

Information / Documentation

Evaluation Criteria



Long-term harmonization

• Full harmonization required for

– Wave loads

– Fatigue

– Finite element analysis

– Buckling

– Prescriptive requirements


Long-term harmonization

• Full Harmonization Plan

• One year application and feedback period before long term

harmonization

• Detail plan for full harmonization will be developed by the

Hull Panel by the end of the one-year feedback period

• Three years of harmonization work

• One year for implementation (industry review)

• Full harmonization in five years from implementation



IACS Project Teams for CSR Maintenance

• IACS implemented 2 Project Teams for CSR Maintenance

– Active from 1st June 2006

– PT for Bulk Carriers: 3 JBP + 1 JTP members

• NK (Chair) + BV + GL + ABS

– PT for Oil Tankers: 3 JTP + 1 JBP members

• DNV (Chair) + ABS + LR + BV

– Rotation among members every 2 years

– Running of the PT governed by IACS procedure IACS COUNCIL

HULL PANEL

Chairman: T. Yoneya

CSR Bulk Carriers

K. ABE

ABS

G. CESARINE

BV

S. HARADA

Project Manager

ClassNK

A. Schulz-Heimbeck

GL

CSR Oil Tankers

P. BAUMANS

BV

F. CHENG

LR

R. NAGAYAMA

ABS

P. SALTVEDT

Project Manager

DNV

PERMANENT

SECRETARIAT

R. Leslie

CSR Secretariat

G-Y Han

SG/CSR

NK, LR, BV, ABS


Objectives of PT / CSR Maintenance

• Objectives: Cover the technical issues on CSR

regarding:

– Questions and Answers (Q&A)

– Common Interpretations (CI)

– Amendments (Errata and Rule Changes)

• Under the responsibility of the IACS Hull Panel

• IACS Permanent Secretariat provides interpretation

and Q&A on Web Site



IACS CSR Knowledge Center/Database

• Centralized mechanism for collecting, categorizing and storing questions and answers, feedback and responses, tasks and pending actions, interpretations and rule changes

• Basis for future improvements

• External access to Q&A and Interpretations from IACS Web Site

Transparency and consistent

implementation of the Rules



DESIGN

II.1 Design life



II.4 Fatigue life



II.6.1 Coating life






CONSTRUCTION


II.12 Survey





II.15 Recycling



II.1 Design life

The design life of 25 years is an input parameter in CSR for:

• the determination of the values of the scantling loads

• fatigue loads

• fatigue life expected

• corrosion wastage allowances

For the scantlings loads, the difference between 20 and 25 years of design life is insignificant (1% difference)

For fatigue and wastage allowances, the influence of extension of design life from 20 to 25 years is important


0

0.2

0.4

0.6

0.8

1

1.2

-9 -8 -7 -6 -5 -4 -3 -2 -1 0

Illustration of design life influence on scantlings loads

108: 25 years = 10x:20years x=7.903

Difference : only 1% (8/7.903=1.012)

Long term extreme loads amplitude is distributed according to a Weibull law,

exponent about 1




DESIGN

II.1 Design life



II.4 Fatigue life



II.6.1 Coating life






CONSTRUCTION


II.12 Survey





II.15 Recycling



Technical Comments

• The functional requirement is covered by CSR

• Rule requirements are based on North Atlantic

environment

• Scatter diagram according to IACS Rec. No. 34

• Rule load formulations based on numerical wave load

analysis



Scatter diagram

• IACS Rec. No. 34 scatter diagram for North Atlantic

• Revised in year 2000

• Wave data obtained from British Marine Technology

• Probability described as occurrences per 100000 observations


Geographical area covered



Derivation of rule loads

Main principles:

• Numerical wave load analysis, using 3D hydrodynamic

calculations

• Envelope values, considering all sea states and headings

• Regression analysis, together with calibration

• Correction factors applied to account for non-linear effects

and operational considerations

• Speed effect included for fatigue loads

• Load formulations covered by existing Unified

Requirements are maintained



• Hydrodynamic calculations:

• Pierson-Moscowitz wave spectrum

• Wave energy spreading function of cos2

• Equal probability of all wave headings

• 30 deg step of ship/wave heading

• Rule load formulations derived for:

• Ship motions and accelerations

• External and internal pressures

• Global wave bending moments and shear forces




Strength Assessment

(ULS/SLS)

Most severe load expected

Equivalent design wave

approach

Rule load at 10-8

*Load combination factors

Selected load cases for maximised responses

Rule load at 10-4

Stress combination factors

Long-term distribution approach

Fatigue Assessment

(FLS)

Expected load history

Weibull distribution for

total stress range



• Design wave approach for strength assessment:

• Dynamic load cases selected to maximize certain

load components using 25 year return period

• For each load case, simultaneously occurring load

components are calculated using load combination

factors (LCF).

• The LCF calculations are based on the equivalent

design wave concept.

• A load combination factor indicates the magnitude of

a secondary response compared to its own maximum

rule value

• Accounts for the relation between the wave heading,

wave period, wave amplitude and phasing of the

dominant response and the secondary responses




• Long-term approach for fatigue assessment:

• Fatigue loads are calculated to represent the

expected stress range history

• Weibull distribution

• 10-4 probability level is chosen as the rule reference

values

• Forward speed of 75% of service speed is applied

• Non-linear correction factors are not applied

• Load combination on stress-level

• Stretching of sea pressure over swl


Validation of rule loads

• Vertical acceleration, VLCC

• Vertical bending moment, VLCC

0

2000000

4000000

6000000

8000000

10000000

0 0.2 0.4 0.6 0.8 1

rel dist from AP

x/L [-]

[kN

m] JTP

JTP Bellshaped

Wadam

0.00

1.00

2.00

3.00

4.00

5.00

6.00

0 0.2 0.4 0.6 0.8 1

rel dist from AP

x/L [-]

[m/s

2]

JTP

Wadam



New data in wave statistics

• Existing scatter diagram is based on visual observations

from ships

• Based on large amount of data

• Some uncertainty connected to the observations

• Some effect of bad weather avoidance included in the

data

• New data available:

• Buoys, wave radars, satellites, ship response,

hindcast/forecast

• All with related uncertainties, and rather large

variation in data

• Ongoing research on modified scatter diagrams


Wave statistics - steep waves

• Effect of steep/rogue waves not covered by CSR

• Several research projects carried out during recent years, e.g. the EU project MAXWAVE

• Steep waves found to occur more frequently than previously believed

• Different physical explanations:

• Wave-current interaction

• Combined seas

• Wave energy focusing

• Wave loads particularly affected by steep waves:

• Bow and bottom slamming

• Green water on deck

• Superstructure impact



Wave statistics - steep waves

• However, steep waves is still considered to be a research

topic

• No consensus on the definition of a steep wave

• Ongoing and planned projects to investigate unresolved

issues

• Need more information regarding:

• Probability of occurrence (statistical model)

• Physical understanding of the phenomenon

• Spatial and time representation of the wave (wave

model)

• Structural behaviour, numerical load model

• Could be covered in future rules as an ALS condition


Speed effect

• No speed effect included in CSR for strength assessment

• Speed reduction in heavy weather due to:

• slamming

• bow submergence

• added wave resistance

• voluntary speed reduction

• Model tests on full form ships: Very small forward speed, even in 5-year storms

• Speed sensitivity studies carried out during CSR Tank development:

• VLCC and product carrier

• Motions and accelerations

• Global loads

• External wave pressure



Speed effect

Speed sensitivity studies:

• Motions and accelerations

Vert Acc FP Sensitivity to Speed

0

1

2

3

4

5

6

7

8

0 5 10 15 20

speed [kn]

Acc [m

/s2]

VLCC (58) Design

VLCC (58) Ballast


Speed effect


• Global loads

VBM Speed Sensitivity VLCC Design

0

2000000

4000000

6000000

8000000

10000000

12000000

14000000

0 0.2 0.4 0.6 0.8 1

x rel AP [-]

[kN

m]

Zero speed

5 knots

10 knots

15 knots

VSF Speed Sensitivity VLCC Ballast

0

20000

40000

60000

80000

100000

120000

140000

160000

0 0.2 0.4 0.6 0.8 1

x rel AP [-]

[kN

]

Zero speed

5 knots

10 knots

15 knots



Speed effect


• External sea pressure

VLCC Scantling Head Sea Pressures - 0.5L

-5

0

5

10

15

20

25

0 10 20 30 40

0 knots

5 knots

10 knots

15 knots

LAN 0kn

Reference line

Product Scantling Head Sea Pressures - 0.5L

-4.00

-2.00

0.00

2.00

4.00

6.00

8.00

10.00

12.00

0.00 5.00 10.00 15.00 20.00

0 knots

5 knots

10 knots

15 knots

LAN 0kn

Reference line


Speed effect

Speed effect findings:

• Very small forward speed possible in extreme weather

• Small effect of speed on the dynamic loads

Conclusion: No need for speed correction for ULS



Load probability level

Probability of exeedance

1.00E-11

1.00E-10

1.00E-09

1.00E-08

1.00E-07

1.00E-06

1.00E-05

1.00E-04

1.00E-03

1.00E-02

1.00E-01

1.00E+00

0 1 2 3 4 5 6 7 8 9 101112131415161718192021222324252627

Hs

Pro

bability

1

2

5

10

20

25

30

• Design life of 25 years used in CSR

• Probability level of 10-8implies a return period in between 20 and 25 years

• If 10-8 represents 25 years, then 10-7.9 represents 20 years

• The difference in long-term extreme load assuming Weibull distribution is 1%

• Actual number of response cycles depends on the ship size and service time

• Extreme sea state:1.4% increase in Hs when return period is increased from 20 to 25 years


Load probability level

• Characteristic load taken as the most probable largest value during the design life (10-8 probability level)

• Simplified design procedure, using Weibull-fit to represent the long-term load distribution

• Assuming a Gumbel probability function for the extreme load, this implies a (1-1/e)=63.2% chance of exceedance

• For large number of occurrences, the Gumbel distribution becomes very narrow

• Probability of exceedance equal to 0.01:

• H0.01/Hmp≈1.1

• Accounted for by safety margins in the rule requirements

1.0E-08

1.0E-07

1.0E-06

1.0E-05

1.0E-04

1.0E-03

1.0E-02

1.0E-01

1.0E+00

0 20 40 60 80 100




DESIGN

II.1 Design life



II.4 Fatigue life



II.6.1 Coating life






CONSTRUCTION


II.12 Survey





II.15 Recycling



Technical Comments

• The functional requirement is covered by CSR

• Tier II items to be addressed in the Rules include:

- Safety Margins

- Strength Assessments

- Ultimate Strength

- Structure Compatibility

- Facilitate Loading/Unloading

- Net Scantlings




.1 Safety Margins

a) Environmental conditions

- 25 year North Atlantic used

- Most vessels trade in more benign environments

b) Loading conditions

- Representative design cargo and ballast loading conditions

- Envelope the actual vessel loading conditions

c) Local loads

- Static and dynamic loads maximized for local applications

- Include occasional overloads during loading/unloading

d) Load combinations

- Combining local and hull girder loads as well as static and dynamic components

- Various loading combinations taken to maximize load effects on different structural components


Route Information



Sample Loading Conditions - Prescriptive

Table 8.2.7 Design Load Sets for Plating and Local Support Members

Structural Member Design Load

Set (1, 2, 3)

Load Component

Draught Comment Diagrammatic Representation

1 Pex Tsc

2 Pex Tsc Sea pressure only

7 Pin – Pex Tbal

Keel, Bottom Shell,

Bilge, Side Shell, Sheer strake

8 Pin – Pex 0.25Tsc

Net pressure difference between water ballast

pressure and sea pressure

1 Pex Tsc Green sea pressure only or other loads on deck

3 Pin 0.6Tsc

4 Pin -

In way of cargo tanks

11 Pin-flood -

Cargo pressure only

1 Pex Tsc Green sea pressure only or other loads on deck

5 Pin Tbal

6 Pin 0.25Tsc

In way of other tanks

11 Pin-flood -

Water ballast or other liquid pressure only

9 Pdk Tbal

Deck

Any location

10 Pdk -

Distributed or concentrated loads

only. Simultaneously occurring green sea pressure may be

ignored


Sample Loading Conditions - FEM

FE Load Cases for Tankers with Two Oil-tight Longitudinal Bulkheads

Still Water Loads Dynamic load cases Strength assessment

(1a)

Strength assessment against hull girder shear loads (1b)

Loading Pattern

Figure Draught

% of Perm.

SWBM(2)

% of Perm. SWSF(2) Midship

region Forward region

Midship and aft regions

Design load combination S + D (Sea-going load cases)

100% (sag)

See note 3 1 \ \

A1

P

S

0.9 Tsc 100% (hog)


2, 5a \ \

100% (sag)

See note 3 1 \ \

A2

P

S

0.9 Tsc 100% (hog)


2, 5a \ \


2 4 2

A3(6)

P

S

0.55 Tsc see note 5

100% (hog) 100%

(-ve fwd) See note 5

5a \ \




Load Scenarios Rule Requirements

Design Load Combination (specified in Section 7/6) Operation

Loads

(that the vessel is exposed to and is to withstand)

Ref. no

Notation

Design Format

(specified in

Sections 8 and 9)

see Note 1

Acceptance Criteria Set

(specified in Sections 8 and 9)

Seagoing operations

1. SG + SL + DG + DL ≤ η2 R1 AC2 Static and dynamic loads in heavy weather

1 S + D 2. γS SG + γD DG ≤ R2/ γR2 AC2

Impact loads in heavy weather

2 Impact SL + Dimp ≤ η3 Rp AC3

Internal sloshing loads 3 Sloshing SG + Dslh ≤ η1 R1 AC1

Transit

Cyclic wave loads 4 Fatigue DM ≤ ∑ηi / Ni -

BWE by flow through or sequential methods

Static and dynamic loads in heavy weather

5 S + D SG+SL+ DG + DL ≤ η2R1 AC2



Harbour and sheltered operations

Loading, unloading and ballasting

Typical maximum loads during loading, unloading and ballasting operations

6 S SG + SL ≤ η1 R1 AC1

Tank testing Typical maximum loads during tank testing operations

7 S SG+ SL1≤ η1 R1 AC1

Special conditions in harbour

Typical maximum loads during special operations in harbour, e.g. propeller inspection afloat or dry-docking loading conditions

8 S SG+ SL ≤ η1 R1 AC1

Accidental condition

for water tight boundaries

1. SL ≤ η2 R1 AC2

Accidental flooding

Typically maximum loads on internal watertight subdivision structure due to accidental flooding

9 A for collision bulkhead

2. SL ≤ η1 R1 AC1

Note




.1 Safety Margins (continued)

e) Structural modeling

- Prescriptive rules

- FEM

f) Fatigue (II.4)

g) Corrosion (II.6)

h) Material imperfections

- Minimum strength properties used, although actual properties typically greater

i) Construction workmanship errors (II.11)

j) Buckling

- s/t ratios, prescriptive buckling and advanced buckling method

k) Residual strength (II.5)


Structural Modeling - Local Scantlings

• Plate

• Stiffener (local support member)

Pressure based formulation

32

cmCm

spZ

yds σ

l=

ydtshr

shrshrnetw

Cd

lsPft

τ=−

yd

hg

sssσ

σαβC - =

mmC

pskt

aa

ydσ0158.0=

yd

hg

aa σ

σαβ -Ca =



• Primary support members

mmfd

Ft

sb

2=32

cmCm

spZ

yds σ

l=


SM, shear area and sectional area of cross tie may be reduced to85% based on satisfactory FEA

Shear70%

0.2ℓ

100%

0.2ℓ

100%

0.2ℓ

100%

50%

0.2ℓ

100%

Bending

Distribution:


Permissible Stress Factors for Plate and Stiffener(subjected to hull girder stress)




- combine Local Stress and hull girder stress at:

• end span and

• face plate side

Floor/WebFloor/Web

Longitudinals

compression compression

Pressure from attached plate side

Floor/WebFloor/Web

Longitudinalstension tension

Pressure from stiffener side

Hull

Girder

Stress

compressioncompression

tensiontension

Stiffener SM



( )310

)( −

−−

−−−

−−

−−−−−

⋅⋅

−⋅+−

=offneth

hwihlc

offnetv

vwivlcpermswnatotalhg

I

yMf

I

MfMzzσ

Hull Girder Stress

N A

Tension

Compression

TensionCompression+

- Total Bending StressSWBM DLCF

for MwvDLCF

for Mwh

Vert. Bending Hor. Bending

“Minus” so that positive “y” makes

positive (tensile) stress

(+)(-)

(+)

(-)




A.P.

Cargo Region

0.85L

Forward cargo

tank region

Forward

end

Mid and aft cargo

tank region

Machinery space

and aft end

Tank

LCG

F.P.

SWBM / WIBM

Internal / External

Pressure

Hull Girder SM

IncreaseIncrease

ReduceReduce

ReduceReduce



Structural Modeling - FEM Analysis

� Strength analysis by FEM to verify the ship structure is within

the class required standard

� Analysis is required as part of the rules

� Midship cargo region 3-tank FE model

� General mesh size following stiffening system, e.g. 900 mm

� Model based on average corroded thickness tgross – 0,5 tcorr



Structural Modeling - FEM Analysis, Fine Mesh

• Fine Mesh Models - with 50x50 mm (or smaller)


• Defines the limits to maximum allowable slenderness for the structure

• The criteria are based on analytical buckling formulas, and non stress based and cover all structural elements

• Application:

– Structural elements with failure modes not covered by the prescriptive buckling requirements or the advanced buckling are designed to be stocky

– Maximum slenderness is also given to failure modes that are covered by the prescriptive buckling requirements or the advanced buckling analysis (baseline floor)

BucklingStiffness and proportion requirements



BucklingPrescriptive buckling requirements

• Analytical formulas categorised according to

structural elements and failure modes

• The buckling stress is defined for all relevant failure

modes

• The prescriptive buckling requirements for plates and

stiffeners based on GL (DIN)

• Application:

– Assessment of the critical buckling stress for individual structural elements (e.g. buckling of plates and pillars)


BucklingPrescriptive buckling requirements

• Prescriptive buckling check based on formulas of GL-rule, which

are developed based on DIN 18800

• Original GL-approach uses different net-thickness approach

• Stresses calculated based on gross thickness of ships

cross section

• Allowable buckling stresses calculated based on net-

thickness of considered panels (- 50% corrosion addition)

• Section modulus of ship cross section will reduce by 10% in

maximum, which gives approximately 10% increased stress

in the considered panel

• Safety factor of 1.1 was introduced to cover this effect in

GL-approach

• In CSR Approach stress calculation is based on net-

thicknesses therefore allowable usage factor is set to 1.0 in

general



BucklingAdvanced buckling analysis

• Based on nonlinear analysis techniques

• CSR rules give general requirements and

specification to:– to the advanced buckling analysis– the application– structural modelling principles– assessment criteria

• Application

– CSR Rules allows the use of the ultimate capacity for certain structural elements subject to lifetime

extreme loading


BucklingAdvanced buckling analysis

• Covers bi-axial compression, shear stress and lateral

pressure

• Physical representation

• Control of ultimate capacity

• The advanced buckling analysis is considered a

better representation than the prescriptive buckling

requirements



a) Weak/thin plate - strong stiffener sideways:

thin plate/wide stiffenerflange

b) Weak stiffener sideways/torsional:

High stiffener/small flange

a) + b) effect interactingc) Weak stiffener out-of-plane: Low stiffener

height/long span/small flange: prevented by

PULS design principles

Buckling Advanced Buckling Software

Graphical presentation of Buckling Modes



.2 Strength Assessment

a) Members to be evaluated

- Covers all strength components of the vessel

b) Failure modes

- Yielding, buckling and fatigue

c) Deflections

- Hull girder inertia

- Buckling

- s/t and d/t ratios



Members to be Evaluated

Aft end& Machinery Room

Fore endCargo Area

Ship in operation renewal criteria 12

Testing procedures 11/5

Equipment 11/4

Support structure and structural appendages 11/3

Crew protection 11/2

Hull openings and closing arrangements 11/1

Topic Sections

Sections

8/5.4

8/5.3

8/5.2

8/5.1

8/4.5-4.8

8/4.4

8/4.3

8/4.2

8/4.1

Aft end deck structure

Aft end shell structure

Aft end bottom structure

Aft end general structure

Machinery internal structure

Machinery deck structure

Machinery side structure

Machinery bottom structure

Machinery general structure

Topic

8/5.5-5.7Aft end internal structure

Sections

9/2

9/1

8/6.2

8/2.6

8/2.5

8/2.4

8/2.3

8/2.2

8/1Hull girder strength

9/3

Strength assessment (FEM)

Hull girder ultimate strength

Sloshing

Primary support members

Bulkheads

Inner bottom

Hull envelope framing

Hull envelope plating

Topic

Fatigue strength

Sections

8/6.4

8/6.3

8/3.5-3.9

8/3.4

8/3.3

8/3.2

8/3.1

Topic

Bow impact

Bottom slamming

Internal structure

Deck structure

Side structure

Bottom structure

General structure

Loads 7

Materials and Welding 6

Structural Arrangement 5

Basic Information 4

Rule Application 3

Rule Principles 2

Introduction 1

Topic Sections


Failure Modes

Principal Acceptance Criteria - Rule Requirements

Plate panels and Local Support Members

Primary Support Members

Hull girder members

Acceptance criteria set

Yield Buckling Yield Buckling Yield Buckling

AC1: 70-80% of

yield stress





Pillar buckling

75% of yield stress

NA




85% of yield stress


Pillar buckling


Usage factor

typically 0.9

AC3: Plastic criteria


Plastic criteria


NA NA

(Static)

(Dynamic)

(Impact)



Failure Modes

Principal Acceptance Criteria - Design Verification - FE Analysis

Global cargo tank analysis Local fine mesh analysis

Acceptance criteria set

Yield Buckling Yield











(Static)

(Dynamic)


Deflections

Buck

lingStress/Yield

Stress



Deflections

• Primary Support member deflection controlled by minimum

web depth requirements based on a percentage of the

unsupported span of the member

• Calibrated and based on existing rules

• Controls the inertia of the member

EI

w

384

4l

=δδ

wl



.3 Ultimate Strength

a) Ultimate strength of the hull girder

b) Ultimate strength of plates and stiffeners




• Hull girder ultimate

strength:

– Partial safety factor

format

– Calibration of safety

factors using

reliability analysis

– Only sagging

considered

Mu

MW

MS

Probability Density

Rule Format Reliability Analysis

Criterion: Criterion: Limit state: g=Mu-MS-MW

Still water moment, MSW • Rule value, empirical

• Actual loading condition

• Other

Wave moment, MWV • Rule value, empirical

• Direct calculation

- Detailed recipe wrt. kind of

analysis, environmental

conditions, probability level etc.

• Other

Moment capacity, MU • E.g. incremental iterative method

• Material strength

• Other

Probability Density

Probability Density

Safety factors, γS, γW, γR Design Points (DP)

MS, DP

MW, DP

- actual loading - model uncertainty

→ MS distribution

- joint environmental model - hydrodynamic analysis - model uncertainty

→ annual extreme response

- random material - geometrical uncertainty - model uncertainty

→ capacity distribution

MU, DP

ettff PP arg,≤

SW

DPS

SM

M ,=γ

WV

DPW

WM

M ,=γ

DPU

UR

M

M

,

=γ

R

UWVWSWS

MMM

γγγ ≤+

R

U

sagwvWswS

MMM

γγγ ≤+ −



– Aim of reliability analysis calibration:

• Ensure a sufficient and consistent overall safety level,

accounting for the uncertainties involved

– Wave bending moment uncertainties:

• Randomness and uncertainty in sea state data

• Wave load prediction

– Hull girder bending capacity uncertainties:

• Material properties

• Strength prediction




Ultimate capacity in sagging:

• quite accurately predicted

• no effect of lateral pressure

• no double bottom bending

• model uncertainty estimated

based on comparisons with

non-linear FEM calculations

Design load combination γS γW γR A) Permissible still water bending

moment 1.00 1.20 1.10

B) Maximum bending moment for homogenous full load condition

1.00 1.30 1.10

0.60

0.70

0.80

0.90

1.00

1.10

1.20

1.30

1.40

1.50

1.60

1.70

1.0E-051.0E-041.0E-031.0E-02

Annual Probability of Failure

Part

ial safe

ty facto

r

SUEZMAX gamma_SW

SUEZMAX gamma_WV

SUEZMAX gamma_R

PRODUCT gamma_SW

PRODUCT gamma_WV

PRODUCT gamma_R

VLCC 1 gamma_SW

VLCC 1 gamma_WV

VLCC 1 gamma_R

VLCC 2 gamma_SW

VLCC 2 gamma_WV

VLCC 2 gamma_R

AFRAMAX gamma_SW

AFRAMAX gamma_WV

AFRAMAX gamma_R

WAVE

STILL

WATER

CAPACITY



.4 Structure Compatibility

a) Purpose of the space

- Designated usage of the space is used in the CSR i.e.

density, corrosion additions (temperature, corrosive

nature), etc.

b) Structural continuity

- The CSR include extensive requirements for continuity

on both the global and local levels




.5 Facilitate Loading/Unloading

a) Load scenarios are included in the design loads in the

CSR

b) Allowable SWBM and SWSF limits include in-port limits

.6 Net Scantlings

a) Definition proposed at MSC 82.


1. Provide a link between the assumed reduction in strength during

newbuilding strength evaluations and the in-service gauging

assessment criteria

2. Today’s in-service gauging assessment criteria covers:

� Global strength corrosion

� General corrosion

� Local (pitting, grooving and edge) corrosion

Net Scantling - Philosophy



Net Scantling – General Corrosion

Predicted corrosion

in 2.5 years (0.5 mm)

Required

Net

Thickness

Corrosion

Addition

Design

Required

Renewal

Thickness

Wastage

Allowance

In Service

Annual

Thickness

Measurements

includes link

between

newbuilding

and in-service

standards

General Corrosion – uniform thickness reduction in mm over

an extensive area.


Net Scantlings - Philosophy

Field Stresses:

Based on hull girder properties reduced by

10% ( Z net50 )

Field Stresses:

Based on gross scantling

Local corrosion:

Allowable % pitting, grooving and edge

corrosion

Local corrosion:


corrosion

General corrosion added to net scantling:

Discrete margins, in millimeters, based on

surface exposure.

General corrosion deducted from as-built:

% deduction or local simplified buckling,

whichever is less

Hull girder properties permitted to reduce by

10% (same as Z net50 ):

Z measured ≥ Z renewal = Z net50


10%:

Z measured ≥ Z renewal = 0.9 x Z gross required

Evaluations made on net scantlingEvaluations made on gross scantling

IACS proposed GBS definitionExisting in-service gauging criteria

( - corrosion deducted) ( + corrosion added)



Net Scantling - Related to Assessment Method

Strength evaluation

General corrosion renewal

As built

Renewal

Strength evaluation

Hull girder renewals

50%

50%

Strength

Time



Strength evaluation


As built

Renewal

50%

50%

Fatigue evaluation

Local properties

Strength evaluation


50%

50%

Strength

Time

Note: only hull girder properties, general and local corrosion have to be evaluated

during the in-service phase




Strength evaluation


As built

Renewal

25%

25%

Fatigue evaluation

Hull girder properties

50%

50%

Fatigue evaluation

Local properties

Strength evaluation


50%

50%

Strength

Time




� Proposed definition of “net scantling” to use in Tier II.3:

"The net scantlings are to provide the structural strength required to sustain the design loads, assuming the structure in intact condition and are to be derived from newbuilding strength evaluations linked to in-service diminution limits as follows:

.1 diminution of the hull girder section modulus is limited to not more than ten percent (10%), corresponding global stress calculations of the hull girder and primary support members may be based on this general scantling reduction,

.2 individual plates and stiffening elements are to have sufficientstrength to sustain design loads excluding additions for corrosion,

.3 fatigue calculations account for scantling variations through the design life,

.4 highly localized pitting, grooving and edge corrosion are to be treated separately and are typically not included in the newbuilding evaluations.”

GBS Net Scantlings



� Question: What would be the impact of using the same net

scantling for all three diminution definitions?

- Assuming that IMO and IACS retains today’s 10% limit on longitudinal strength diminution.

- Very crude estimations indicate an increase of about 6 percent in steel weight over CSR impact (does not include

fatigue impacts)

- For deck plate, if global 10% diminution governs, which is very rare, today’s as-built thickness will be tomorrow’s

renewal thickness

- It could be proposed that IMO and IACS increase longitudinal strength diminution from 10% to 20% to match newbuilding

standard, but today’s 10% is very rarely governing.

GBS Net Scantlings - Impact


� Summary of over 2000 cross sections

GBS Net Scantlings - Impact

Loss of Hull Girder Section Modulus

0%

1%

2%

3%

4%

5%

6%

7%

8%

9%

10%

11%

10 12 14 16 18 20 22 24 26 28 30 32 34

Ship Age

HG

SM

Loss

(As G

auged/A

s B

uilt) (%

)

mean

mean+stdv

mean+2stdv

data




DESIGN

II.1 Design life



II.4 Fatigue life



II.6.1 Coating life






CONSTRUCTION


II.12 Survey





II.15 Recycling


II.4 Fatigue life

Technical Comments

• The functional requirement is fully covered in CSR

• The long term distribution of stresses in the structure of the ship sailing in North-Atlantic environment is represented by a two-parameter Weibull law. The best fit of the Weibull distribution to the North-Atlantic scatter diagram is obtained by selecting a low probability of occurrence (10-4) for the scale parameter of the Weibull law

• The design S-N curve used is two standard deviations below the mean experimental ( 50%) S-N curve (2.5% under the curve)

• The linear damage accumulation rule of Miner’s sum is applied and a unit value of the damage ratio D corresponds to fatigue cracking

• The expected fatigue life is to be greater or equal to the design

life (i.e. 25 years)



Fatigue assessment procedure in CSR for Oil Tankers


• Important issues related to the Requirements

– Wave-induced loads at 10-4 probability level

• Vertical bending moment

• Horizontal bending moment

• External pressure

• Internal tank pressure

– Two loading conditions: full load (design draft)/normal ballast

– Net thickness concept used

– Palmgren-Miner’s linear damage model

– Long term stress range distribution described by Weibull distribution

– Two approaches

• Nominal stress approach – longitudinal end connections

• Hot spot stress approach – hopper knuckle connection

Fatigue Strength Assessment



• Example of Requirement

– Damage model


12

1

≤=∑=i

iDMDM

)ξ

mΓ(µ

)(lnN

S

K

NαDM i

m/ξR

mRiLi

i += 12

Fatigue life = 25/DM

10

100

1000

1.00E+04 1.00E+05 1.00E+06 1.00E+07 1.00E+08

N

Str

ess R

ange (M

Pa)

B

C

W

D

E F

F2

G


• Example of Requirement

– Joint classification

Critical Locations ID Connection type

A B

1

A B

d

leff

leff

F2 F2

2

A B

d

leff

leff

F2 F2

(see note iv)

3

A B

d

leff

leff

d/2

F F2

100R

30R

30

15

max.

15 mm

R ≥ 300 mm

R ≥ 2X/3R ≥ 400 mm

max. 15 mm

min. X/2

min. 300 mmX





• Comparison with current fatigue

rules

0

25

50

75

VLCC1

VLCC1

VLCC1

VLCC1

VLCC1

VLCC1

VLCC2

VLCC2

VLCC2

VLCC2

VLCC2

VLCC2

VLCC2

VLCC2

Suezmax

Suezmax

Suezmax

Suezmax

Suezmax

Suezmax

Suezmax

Suezmax

Aframax

Aframax

Aframax

Aframax

Aframax

Aframax

Product Carrier

Product Carrier

Product Carrier

Product Carrier

Life

Lower range - existing N.A.

Upper range - existing N.A.

JTP proposed Weibull

IACS proposed Weibull



DESIGN

II.1 Design life



II.4 Fatigue life



II.6.1 Coating life






CONSTRUCTION


II.12 Survey





II.15 Recycling




Technical Comments

• The functional requirement is partially covered by CSR

• Not explicitly covered, since the rule requirements only consider

intact structure

• Implicitly covered, based on experience from existing ships

upon which the rules were calibrated

• Flooding is included as an accidental load, but only the effect of

local pressure is checked

• Effect of flooding on global strength is not covered

• Post-buckling behaviour is included in hull girder check, but only

for intact structure

• Structural damage due to collision or grounding not considered

• Studies have indicated that damage condition is not

dimensioning for double hull oil tankers



Extent of structural damage

to consider

• Should be based on statistical data

from reported collision and

grounding damages

• IMO damage database

• EU-project HARDER

• Damage assumptions made

for oil outflow analysis in

MARPOL

• Need to select damage probability

level

• Need to select probability level for

environmental loads in damaged

condition

• Requirements should be calibrated

against existing designs

Probability Distribution of Damage Heigth

Harder Project, 460 Collision Cases

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

0.00 0.05 0.10 0.15 0.20 0.25

Damage Heigth, h/L

Cum

ula

tive P

robability

Probability Distribution of Damage Length

Harder Project, 542 Collision Cases

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45

Damage Length, ld/L

Cum

ula

tive P

robability




Individual class Rules contain criteria for structural damage

extent, but not placed in CSR yet

Samples:

Class notation CSA-2 Class notation RES

•Collision damage:

•Grounding damage:



DESIGN

II.1 Design life



II.4 Fatigue life



II.6.1 Coating life






CONSTRUCTION


II.12 Survey





II.15 Recycling




Technical Comments

• Present Description of GBS

– Measures to protect against corrosion are to be applied to ensure that net scantlings required to meet structural strength provisions are maintained throughout the specified design life.

– Additional measures include, but are not limited to, coatings, cathodic protection, impressed current systems, etc.

• “II.6” consists of following two sub-functions

– II.6.1 Coating life

– II.6.2 Corrosion addition

• The above pertain to providing protection against corrosion or anticipating corrosion in the strength calculations.

• The overall goal: Required scantlings meet the intended strength provisions throughout the specified design life.


II.6.1 Coating life

Technical Comments

• Present Description of GBS:

– Coatings should be applied and maintained in accordance with manufacturers’ specifications concerning surface preparation, coating selection, application and maintenance.

– Where coating is required to be applied, the design coating life is to be specified.

– The actual coating life may be longer or shorter than the design coating life, depending on the actual conditions and maintenance of the ship.

– Coatings should be selected as a function of the intended use of the compartment, materials and application of other corrosion prevention systems, e.g. cathodic protection or other alternatives.



II.6 Protection against corrosionII.6.1 Coating life

Technical Comments (continued)

• Coating Requirements of IMO

� the requirements of SOLAS Reg. II-1/3-2, IMO Resolution A.798(19) and IACS UI SC 122.

� Amendments of SOLAS Regulation II-1/3-2, IMO Resolution MSC.216(82)

� IMO performance standard - “Performance standard for

protective coatings for dedicated seawater ballast tanks in all

types of ships and double-side skin spaces of bulk carriers”, IMO Resolution MSC.215(82)

• CSR: Reference o the IMO instruments to ensure that the Rules

are inline with the SOLAS requirement with respect to corrosion prevention of ballast tanks

• CSR: Early implementation of IMO performance standard for ships

contracted for construction on or after 8 December 2006


II.6 Protection against corrosionII.6.1 Coating life

Verification

• Functional Requirements II.6.1 Coating Life has

been/will be covered by IMO mandatory instruments such as PSPC

• The referenced requirements such as SOLAS Reg. II-

1/3-2 cover the following items related to information and documentation for II.6.

.1 Locations and/or spaces where coatings are required to be used

.2 Types of coating to be used for the various spaces

.3 Reference coating performance standards

• To verify if all the necessary IMO mandatory

instruments are properly referred to




1. Basic Concept

• Net Scantling Concept

2. Stochastic Corrosion Propagation Model

3. Statistical processing of thickness

measurements data

4. Example of Corrosion Addition


1. Basic Concept of Corrosion Addition

• General philosophy for establishing “corrosion additions”

(a) based, in general, on the premise that today’s practice

is a reference point, and departures from today’s

practice will need to be backed-up with technical justification;

(b) established based on the basic assumption of coatings provided (where required) at time of newbuilding,

(c) appropriate for a 25-year service life;

(d) based on absolute numbers, i.e., 4.0mm (not 25%);

(e) independent of type of local failure mode employed, i.e., yielding, buckling, or fatigue;

(f) based on published data and recent experience of IACS member societies;



1. Basic Concept of Corrosion Addition

• Adoption of Net Scantling Concept

– Corrosion additions should be completely consistent with wastage allowances

– “Corrosion addition” = “Wastage Allowance” + 0.5mm

Note: The 0.5mm is added in reserve for the wastage

occurring between the inspection intervals of approximately 2.5 years

• Based on the stochastic corrosion propagation model and data on record of gauging, etc.

• A extra margin in some areas of the structure was

added to account for the variability of corrosion based on service experience.


Predicted corrosion

in 2.5 years (0.5 mm)

Required

Net

Thickness

Corrosion

Addition

Design

Required

Renewal

Thickness

Wastage

Allowance

In Service

Annual

Thickness

Measurements

includes link

between

newbuilding

and in-service

standards

General Corrosion – uniform thickness diminution/reduction in mm over an extensive area.

Adoption of Net Scantling Concept- Measures to deal with General Corrosion -



2. Stochastic Corrosion Propagation Model

Contents of Explanation of Stochastic Corrosion Propagation Model

1. Evaluation of Generation and Progress of Corrosion

� Corrosion Rate Approach

� Stochastic Corrosion Propagation Model

2. Concept of Flexible Probabilistic Corrosion Model

3. Consideration on General Corrosion Propagation

4. Example of Evaluated Behaviors

5. Illustration of state of corrosion simulated by the corrosion model


Evaluation of Generation and Progress of Corrosion

Annual corrosion rates scatter widely.

Actual corrosion generation and progress cannot be explained by the annual corrosion rates.

0 0.2 0.4 0.6 0.80

0.1

0.2

0.3

0.4

0.5

Annual Corrosion Rate (mm/year)

Frequency

5 years

0 0.2 0.4 0.6 0.80

0.1

0.2

0.3

0.4


Frequency

10 years

0 0.2 0.4 0.6 0.80

0.1

0.2

0.3


Frequency

15 years

0 0.2 0.4 0.6 0.80

0.1

0.2

0.3

0.4


Frequency

20 years

0 0.2 0.4 0.6 0.80

0.1

0.2

0.3

0.4

0.5

0.6

0.7


Frequency

25 years



Elapsed TimeElapsed TimeD

epth

of

Co

rro

sio

nD

epth

of

Co

rro

sio

nIntact ConditionIntact Condition

First it progresses First it progresses depthdepth--wisewise

Then it spreadsThen it spreads

Annual Wastage Rate

StartsStarts

Usual method of getting corrosion

rate using linear line

Actual corrosion generation and progress

cannot be explained by the annual

corrosion rates.

Evaluation of Generation and Progress of Corrosion


Time

Distribution of transition time

Distribution of corrosion depth

Time

Distribution for effectiveness of paint coatingsGeneration of active

pitting points

Transition to pitting points from active pitting points

Progress of pitting points

Concept ofFlexible Probabilistic Corrosion Model

Time

Corrosion depth

Probability Density

PD

PD

Corr

osio

n d

epth



Time



Time


pitting points




Time

Corrosion depth

Probability Density

PD

PD

Corr

osio

n d

epth


Time



Time


pitting points



Variety of corrosion progress patterns can be described by the probabilistic corrosion model.


Time

Corrosion depth

Probability Density

PD

PD

Corr

osio

n d

epth



Time



Time


pitting points



a special case


Time

Corrosion depth

Probability Density

PD

PD

Corr

osio

n d

epth


Time



Time


pitting points



a special case


Time

Corrosion depth

Probability Density

PD

PD

Corr

osio

n d

epth

The probabilistic corrosion model can describe the corrosion

diminution behavior, even if thickness diminishes at the constant rate from the beginning of the service.



Time



Time


pitting points



a special case


Time

Corrosion depth

Probability Density

PD

PD

Corr

osio

n d

epth

The probabilistic corrosion model can describe the corrosion diminution behavior, even if diminution develops exponentially.


Time



Time


pitting points




Time

Corrosion depth

Probability Density

PD

PD

Corr

osio

n d

epth

� Parameters in the corrosion model were determined based on the actual thickness measurement data.

� It turns out that

�period of no corrosion exists,

�annual corrosion rates are NOT constant and

�diminution does NOT develop exponentially.


� It turns out that







� It turns out that, in case of GENERAL CORROSION, i.e. uniform thickness diminution/reduction over an extensive area,





� It turns out that, in case of GENERAL CORROSION, i.e. uniform thickness diminution/reduction over an extensive area,




Consideration on General Corrosion Propagation

Time

Dim

inution

Trend of actual corrosion behavior

Such a line does NOT reflect

the actual corrosion behavior


Example of Evaluated Behaviors

Frequency

distribution

at 25 years

at 20 years

at 15 years

at 10 years

at 5 years

5 10 15 20 25 30

25

50

75

1000

Age (Year)

Pitting Intensity (%)

5 10 15 20 25 30

1

2

3

4

0

Diminution (mm)

95%

Average

Pitting Intensity: Ratio of the corroded surface area to the entire surface area



25 years20 years15 years

10 years5 years

Illustration of state of corrosion simulated by the corrosion model


3. Statistical Processing of Thickness Measurements Data

• To collect thickness measurements data

• To transfer the data to electronic form

• To categorize the electronic data on the basis of

– exposure to corrosive agents such as water, cargo or corrosive atmosphere, and

– whether the structure is protected by corrosion prevention systems, e.g. coating, cathodic protection

• To store data into the structured DATABSE

• To estimate thickness diminution due to general

corrosion by Statistical Analysis according to the Categorization and the corrosion propagation model

• To determine corrosion addition values for one side of plates based on the categorization




• To store data into the structured DATABSE�Number of Gauging Report: over 500

�Number of Data: about 600,000






4. Example of Corrosion Addition - Measures to deal with General Corrosion -




DESIGN

II.1 Design life



II.4 Fatigue life



II.6.1 Coating life






CONSTRUCTION


II.12 Survey





II.15 Recycling



Technical Comments

• The functional requirement is partially covered by CSR

• No explicit requirements to structural redundancy

• Implicitly covered through:

• Inherent redundancy of stiffened panels

• Inherent redundancy of double hull tankers

• Criticality class considerations made during the rule

development

• Advanced buckling methods used for strength

assessment of stiffened panels




Criticality class:

• Criticality of each

structural element

• Acceptance criteria

according to criticality

• Less critical elements

fail first

LOCALPRIMARYMAJORGLOBAL

Criticality

color code:

Low

High

Medium

Hull girderPL E

DeckPL E

Double bottomPL E

Double sidePL E

Long. bulkheadPL E

Tr. bulkheadPL E

Deck panel

PL E

Deck long.

PL E

Deck plate

PL E

Deck girders

PL E

Deck girder web

PL E

Inner btm. panel

PL E

Btm. shell panel

PL E

Bottom girders

PL E

Inner btm long.

PL E

Btm. shell long.

PL E

Inner btm. plate

PL E

Btm. shell plate

PL E

Btm. girder web

PL E

Inner side panel

PL E

Inner side long.

PL E

Inner side plate

PL E

Side shell panel

PL E

Side shell long.

PL E

Side shell plate

PL E

Side girders

PL E

Side girder web

PL E

Long. bhd. panel

PL E

Long. bhd. long.

PL E

Long. bhd. plate

PL E

L. bhd. girders

PL E

Lbhd. girder web

PL E

Tr. bhd. panel

PL E

Tr. bhd. stiff.

PL E

Tr. bhd. plate

PL E

Tr. bhd. girders

PL E

Tbhd. girder web

PL E

3

2

0

1

4

5

1.1

1.2 & 1.3

2.1

2.2

2.3 & 2.4

3.1

3.2

3.3 & 3.4

4.1

4.2

5.1

5.2

1.1.1 1.1.2

1.2.2 & 1.3.2

2.1.1 2.1.2

2.2.1 2.2.2

2.3.2 & 2.4.2

3.1.1 3.1.2

3.2.1 3.2.2

3.3.2 & 3.4.2

4.1.1 4.1.2

4.2.2

5.1.1 5.1.2

5.2.2



Advanced buckling methods:

• Elastic buckling of plates allowed

• Stiffeners required to sustain the redistributed load

• Gives redundant panels




Exception:

• Corrugated bulkheads do generally not have the

same redundancy as stiffened panels



DESIGN

II.1 Design life



II.4 Fatigue life



II.6.1 Coating life






CONSTRUCTION


II.12 Survey





II.15 Recycling



II.8 Watertight and Weatherthight Intergrity

Technical Comments

• Subdivision of ship and tank size influenced by

- floodability and damage stability (SOLAS,ICLL)

- oil-outflow restrictions (MARPOL)

• Hull opening and closing arrangements regulated by ICLL

and SOLAS requirements

• Strength requirements of opening and closing

arrangements given in ICLL and URS 26 and URS 27

(fore deck) used as basis for CSR


II.8 Watertight and Weatherthight Intergrity

Technical Comments (cont.)

• Scantlings of watertight boundaries defined under

consideration of

• International conventions and rules, IACS URS, if

appropriate

• Static and dynamic loads of loading conditions

(combined global and local loads) and flooding

conditions, if appropriate




DESIGN

II.1 Design life



II.4 Fatigue life



II.6.1 Coating life






CONSTRUCTION


II.12 Survey





II.15 Recycling


II.9 Human Element Considerations

1. Technical Comments

• In classification point of view related only to sufficient

space and accessibility for safe inspection/survey,

maintenance, repair and rescue operations

• Some specific requirements regarding the protection of

crew members are included in ICLL 1966 and UI LL14 of

IACS (e.g. guard rails)

• requirements related to accidental protection and

ergonomics defined in national regulations and Tier V

standards

• It is the responsibility of the owner/designer/builder to

ensure that regulations of international, national, canal

and other authorities which may affect structural aspects

are considered




DESIGN

II.1 Design life



II.4 Fatigue life



II.6.1 Coating life






CONSTRUCTION


II.12 Survey





II.15 Recycling


III.10 Design transparency

Technical comments• The builder is responsible for providing design documentation according

to requirements specified in the Rules

• Quality systems are applied to the design, construction, operation and maintenance activities to assist compliance with the requirements of the Rules.

• it is the responsibility of the owner to specify the intended use of the ship, and the responsibility of the builder to ensure that the operational capability of the design fulfils the owner’s requirements as well as the structural requirements given in the Rules

• the builder shall identify and document the operational limits for the ship so that the ship can be safely and efficiently operated within these limits

• verification of the design is performed by the builder to check compliance with provisions contained in the Rules in addition to national and international regulations

• the design is performed by appropriately qualified, competent and experienced personnel

• the classification society is responsible for a technical review and audit of the design plans and related documents for a ship to verify compliance with the appropriate classification rules.



III.10 Design transparency

Technical comments

• The alternative arrangements are considered on the basis of equivalency.

• Information is be submitted to demonstrate that the structural safety of the novel design is at least equivalent to that intended by the Rules.

– Dependent on the nature of the deviation, a systematic review may be required to document equivalence with the Rules.

• Alternative calculation methods may be accepted provided it is demonstrated that the scantlings and arrangements are of at least equivalent strength to those derived using the Rule calculation method.

• Ship Construction File

• IMO is yet to defined the contents of the File

• IACS UR Z23 already does



DESIGN

II.1 Design life



II.4 Fatigue life



II.6.1 Coating life






CONSTRUCTION


II.12 Survey





II.15 Recycling



III.11 Construction quality procedures

Technical comments: The Rules and UR Z23 state:

• the builder is responsible for ensuring that adequate supervision and

quality control is provided during the construction

• construction is to be carried out by qualified and experienced

personnel

• workmanship, including alignment and tolerances, is to be in

accordance with acceptable shipbuilding standards

• the Classification Society is responsible for auditing to verify that the

construction and quality control are in accordance with the plans and

procedures

• shipbuilding quality standards for the hull structure during new

construction are to be reviewed and agreed during the kick-off

meeting between the builder and class

• the structural fabrication is to be carried out, in accordance with

IACS Rec. 47 or a recognised fabrication standard accepted by the

Classification Society



Technical comments: The Rules and UR Z23

state…cont...:

• additional attention is paid during construction and repair of critical areas of

the structure

• the meeting prior to commencing any newbuilding project to assess the

degree of compliance of the shipyard with the items in Table 1 of UR Z23

• provides a list of surveyable items in Table 1 for the hull structure covered by

UR Z23

• increased inspection requirements are deemed necessary as a result of a

substantial non-conformance

• the builder is to be requested to agree to suspend relevant construction

activities if warranted by the severity of the problem under investigation,

which was discovered during the construction process.




Question:

How does a classification society determine that a shipyard is

qualified to construct a vessel to its Rules?



Answer: UR Z23:

6. Review of the construction facility*

6.1 The society is to review the construction facilities prior to any steelwork or construction taking place in the following circumstances:

6.1.1 where the society has none or no recent experience of the construction facilities – typically after a one year lapse - or when significant new infrastructure has been added,

6.1.2 where there has been a significant management or personnel re-structuring having an impact on the ship construction process,

6.1.3 or where the shipbuilder contracts to construct a vessel of a different type or substantially different in design.

*Footnote: Reference is made to Appendix 1 “Shipyard review record”, as an example.




Answer:

UR Z23:

7.1 Prior to commencing any newbuilding project, the society is

to discuss with the shipbuilder at a kick off meeting the items

listed in Table 1. The purpose of the meeting is to agree how the

list of specific activities shown in Table 1 is to be addressed



Answer:

UR Z23:

9. Proof of the consistency of surveys

9.1 The classification society is to be able to provide evidence,

e.g. through records, check lists, inspection and test records, etc.

that its surveyors have complied with the requirements of the

newbuilding survey planning and duly participated in the relevant

activities shown in the shipbuilder’s examination and test plans.

9.2 For audit purposes, the information specified in 9.1 is to be

made available.




DESIGN

II.1 Design life



II.4 Fatigue life



II.6.1 Coating life






CONSTRUCTION


II.12 Survey





II.15 Recycling


III.12 Survey

Technical comments: UR Z23 states:

• Prior to commencing any newbuilding project, the society is to discuss with the shipbuilder at a kick off meeting the items listed in Table 1.

• The purpose of the meeting is to agree how the list of specific activities shown in Table 1 is to be addressed.

• The meeting is to take into account the shipbuilders construction facilities and ship type and deal with sub-contractors.

• The shipyard is to be informed of likely intervals for sampling and patrol activities.

• A record of the meeting is to be made, based upon the contents of the Table

• The record of the meeting is to be updated as the construction process progresses in the light of changing circumstances

• The builder is to be asked to agree to undertake ad hoc investigations during construction where areas of concern arise and for the builder to agree to keep the classification society advised of the progress of any investigation.




DESIGN

II.1 Design life



II.4 Fatigue life



II.6.1 Coating life






CONSTRUCTION


II.12 Survey





II.15 Recycling



Technical Comments

• First part of this requirement covered by II.9 Human

element (accessibility)

• Reference to UR Z10.4 for thickness measurements

• Renewal limit for steel structure parts defined based on

net-thickness concept

• CSR do not include requirements with respect to survey

related to the verification of compliance with the rules

during construction and operation

• Owners are responsible for maintaining the ship and the

individual classification societies verify the compliance

with the class requirements in accordance with the

classification society survey scheme




Technical Comments (cont.)

• Extent and frequency of Survey for double hull oil tankers

defined in UR Z10.4

• Extent of survey dependent of structure to be surveyed,

coating condition and age of the ship

• Introduction of criteria for planning survey and

maintenance for ship-in-service to be discussed

(identification and consideration of areas that need

special attention) In general this is committed to the

surveyors / societies experience



DESIGN

II.1 Design life



II.4 Fatigue life



II.6.1 Coating life






CONSTRUCTION


II.12 Survey





II.15 Recycling




1. Technical Comments

• CSR for oil tankers refer to SOLAS Ch II-1, Part A-1,

regulation 3-6, and the checking is mainly a statutory matter

• with a direct reference in CSR for oil tankers Sec 5

[5]

2. Additional Criteria

CSR for oil tankers add requirements for access to

specific areas: duct keel and pipe tunnel.



DESIGN

II.1 Design life



II.4 Fatigue life



II.6.1 Coating life






CONSTRUCTION


II.12 Survey





II.15 Recycling



II.15 Recycling

Technical Comments

• Recycling not in scope of class rules

• Convention on Safe and Environmentally Sound

Recycling of Ships under development at MEPC


Thank you

CSR Aim:

To develop a set of unified Rules and Procedures for

the determination of the structural requirements for

oil tankers and bulk carriers


***

7 March 2007 Page 1

IMO Pilot Panel Questions to IACS for March 12 Meeting

General:

1) IACS maintains a series of Unified Requirements that apply to all their member

societies. It would seem that these requirements have been decided and agreed by a

large portion of the industry and as such these should probably qualify as Tier II

requirements. We would appreciate a comment from IACS as to whether they agree.

Ans: IACS URs are proprietary documents of the International Association of

Classification Societies and do not have application outside IACS. The URs

are not associated with an “IACS class,” instead it is a requirement of

membership that the URs be introduced into Members’ individual Rules.

Classification is not assigned to a ship based on application of UR(s), only

classification Rules of an individual Member can be applied to a design. The

Tier II requirements of GBS are requirements for rulemaking and define which

topics have to be covered in the appropriate rules to fulfill or reach the goal of

Tier I of GBS. Therefore while some of the URs are elements of Tier VI of

GBS, they will actually be covered within the process of accepting the

individual rules under GBS.

2) The developing status of rule programs of IACS members to be summarized and

explained to the pilot panel members if possible with case studies since CSR rules for

Tankers and Bulk Carriers came into force on 1st Apr. 2006. Countermeasures should

be provided. For example, if nonconformity caused by the misunderstanding,

misinterpretation and human errors in programming of rule appears between IACS

members it should be immediately clarified and corrected. Considering the actual ship

design contracted based on CSR, it will become very urgent and critical situation to

shipyards.

Ans: IACS anticipated issues and difficulties related to the application of the CSR,

which is a first joint rule development on such a large scale, and consequently

implemented a set of measures to cope with their implementation. This

includes extensive maintenance, maintaining a question and answer database as

well as an interpretation database. This system is used to document application

issues and provide immediate clarification when issues arise as well as during

subsequent rule development activities.

While this is an interesting question and we believe that IACS has a system in

place to address these issues, it is our understanding that GBS deals mainly

with the development of the rule requirements and not with implementation

perse.

3) Please clarify what extent IACS CSR for Tankers covers the requirements of Tier II.

We recognize that IACS CSR does not cover all the requirements of Tier II of GBS.

Ans: Correct. Not all requirements of Tier II of GBS are classification items

therefore not all requirements are covered by the IACS CSR. A table of

MSC 83/INF.5

ANNEX 3

7 March 2007 Page 2

covered requirements is given at the beginning of the presentation (slide 3 and

4). Therefore as part of Tier III we need a clarification on how to address GBS

topics that are covered by IMO regulations or industry standard and not in the

Class rules.

Tier II.1:

4) Recognizing that Tier I contains the clause "...when properly operated and maintained..." We would like to see a comment from IACS as to their expectation,

relative to their societies' rules, for the role that in-service survey and maintenance

plays in achieving the design service life of a vessel. We would also appreciate a

summary description of the inspection regime for tankers particularly as it relates to

increasing frequency and intensity of survey as a vessel ages.

Ans: The expectation is that a ship is to be maintained in good condition in

accordance the Classification Society survey scheme and also with

international and national regulations and requirements. In addition the

operation’s personnel are to be provided with sufficient training such that the

ship is properly handled to ensure that the loads and resulting stresses imposed

on the structure are minimized, or, certainly that the structure is not

overstressed. The survey inspection scheme generally requires more frequent

and increased scope of surveys as the vessels become older, please refer to the

IACS UR Z10.2, 10.4 and 10.5 which may be obtained from www.iacs.org.uk .

Tier II.2:

5) What is the basis of design life of oil tankers in relation to sea state conditions in Tier

I?

Ans: The North Atlantic wave environment is represented by a wave scatter diagram

that gives the probability of each sea state as the number of occurrences per

100,000 observations. Using the scatter diagram, the long-term value of the

load is obtained as the most probable largest value occurring with a certain

return period. The return period of the load is taken as equal to the design life

of 25 years. The probability level for the design load is then 1/N, where N is

number of load cycles during the design life. The actual number of wave load

cycles for a certain ship will depend on the ship size, speed and port time. The

number of wave load cycles corresponding to a design life of 25 years is

assumed constant and equal to 108. Previously, this value was assumed to

represent 20 years, but it is found to be more representative for 25 years. The

difference in load magnitude between loads based on a 25 year and a 20 year

return period is small. For example; if the 10-8 probability level relates exactly

to a 25 year return period, then corresponding 20 year return period would be

given at a probability level of 10-7.9, assuming that the long-term distribution of

the load can be represented by a Weibull distribution with a shape parameter

equal to 1.0. The corresponding difference in actual load value is

approximately 1%.


7 March 2007 Page 3

6) With respect to rouge and/or steep waves, what is IACS’ opinion of the state-of-the-

art and how these are treated/should be treated in the rules?

Ans: The effect of steep/rogue waves is presently not covered by CSR or other class

requirements. There has been several research projects carried out in recent

years on the topic, such as the EU project MAXWAVE. Among the findings

from this research are that steep waves can have an important effect on ships,

and that such waves seem to occur more frequently than previously assumed.

The load categories that are particularly affected by steep waves are bow and

bottom slamming, green water loads on deck, and superstructure impact.

Despite the efforts made so far, it is considered that more work is needed

before class requirements on steep waves can be formulated. There has not yet

been agreement on a definition of steep waves, and there exist different

theories for the physical explanation of the phenomenon, such as wave-current

interaction, combined seas, and wave energy focusing. There are at the

moment ongoing and planned research projects that will investigate the

unresolved issues. Especially, it is important to obtain more information on the

probability of occurrence (a statistical model), the spatial and time

representation of steep waves (a wave model), and the structural response

under the action of such waves (numerical load analysis). Also, a better

physical understanding of the phenomenon is desirable.

When the above issues have been resolved, requirements for consideration of

steep waves could be formulated in the rules. It would be natural to treat such

abnormal loads as an ALS (accidental limit state) condition, meaning that

reduced safety factors can be accepted.

7) In IACS Documentation Package (Para II.1 - Design life and II-2 -Environmental

loads) it is mentioned that in CSR the "characteristic value of loads in ultimate

strength" are based on a probability of exceedance of 10-8. However, the probability

that a largest peak value may exceed the probable extreme value is quite large and

hence it is not seems appropriate to use this value for engineering purpose since that it

is known that in a perfectly narrow-banded process the probability that the

characteristic value calculated for 10-8 exceedance is 63.2%. For purposes of

structural design we must obtain an extreme value for which the probability of being

exceeded is some acceptably small value (typically 0.01). We would appreciate

additional comments regarding this issue from IACS including the considerations

adopted for the non-linear effects in maximum loads.

Ans: In principle, any value can be used as the characteristic load, since the safety

factors will be adjusted accordingly to achieve the overall target safety level.

The use of the most probable largest value of the load is a practical approach,

because this value can easily be determined from the long-term load

distribution for any return period, and does not require knowledge about the

probability distribution for the load corresponding to the design life. Assuming


7 March 2007 Page 4

that the surface elevation can be described by a narrow-banded Gaussian

process, the extreme load can be described by a Gumbel probability function.

For a Gumbel distribution, there is a 63.2% probability of exceeding the most

probable largest value. However, for large number of occurences the Gumbel

distribution becomes narrow, meaning that the difference between the most

probable largest value and for instance the value with 1% probability of

exceedance becomes smaller.

Whichever value is used as the characteristic load, the acceptance criteria need

to be calibrated to achieve the overall target reliability. The difference between

the most probable largest value and the value with a small probability of

exceedance will therefore be accounted for in the safety margins. For the hull

girder strength criteria, the partial safety factors have been calibrated using

reliability analysis, with the actual probability distribution of the loads used as

input to the calibration. This means that the end result should be the same,

whether the characteristic load is taken as the most probable largest value or a

value with a small probability of exceedance.

Tier II.3:

8) Provide information to justify the following:

a) Values of the “Usage factors” introduced in the assessment of acceptability

against buckling failure.

b) Relevance of the parameter “Depth-to-thickness” ratio to control the deflection.

c) Data showing that the method used to evaluate the maximum local stresses on the

stiffeners is adequate to cover all the common arrangements and its accuracy.

Ans: Thank you for this general question. This was covered during the PP meeting,

please refer to the IACS presentation slide pages 35 to 63.

9) The ultimate strength requirement in the CSR calls for a 1.2 partial safety factor to be

applied against the wave bending moment, and a 1.1 partial safety factor against the

hull girder structural capacity. This results in an overall "safety factor" of approx.

1.2, which seems reasonable and represents a substantial enhancement over some of

the double hull tankers built to pre-CSR rules. However, the 1.2 factor applied on the

wave bending moment seems disproportionate compared to the 1.1 factor on capacity.

We would expect a greater confidence level in projecting wave bending moments

than computing hull girder structural capacity. With regard to long term distribution

of wave bending moments -- its rather narrow banded and our experience is that

increasing to 40 or 50 year life only increases the wave bending moments by a few

percent.

Ans: The partial safety factors specified for the ultimate hull girder strength check

are calibrated using reliability analysis. The aim of the calibration is to ensure a

sufficient and consistent overall safety level for all ships, accounting for the

uncertainties and randomness related to the calculation of load and strength.

For the wave bending moment, the uncertainties accounted for are the

randomness and uncertainty in the sea state data, and uncertainties related to


7 March 2007 Page 5

the wave load prediction. For the hull girder bending capacity, the randomness

and uncertainty in the material properties and the uncertainties related to the

capacity model are accounted for.

The characteristic value of the wave bending moment is calculated as the most

probable largest value, while the characteristic value of the hull girder capacity

is based on minimum values of the material strength. Also, it is found that the

estimation of hull girder capacity in the sagging condition is quite accurate,

since the collapse in sagging takes place in the deck. This means that the

loading in the deck panels is uni-axial, in contrast to bottom failure, where the

effect of lateral pressure and double bottom bending must be accounted for.

Due to these effects, a larger safety factor is needed for the wave bending

moment than for the hull girder strength.

The calibration process carried out for the partial safety factors is described in

detail in Section 9.1 of the Background Documentation to the CSR.

10) The CSR does not require analysis to demonstrate that suitable continuity is applied

at the ends of the cargo block, and other areas of discontinuity in the hull girder

primary structure. Rather, it has statements such as "... due consideration is to be

given to the arrangement of major longitudinal members in order to avoid abrupt

changes in section" and "... due consideration is to be given to the tapering of primary

support members". Please explain why IACS is comfortable that a global ship FEA

or at least local FEA in way of transitions is not needed. It would be very helpful if

IACS could provide examples of acceptable and unacceptable levels of transition for,

say, a representative AFRAMAX tanker.

Ans: The objective of the structural continuity requirements is to avoid hard spots,

notches and stress concentrations in the structure. Requirements for large hull

girder longitudinal members as well as for the end termination of primary and

local members are included in the CSR. These general requirements have been

in the rules for many years with satisfactory result. One of the reasons we did

not include sample figures in the CSR, is due to extensive feedback requesting

us not to do so, many designers feel that if a sample is given it becomes a

quasi-requirement and thereby limits their flexibility. For the sake of the PP we

can provide such an example.

11) With respect to Tier III.3, the focus in our discussion so far has tended to be on

bending moment with little discussion of shear force. We would appreciate a

comment from IACS as to how shear should be treated within this Tier.

Ans: While the GBS may be concentrating on bending moments, all class rules

including the CSR, include extensive requirements for the evaluation of shear

forces.

12) In the GBS Correspondence Group documentation, there is much discussion of

'excessive deflection' and 'limits of deformation' yet we are not aware of any specific

rules. Deflections seem to be controlled by scantlings, aspect ratio control, and


7 March 2007 Page 6

section modulus requirements. However, IACS uses both terms in their alternative

proposal. We would appreciate a comment from IACS as to how

deflection/deformation is handled in their rules.

Ans: This was covered during the PP meeting, please refer to the IACS presentation

slide pages 52 to 63.

13) Also in the CG correspondence and in our discussions, there appears to be varying interpretations and perhaps disagreement as to what is meant by 'net scantlings'.

IACS has provided their presentation on 'GBS Net Scantlings' and on slide 14 have

proposed a definition. The definition presumes three different levels of scantling

reduction (hull girder, local, and fatigue) but unfortunately the ppt slides do not

contain a justification for this approach. We request that IACS provide a summary

justification. Should IMO decide that the same net scantling margin be used for all

three (hull girder, local and fatigue), what would be the impact on hull design/steel

weight?

Ans: The justification for the two levels hull girder and local is that they represent,

and are directly linked, to the gauging thickness measurements used in-service

as of today as required by the rules and SOLAS. The fatigue level simply

averages the condition half way between the newbuilding condition and the

minimum permissible renewal condition, since fatigue is a time-dependent

phenomenon that will span both conditions of the vessel. Should IMO decide

to use a simplified “pure net” definition for newbuilding scantling

determination but retain the current two levels for hull girder and local

thickness measurements, the impact would be that the steel weight would be

increased by roughly 6 percent. (Note, this study was later expanded and

results ranged from 3.65% to 7.8%.) The difference mainly being that current

thickness measurement allowables associated with the hull girder permits a

10% reduction in section modulus which has been used for about the last 30

years and as shown in slide page 78 is rarely governing, and the “pure net”

definition would require about 20% margin be built in. See slide pages 70 to

78.

Tier II.4:

14) Regarding Tier II.4 Fatigue Life, there are different acceptable methodologies to

carry out fatigue life calculations, and methodologies used in CSR for tankers and

bulk carriers differ. However what is important is that the different methodologies

used, assuming similar basic inputs, would give similar results. Has IACS carried out

any comparisons of the two methods and identified any significant differences in

results? Have the fatigue life methodologies been calibrated or benchmarked against

experimental test data or full scale ship damage data?

Ans: The two methodologies have been checked and give similar results given the

same application parameters. The area of fatigue is a very complex one and

the two project teams that originally developed the rules used two different

approaches that happen to fit the individual application parameters for the


7 March 2007 Page 7

individual vessel type. The main calibration of the new CSR was made against

the latest rules of the individual class societies. The individual class society

rules have been calibrated against know failures and success over the years as

well as being calibrated against extensive direct calculations using spectral

fatigue or stochastic methods. The calibration took into account the increased

criteria associated with using the North Atlantic environment and a 25 year

fatigue life, both of which served to generally increase the fatigue related

requirements.

15) Provide information to justify the data of the benchmark studies carried out to

ascertain the accuracy of the simplified fatigue analysis method included in the rules.

Ans: Please refer to the answer to question 14.

16) With respect to Tier III.4, Fatigue, it is not clear that everyone has the same

understanding of what is meant by fatigue life. Therefore, please define what IACS

means when they say that they have designed to a fatigue life of 25 years. (Our

simple understanding is that a 25 year fatigue life means that there is a 97%

confidence level that the detail under consideration will not exhibit a detectable crack

(6 mm?) before age 25 when exposed to North Atlantic environmental conditions

over its life time.) Once a fatigue crack appears, how quickly does IACS expect it to

grow through-thickness, and then to a length that would affect structural integrity of

the vessel?

Ans: The definition of fatigue life offered within the parentheses ( ) in this question

is generally used by IACS. Indicating the time a crack will grow is not a

simple task, however it is correct that in most instances there is period of time

between when a crack first appears and when it would propagate to a point that

it would affect the structural integrity of the local structure or even the vessel.

Typically once a crack appears it is generally repaired. If a local temporary

repair or drill stop can not be used and it not obvious that there is sufficient

time to repair the crack at the next schedule repair date, a crack propagation

analysis may be carried out to determine if the repair may be postponed, i.e.

determine how quickly it will grow.

17) Also with respect to Fatigue, the Tier II requirement appears to be lacking better

definition of the determination of fatigue. We would appreciate IACS comments on

what should be included as the basis for the fatigue calculation (hydrodynamic load

analysis at various wave headings, Miner's rule, -2 sigma S-N curves, etc.)

Ans: The area of fatigue is very complex and there are many different approaches

used which take into consideration the problems associated with the

application. For example the approach used in repetitive tanker structure is not

the same as that used in isolated offshore connections. The main measurement

is the calibration of the approach or a comparison of the approach with existing

results that have proven to be acceptable over time. As the current task is to

develop goal-based standards there should be sufficient flexibility built into the

Tier II requirements such that new rule development is not stifled. Having said


7 March 2007 Page 8

that, there are some basic parameters which may be referenced such as using

the North Atlantic environment, 25 year life, Miner's rule, and -2 sigma S-N

curves.

18) In IACS Documentation Package (Para II.4 - Fatigue life) is mentioned the "damage

ratio". To prevent fatigue fracture this ratio must not exceed 1.0. In practice, because

of the various uncertainties the limit value is substantially less than 1.0 (typical values

are in the range from 0.1 to 0.3). It should be useful to clarify the values adopted in

CSR.

Ans: The CSR was developed with individual margins built in to the various steps,

such as the environment, SN curve, stress determination, etc. and calibrated

such that the usage factor of 1.0 represents the acceptance limit. We are aware

that other industries as well as the offshore industry calculates fatigue without

individual margins but then in the end introduce an overall factor such as 2, 3

or 10 which would correspond to the 0.1 to 0.3 as you state. The two

approaches are simply looking at the same problem but solving it in different

ways.

Tier II.5:

19) With respect to Tier II.5, the term 'specified damaged conditions' is not defined and as

indicated by our discussions it will be difficult to verify without a better definition.

We would appreciate IACS opinion of what would be a reasonable definition (e.g.

loss of all longitudinal material between two adjacent crack arrestors anywhere within

the midship section, at the maximum environmental condition.)

Ans: Since the rule requirements only consider intact structure, specified damage

conditions have not been defined in the rules. The effect of structural damage

has traditionally been considered as outside the main scope of class, but some

class societies have introduced such requirements as part of additional class

notations.

If specified damage conditions were to be included in the rules, the extent of

damage to consider should be defined based on statistical data available from

reported collision and grounding damages. Also, the damage conditions

specfied should be seen in relation to the damage assumptions made for

specifications for oil outflow analysis in MARPOL. Damage data are available

in the IMO damage database, and were used in the EU project HARDER to

produce probability distributions for damage extents.

In order to use the damage statistics, a damage probability level must be

defined. The probability level should be determined based on a target

probability of survival in the case of collision or grounding. At the same time,

the probability level to be used for the environmental loads in the damaged

condition must be defined. The return period of the loads used for this

condition should be reduced compared with the intact condition, to account for


7 March 2007 Page 9

the fact that most collisions and groundings occur close to shore, where the

environmental conditions are expected to be less severe.

Finally, the requirements determined using the above considerations need to be

calibrated against existing designs.

Tier II.6:

20) It should be very important to keep in a GOOD condition of cargo hold against

corrosion to conduct relevant and sufficient inspection. This will be discussed in IMO

DE50 for amendments to IMO A744(18) EPS programme.

Ans: Thank you for your comments. IACS is well aware of the fact that the

compulsory coating of cargo tanks will be an agenda of IMO.

21) What is the basis of the CSR for the life of the protection against corrosion of main

structural members of tankers? It should be recognized that corrosion debris in the

cargo hold of oil tankers are mainly the effluents of chemical reaction (mainly by

sulphur contents of the cargo) and does not the fall out of the construction members

itself.

Ans: As there have not been mandatory coating requirements of cargo tanks but

ships whose cargo tanks were voluntarily coated partly (e.g. upper deck and

longitudinals) have been constructed. This fact is automatically taken into

account in CSR because thickness measurements data inevitably includes both

data.

22) Provide information to justify the data on the statistical analyses used to develop the

corrosion additions included in the rules.

Ans: IACS will prepare a technical background document which includes such

information.

23) We would like to hear a more comprehensive description of the statistical basis for

the corrosion allowance. Based on their statistical work and experience, we are

interested in IACS' best estimate of the expected % of steel replacement for a tanker

built to minimum CSR requirements, assuming a typical (average) level of

maintenance and a 25 year life. Does the 95% assumption translate into an estimated

5% steel replacement as a mean value?

Ans: The estimated 5% steel replacement does not imply that ships which are

normally operated and maintained will have to replace 5% of total steel. As

ship’s conditions depend on their operation and maintenance, the ship group

which is poorly maintained will have to replace steel of much larger than 5%.

Instead, another ship group which is normally maintained will not need to

replace 5% of total steel.

Tier II.11:


7 March 2007 Page 10

24) With respect to Tier II.11, Construction Quality, and based on some of the CG and

group discussion, we would appreciate a comment from IACS as to how it is

determined that a shipyard is qualified to construct a vessel to their rules.

Ans: UR Z23 contains requirements for the review of the construction facility. Also

it specifies that prior to commencing any newbuilding project, the

classification society is to discuss with the shipbuilder at a kick off meeting the

items listed in Table 1. The purpose of the meeting is to agree how the list of

specific activities shown in Table 1 is to be addressed. Further, the

classification society is required to provide evidence to prove the consistency

of its surveys (e.g. through records, check lists, inspection and test records,

etc.).


***

PP Questions to IACS during the March 2007 meeting 28-Jun-2007.doc

Page 1 of 11

Questions/Comments to IACS During Presentation on March 12

1. TSCF has been drafting a document that will establish a standard for the minimum

level of maintenance expected from class and owners. The document was established

because a consistent definition was found to be lacking throughout industry. The

document is going to print shortly and should be out sometime in the summer.

Ans: This will be an interesting document to review and we look forward to

receiving it. When IACS was formulating the CSR, we wrestled with such a

definition, but in the end simply referred to the existing survey requirements of

the individual class societies.

Tier III.2

2. Does the presentation on Environmental Conditions cover CSR or IACS UR 34?

Ans: The presentation includes some background and assumptions for the

Environmental Conditions specified in IACS UR 34, as well as the procedure

followed to derive the rule loads used in CSR using the conditions specified in

IACS UR 34. Please note that IACS UR 34 only covers the wave statistics and

how to use them, while CSR includes prescriptive load formuleas that are

derived based on these data.

3. Regarding speed for wave encounters, is it enough to say that speed is included, or

should there be some guidance to the master, for example, beyond simple ‘due

diligence’?

Ans: Speed is included in the wave encounters for the fatigue loads, but not for the

loads used for strength assessment. This approach was based on model tests

showing that full form ships are only able to maintain very small forward

speeds, even in 5-year storms. In addition, speed sensitivity studies were

carried out during the rule development, showing that the effect of speed on the

dynamic loads is small. Consequently, the load formulations in the rules are

not based on the assumption that the master voluntarily reduces the speed in

heavy weather, and it is not considered necessary to give any specific guidance

related to speed reduction.

4. What about new types of wave data?

Ans: The existing scatter diagram is based on visual observations from ships. The

advantage of this scatter diagram is that it is based on a large amount of data,

but there is some uncertainty connected to the observations. There is also some

effect of bad weather avoidance included in the data. New wave data are now

available, based on information from buoys, satellites, and wave radars.

However, all these methods have uncertainties related to them, and so far the

different data sets show large variation. Therefore, more work is needed before

a modified scatter diagram can be adopted. See slide page 25.

MSC 83/INF.5 ANNEX 4


Page 2 of 11

5. Are the effects of steep waves included?

Ans: The effect of steep waves is not included, since this is considered to be a

research topic. More informations is required regarding a physical

understanding of the problem, the probability of occurrence (statistical model),

spatial and time distribution of the wave (wave model), and a numerical load

model. See slide page 26-27.

6. Why does slide (??) show slightly more bending aft of midship than forward?

Ans: This was a question as to why the effect of the speed on the global bending

moment is larger aft of midship than forward. It is difficult to give the exact

reason for this, since the global bending moment is affected by a number of

factors. One factor is that the effect of speed on the sinkage and trim have been

accounted for when applying forward speed. This will give slightly different

buoyancy and force. Another factor is the wave heading. The vertical bending

moment may, for some vessels, be worse for following seas in the aft region of

the vessel. The results shown in the figure are envelope values, calculated as

long term values accounting for all wave headings and all sea states in the

scatter diagram for a particular vessel. A third factor is that in case of forward

speed the encounter periods are changed for encountering and following

waves, changing the "spread" of the RAO (eg "shorter", "wider"

peak). These RAO's are combined with longterm sea-data giving larger or

smaller response, depending on typical wave-length of the sea-state. Length of

vessel in addition to the encounter frequency and the sea-state will then decide

if the response increases or decreases. Therefore, it is difficult to say why the

results for this case show a larger effect in the aft part than in the fore part.

7. Does the rule envelope cover the calculations?

Ans: Yes. See slide page 24.

8. How many years does 10-8 probability of exceedance equate to: 20 or 25 years?

Ans: The probability level for the design load is 1/N, where N is number of load

cycles during the design life. The actual number of wave load cycles for a certain ship

will depend on the ship size, speed and port time. The number of wave load cycles

corresponding to a design life of 25 years is assumed constant and equal to 108.

Previously, this value was assumed to represent 20 years, but it is found to more

representative for 25 years. The difference in load magnitude between loads based on

a 25 year and a 20 year return period is small. For example; if the 10-8 probability

level relates exactly to a 25 year return period, then corresponding 20 year return

period would be given at a probability level of 10-7.9, assuming that the long-term

distribution of the load can be represented by a Weibull distribution with a shape

parameter equal to 1.0. The corresponding difference in actual load value is

approximately 1%. See slide page 33.

9. What is the most probable largest value for load probability



Page 3 of 11

Ans: The most probable largest load is the load level that is most likely to be the

largest value occuring during the design life. This corresponds to the maximum

value of the probability density distribution for the load. See slide page 34.

10. How does the 1.2 safety factor on ultimate strength match up with the load slide?

Ans: (ref. Q&A no. 9 in “PP Questions to IACS prior to….”) The partial safety

factors specified for the ultimate hull girder strength check are calibrated using

reliability analysis. The aim of the calibration is to ensure a sufficient and

consistent overall safety level for all ships, accounting for the uncertainties and

randomness related to the calculation of load and strength. For the wave

bending moment, the uncertainties accounted for are the randomness and

uncertainty in the sea state data, and uncertainties related to the wave load

prediction. For the hull girder bending capacity, the randomness and

uncertainty in the material properties and the uncertainties related to the

capacity model are accounted for.

The characteristic value of the wave bending moment is calculated as the most

probable largest value, while the characteristic value of the hull girder capacity

is based on minimum values of the material strength.

The calibration process carried out for the partial safety factors is described in

detail in Section 9.1 of the Background Documentation to the CSR.

11. Appears that exceedance on one can wipe out the safety factor on the other. There

appears to be no linkage in the rules to what’s actually happening in practice.

Ans: This statement is not correct. The safety factors have been determined using a

systematic calibration procedure, using reliability analysis to arrive at the target

safety level. This procedure accounts for the uncertainties related to both load

and strength, including the probability of exceeding the most probable largest

load value. Ref. also Q&A no. 10.

12. Is the presentation for just CSR or for other class rules as well?

Ans: This is just a general approach, but it has existed in class rules long before

CSR.

13. The discussion is about global loads. What about local loads?

Ans: In principle, safety factors for local loads are calibrated in a similar way as for

the global loads. However, the partial safety factor (PF) format is only applied

for the hull girder strength criterion. For the other criteria, the Working Stress

Design (WSD) method is applied, meaning that a single safety factor is used to

account for uncertainties related to both loads and strength.

14. How is the load from the pressure profile treated above the waterline, especially

considering the stress/load path discontinuity?

Ans: The stretching above waterline for is found by linear interpolation from where

the dynamic pressure is zero and the still waterline. The point where the



Page 4 of 11

dynamic pressure is zero is taken as Pwl/10, where Pwl is the dynamic pressure

at still waterline. Reference is made to Section 7.3 of CSR, and Section 7.3 of

the External Background documentation.

15. What are the implicit safety factors for actual sailing conditions? If acceptable for

only one area, they aren’t acceptable.

Ans: Actual sailing conditions are not considered in the calibration of the safety

factors. Since the rules are developed for unrestricted operation, continuous

operation in the North Atlantic for the entire design life is assumed. The safety

margins are set to be sufficient even for the worst possible trading.

Consequently, when considering implicit safety factors in general, actual

sailing conditions are not accounted for. In reality, however, the safety level for

each ship will depend on the trading route of the ship, and ships operating in

benign waters will obviously have a higher implicit safety margin than ships

trading in more severe environment.

Tier III.3

16. Why 0.9T?

Ans: There are empty tanks in this particular loading condition, so the ship can not

be at maximum draft. However, these are just rule cases. If there is a unique

load case that results in full draft and empty tanks, these must be analyzed as

well.

17. There does not appear to be a full load case.

Ans: It is generally not a governing condition for global FEM conditions where

checkerboard full and empty tanks are more critical.

18. Why do the calculations begin with one half of the corrosion margin used?

Ans: This will be discussed later under the net scantlings topic.

19. On slide page 44, what does ‘M’ consider? The properties of different shapes should

be considered, especially for those that are non-symmetrical. The stress increase

should be considered for local scantlings.

Ans: The factor ‘M’ is the bending moment factor considering end fixity and

relation to the hull girder bending, e.g. horizontal or vertical orientation. You

make a good point regarding stress concentrations due to non-symmetrical

sections, this factor is covered in fatigue where the symmetric arrangement of

the stiffener has a high influence, but not strength.

20. What shape is the coefficient biased towards?

Ans: Angles.

21. It should be noted that most failures are in fatigue.



Page 5 of 11

Ans: Agree, if number of failures is considered rather than possible consequences.

22. On slide page 61, has 170% of the yield stress been validated using non-linear

analysis, especially with respect to high strength steel?

Ans: Yes. Non-linear analysis was used to validate this. Additionally, application

of 170% is controlled to places such as local high stress areas and not in way of

welds. If the size of the higher stress area is controlled, the assumptions are

okay. Also, we use mild steel and H32 allowable stress levels even if H36 or

higher strength material is used.

23. On slides page 65-67, and based upon the discussion, there seems to be less

uncertainty about the response and more about the loading.

Ans: Agree. In general the loads have a higher degree of uncertainty than the

properties of the material or fabrication which are controlled during yard Q/C

and class inspections. In addition the engineering modeling of the structural

response has been well known and used for a long time.

24. How are transitions addressed in the rules to ensure continuity? As written, the rules

appear to end at the cargo block. Is that acceptable practice? With no examples or

analysis, how is it interpreted or left to the Group of Experts to decipher?

Ans: The CSR cover the whole vessel structure and include prescriptive rules

covering continuity of structure, especially at the ends of cargo block

longitudinal members extending into the end structure of the vessel. At this

point in time IACS does not require FEA at transitions. It is a good suggestion

to put examples in the rules. Some people like to have examples showing

accepted arrangements, however others feel that examples are regarding as the

only solution that that they tie their hands.

* At this point, there was a general discussion among PP members regarding whether

or not this was a problem and how it should be treated within GBS. Mr. Kim

explained how the yards handled this issue in practice.

25. Does a 50% reduction in net thickness equate to a 10% reduction in hull girder

strength?

Ans: In general yes. Based on the initial studies for tankers performed during rule

development, it’s a narrow band from about 9 to 11%.

26. If many members were reduced, but not to renewal thickness, would you still have to

renew steel if there was a 10% reduction in overall hull girder strength?

Ans: Yes. It is an IMO and class requirements.

27. Owner’s extra can skew average thicknesses and hence the calculations.

Ans: CSR does not include any owner’s extra thickness in the FEA or local strength

analysis, meaning that extra thicknesses requested on net scantlings by the

Owner above rules corrosion additions are fully available for corrosion.



Page 6 of 11

28. How has the mode for fatigue evaluation properties with respect to net scantlings

been evaluated?

Ans: Fatigue is evaluated differently than the strength calculations. For strength

calculations, we are assessing the scantlings in the worst condition permitted

during in-service thickness measurements against the design extreme loads.

Fatigue is a cumulative process that begins on the first day of delivery when

the vessel is in the as-built condition and the process ends when the vessel is in

the worst condition permitted during in-service thickness measurements. Due

the variations between these two conditions the net scantlings for fatigue

simply takes the average between the two. There are some differences between

tanker and bulk carrier CSR in this regard which IACS is in the process of

solving.

Tier III.4

29. How do you account for variations in the shape of structural members due to

corrosion along with subsequent changes to the stress path?

Ans: A coefficient has been introduced in the fatigue calculation to take corrosion

effects into account.

30. What steps have been taken since JTP to sharpen the safety factors, margins, etc?

Ans: See slide page 85. CSR is more stringent that the current standard.

31. How are you accommodating (or assessing) data from pre-CSR ships?

Ans: We are looking at it.

Tier III.5

32. What is a reasonable extent of damage?

Ans: Structural damage due to collision or damage is not considered in the rules, and

the extent of damage has therefore not been defined.

33. Regarding the slide on page 89, what are the environmental conditions?

Ans: The slide on page 89 shows examples of damage conditions that are considered

in voluntary class notations offered by some class societies. In the first

example a 3 month storm is assumed, while in the other example a one year

storm is assumed. For both cases, a safety factor of 1.0 is used.

34. Regarding flooding and global strength, it is possible to be in a more severe sea state.

Have you looked at bending in a more probabilistic manner? If not, how confident

are you that it’s already addressed?



Page 7 of 11

Ans: The global strength in the flooded condition is considered to be implicitly

covered by the rules, based on typical inherent residual strength exhibited by

existing vessels upon which the rules were calibrated.

35. On page 31 of the Demonstration Package (3rd paragraph from the bottom), it refers

to “effect of local fluid pressure …. not counted in the hull girder assessment.” What

does that mean?

Ans: Flooding of a compartment will lead to additional local pressure acting on the

structure in the compartment. In the rules, the structure is assessed considering

this local pressure resulting from flooding. However, the flooding will also

possibly lead to a change in the stillwater bending moment distribution for the

hull girder. This effect is not considered in the hull girder strength assessment.

Tier II.6

36. When JTP was first developed, coating was considered redundant, yet now it seems

to have become an integral part of the process. Therefore, it seems that it should

contribute to the scatter on the diagram.

Ans: Once the new coating requirements come into force, the scatter on the graphs

could be reduced.

37. Was the corrosion data culled?

Ans: No. All data was stored in the database and categorized according to their

corrosive environments such as type of cargo, locations, temperature etc. The

corrosion additions of DH Tanker CSR were determined by the categorized

data. As a result some data could not be utilized in the rules because some

corrosive environments do not exist within DH Tankers. For example,

thickness measurements data of pre-MARPOL ballast tanks with abrasive

cargo could not be used.

38. There seem to be few IACS data point for ships over 16 years in age.

Ans: When IACS collected data, the number of tankers over 16 years in age and

whose corrosive environments are close to those of CSR tankers were few.

Hence thickness measurement data is few.

39. Were the ships in the data set recoated during their life?

Ans: We don’t know really, we consider this in the statistical sense. However, most

ships do not recoat after 10 years.

40. After 25 years, is 5% of the steel wasted beyond 3.5 mm? Does that apply anywhere

or just in the tanks?

Ans: Yes

41. Does it vary from vessel to vessel and by type of vessel?



Page 8 of 11

Ans: Yes. These are fleet statistics.

42. On slide page 110, are the data on this slide the same as from the previous slide (100),

as they do not appear to have the same shape?

Ans: No. The figure on slide page 110 illustrates an example of thickness diminution

of hold frames of single side skin bulk carriers.

43. What happens when the new IMO coating standards are implemented? Does the

graph shift to the right?

Ans: Yes. It is likely that the start point of thickness diminution will shift to the right

because duration of intact condition can be longer than the present one in the

figure on slide page 110.

44. Once a plate loses it’s coatings, corrosion growth is exponential and does not level

out per the IACS model.

Ans: In case of general corrosion (uniform thickness diminution), one of whose

counter measures is to add “corrosion addition”, the corrosion growth is not

exponential according to the outcome of statistical analysis based on real

thickness measurement data. In case of local corrosion whose corroded area is

limited, the corrosion growth is close to exponential because in the first stage it

progresses depth-wise as illustrated in the figure on the slide page 101.

However, the counter measure of this kind of corrosion is not to add “corrosion

addition”.

45. It depends on the type of corrosion.

Ans: Yes. In order to determine thickness of corrosion addition, general corrosion,

i.e. uniform diminution/reduction over an extensive area should be taken as

explained in the slide page 109.

46. Flexing of a plate can cause flakes of corrosion to fall off, which can open up new

areas for corrosion growth.

Ans: IACS agrees with your comments. It should be noted that it is a local

phenomenon of corrosion progress. Timing of falling off of flakes varies

within an extensive area. IACS believe that this phenomenon is reflected to

real thickness measurement.

47. Please provide additional information on who conducted the referenced corrosion

study, how many measurements were considered, when was it published, provide

additional details, etc?

Ans: See slide pages 112-113. The IACS database was established in the late

1990’s. The study consistent of data from 500 ships with 600,000 data points.

The data was published by individual classification societies. The ships range

from 3 to 27 years old.



Page 9 of 11

48. 30,000 3,000 data points per ship implies not very many data points, and means a

limited data set.

Ans: IACS understands that 3, 000 comes from dividing 600,000 by 500 but

believes that the average value is not appropriate to consider the volumes of

data set. The important thing is that the data set consists of about 500 real

thickness measurements reports collected by IACS. The number of data points

of thickness measurement reports vary very much depending on ship’s size,

ship’s age and applicable rules when measurements were carried out. It should

be recalled that minimum number of thickness measurements required by rules

is increasing time to time. In addition, it should be born in mind that if ship’s

conditions are very poor and there are many suspected area, data points are to

be increased according to rules for survey and inspection. Oppositely in case

ship’s conditions are very good, data points need not to be increased.

49. The data set is not so limited, as there are more ships and data points than implied.

Ans: Yes, it is true. In addition, it should be noted that the minimum number of data

points required by present rules is much larger than those in the past. It implies

that one older data point tends to represent wider area than now.

Tier III.7

50. Would localized damage weaken a corrugated bulkhead?

Ans: It depends on the extent of damage. Small, local indents will have a marginal

effect, while larger damages will have a larger weakening effect.

51. Have classification societies agreed whether corrugated bulkheads carry shear loads?

Ans: Yes. There is a UR that covers the topic.

Tier III.10

52. In CSR, there is no requirement for the evaluation of alternate methods. Without

such a requirement, how can it be ensured that such an alternate method produces

equal results?

Ans: The requirements are written generally because of the wide variety of cases.

However, the evaluation of alternate methods is carried out based on

equivalency principle. It has to be demonstrated that the proposed alternative

method produces equivalent results as the one in the Rules.

53. There is no mention of intellectual property rights in CSR. Too much emphasis on

design transparency could negatively impact shipyards.

Ans: CSR do not address intellectual property rights. These are outside of

classification society’s responsibilities and should be regulated through



Page 10 of 11

contractual arrangements between the involved parties. Protection of these

rights should be pursued through the normal legal channels.

Tier III.11

54. Please identify an example of a substantial non-conformance? (ref slide page 132)

Ans: The surveyor identifies that the scope of the agreed NDT requirements is not

being followed.

55. What happens at that point? Increased inspections? When?

Ans: At that point, the classification society will review the situation with the yard

and agree remedial measures. Increased inspections could be required as one of

such measures.

56. What about shipyard qualification schemes?

Ans: They are covered by some individual classification societies, but not IACS.

However, UR Z23 does contain an assessment form which can be used by

IACS members in assessing the capabilities of the yard.

Tier III.12

57. It’s not clear how class adjusts manpower to meet the shipyard construction schedule.

Ans: This is one of the purposes on Table 1 of UR Z23 referenced on slide page 138

and the meeting between the yard and classification society, as specified in UR

Z23. The scope of work and the experience of the shipyard will determine the

required manpower

58. What are the requirements under CSR for testing?

Ans: Some requirements are contained within the rules. Table 1 in UR Z.23

contains many more.

Tier III.15

59. Believe that it is a class responsibility to certify the existence and position of harmful

substances on board the vessel (at least at the beginning).

Ans: This is not a responsibility of classification society. It is the responsibility of a

Recognized Organization as regulated by IMO Convention under development

at MEPC. A classification society may choose to take on the role of the

Recognized Organization under this future Convention. When it does, then it

becomes its responsibility. Some classification societies provide a service to



Page 11 of 11

their clients at their request and in accordance with IMO and Industry

Guidelines on Recycling until the Convention is adopted and enters into force

General

60. The CSR Demonstration Package describes requirements in different places. Will

IACS combine them?

Ans: Good point.

61. Should IACS URs be incorporated into Tier II?

Ans: URs are brought into the rules themselves at certain points, therefore, they are

more appropriate for Tier IV. << Please refer also to the answers to the

questions prior the meeting. >> IACS URs are proprietary documents of the

International Association of Classification Societies and do not have

application outside IACS – there is no such thing as “IACS class”. It is a

requirement of membership that the URs have to be introduced into Members’

Rules. Classification cannot be assigned to a ship based on application of a UR

(s) – only classification Rules of an individual Member can be applied to a

design.

62. The fatigue assessment for CSR tankers and bulk carriers is different. Has there been

any study to assess potential differences in outcome from the different methods?

Ans: No. The goal is to try and harmonize in the next 5 to 6 years.

63. Please describe how the use of speed for wave encounters differs for tankers and bulk

carriers.

Ans: Speed is only considered for fatigue. It is not considered for maximum wave

loading for either tankers or bulk carriers. Therefore as mentioned in the

answer to question 62, the goal is to try and harmonize this in the next 5 to 6

years.


***

MSC82 GBS Net Scantlings 1

MSC 82/5/11

Submitted by IACS

Goal-Based New Ship Construction

Standards

Tier II.2

“Net Scantlings”


1. Provide a link between the assumed reduction in strength during

newbuilding strength evaluations and the in-service gauging

assessment criteria

2. Today’s in-service gauging assessment criteria covers:

� Global strength corrosion

� General corrosion

� Local (pitting, grooving and edge) corrosion

Net Scantling - Philosophy

MSC 83/INF.5

ANNEX 5



Predicted

corrosion

in 2.5 years

(0.5 mm)

Required

Net

Thickness

Corrosion

Addition

Design

Required

Renewal

Thickness

Wastage

Allowance

In Service

Annual

Thickness

Measurements

includes link

between

newbuilding

and in-service

standards

General Corrosion – uniform thickness reduction in mm over

an extensive area.


Net Scantlings - Philosophy

Field Stresses:

Based on hull girder properties reduced by

10% ( Z net50 )

Field Stresses:

Based on gross scantling

Local corrosion:


corrosion

Local corrosion:


corrosion

General corrosion added to net scantling:

Discrete margins, in millimeters, based on

surface exposure.

General corrosion deducted from as-built:

% deduction or local simplified buckling,

whichever is less


10% (same as Z net50 ):

Z measured ≥ Z renewal = Z net50


10%:

Z measured ≥ Z renewal = 0.9 x Z gross required

Evaluations made on net scantlingEvaluations made on gross scantling

IACS proposed GBS definitionExisting in-service gauging criteria

( - corrosion deducted) ( + corrosion added)




Strength evaluation


As built

Renewal

Strength evaluation


50%

50%

Strength

Time



Strength evaluation


As built

Renewal

50%

50%

Fatigue evaluation

Local properties

Strength evaluation


50%

50%

Strength

Time






Strength evaluation


As built

Renewal

25%

25%

Fatigue evaluation

Hull girder properties

50%

50%

Fatigue evaluation

Local properties

Strength evaluation


50%

50%

Strength

Time









• Local Corrosion – local pitting, edge or groove thickness reduction.

Net Scantling – Local Corrosion




( )ownbuiltastmttt −≥ −7.0

Pitting (mm)

• Individual thickness

measurement is to meet

the lesser of the formula

• Pitting intensity less than

20%

• Pitting (Tankers)

ttm≥ t

ren− 1

ttm

measured thickness (gauged)

tren

thickness at which renewals are required based on general corrosion




Edge (mm)




• Edge (Tankers)

ttm≥ t

ren− 1

ttm


tren





Groove (mm)




• Groove (Tankers)

ttm≥ t

ren− 0.5

ttm= 6


ttm


tren



� Proposed definition of “net scantling” to use in Tier II.3:

"The net scantlings are to provide the structural strength required to sustain the design loads, assuming the structure in intact condition and are to be derived from newbuilding strength evaluations linked to in-service diminution limits as follows:

.1 diminution of the hull girder section modulus is limited to not more than ten percent (10%), corresponding global stress calculations of the hull girder and primary support members may be based on this general scantling reduction,

.2 individual plates and stiffening elements are to have sufficientstrength to sustain design loads excluding additions for corrosion,

.3 fatigue calculations account for scantling variations through the design life,

.4 highly localized pitting, grooving and edge corrosion are to be treated separately and are typically not included in the newbuilding evaluations.”

GBS Net Scantlings



MSC 82/5/11

Submitted by IACS

Net Thickness

Q / A


***

IACS INTERNATIONAL ASSOCIATION

OF CLASSIFICATION SOCIETIES 36 Broadway London, SW1H 0BH, U.K. Tel: +44 (0)20 7976 0660 Email: [email protected]

IACS Study Steel Weight Impact from Net

Scantling Definition

24 April 2007

Submitted to:

INTERNATIONAL MARITIME ORGANIZATION Maritime Safety Committee

IMO Pilot Project


MSC 83/INF.5

ANNEX 6

IACS - International Association of Classification Societies ©

All rights reserved. Except as permitted under current legislation no part of this work may be photocopied, stored in a retrieval system, published, performed in public, adapted, broadcast, transmitted, recorded or reproduced in any form or by any means, without prior permission of the copyright owner. Where IACS has granted written permission for any part of this publication to be quoted such quotation must include acknowledgment to IACS. Enquiries should be addressed to The Permanent Secretary, International Association of Classification Societies, 36 Broadway, London, SW1H 0BH Telephone: +44-(0)207 976 0660 Fax: +44-(0)207-808 11007 E-mail: [email protected] TERMS AND CONDITIONS “The International Association of Classification Societies (IACS), its Member Societies and their officers, members, employees and agents (on behalf of whom this notice is issued) shall be under no liability or responsibility in negligence or otherwise to any person in respect of any information or advice expressly or impliedly given in this document, or in respect of any inaccuracy herein or omission herefrom or in respect of any act or omission which has caused or contributed to this document being issued with the information or advice it contains (if any).Without derogating from the generality of the foregoing, neither IACS nor its Member Societies and their officers, members, employees or agents shall be liable in negligence or otherwise howsoever for any indirect or consequential loss to any person caused by or arising from any information, advice, inaccuracy or omission being given or contained herein or any act or omission causing or contributing to any such information, advice, inaccuracy or omission being given or contained herein.” Produced in April 2007 for the International Association of Classification Societies.


IACS Study: Steel Weight Impact from Net Scantling Definition 24 April 2007 Page 3

IACS Study

Steel Weight Impact from Net Scantling Definition

I. Introduction

This document has been assembled to illustrate the impact on the vessel steel

weight of the two different definitions for net scantlings proposed for use in the

IMO Goal-based Ship Construction Standards (GBS). In submittal MSC 82/5/11,

IACS pointed out that not all parties seemed to have a common understanding or

interpretation of the definition of net scantling in Tier II as currently written in

Tier II, Section II.3.

The goal of this document is to share preliminary estimates of the steel weight

impact of two different interpretations of the definition of net scantlings so that an

informed decision can be made on the way forward. This is a preliminary study

which was performed using typical tanker designs.

The outcome is, if the current wording and interpretation as contained in GBS

Tier II is used, the steel weight of tankers will generally be increased by 3.65% to

7.8% over that of the IACS proposal. This would be in addition to the general

steel weight increases as brought about by the new IACS CSRs. The percentage

increase is calculated based on the original steel weight and the associated

increases in way of the cargo block structure only. This increase will generally

have to be provided in the longitudinal deck and bottom areas as well as to all of

the primary support members. It should be noted that the increase in steel weight

will only increase the magnitude of the required net and associated gross

scantlings and will not affect the magnitude of the wastage allowances used in

service to assess thickness measurements, the wastage allowances will remain the

same between the two definitions.

II. Net Scantling Definitions

The GBS Tier II.3 contains the text “Ships should be designed with suitable

safety… to withstand, at net scantlings**, in the intact condition, the

environmental conditions anticipated for the ship’s design life and the loading

conditions appropriate for them…”.

The following are the two different proposals for the footnote (**) which is used

to define what is meant by the term “net scantlings”.

i. the current Tier II, Section II.3 indicates:

** The net scantlings should provide the structural strength required to sustain

the design loads, assuming the structure in intact condition and excluding any

addition for corrosion.

ii. IACS proposal contained in MSC 82/5/11 indicates:



** The net scantlings are to provide the structural strength required to sustain the

design loads, assuming the structure in intact condition and are to be derived

from newbuilding strength evaluations linked to in-service diminution limits

as follows:

a) diminution of the hull girder section modulus is limited to not more than

ten percent (10%), corresponding global stress calculations of the hull

girder and primary support members may be based on this general

scantling reduction,

b) individual plates and stiffening elements are to have sufficient strength to

sustain design loads excluding additions for corrosion,

c) fatigue calculations account for scantling variations through the design life,

d) highly localized pitting, grooving and edge corrosion are to be treated

separately and are typically not included in the newbuilding evaluations.

III. General Discussion

In summary the IACS proposal has adopted an approach which is believed to

realistically model the corrosion behavior and structural strength of actual ships

and which links the corrosion margin at new construction to the corrosion

allowance for ships in service.

The current wording in GBS Tier II, on the other hand, simply states that “the net

scantlings should provide the structural strength required to sustain the design

loads, assuming the structure in intact condition and excluding any addition for

corrosion.” This is essentially the same as the IACS proposal treatment of “net

scantlings” for individual structural elements (item b in the IACS proposal).

However, this “simple” definition is interpreted by some to mean that all strength

calculations, including hull girder strength and fatigue strength are to be

performed assuming that all the individual structural elements are at their net

scantlings simultaneously, from the outset, without any corrosion additions. This

interpretation ignores the reality that all structural elements do not corrode

uniformly with time, or from another point of view, requires that no account is to

be taken of the corrosion additions/margins which are built into the ship when it is

delivered. It also ignores the reality that fatigue damage and corrosion are inter-

related time dependent processes and requires that the newly built ship have a

minimum fatigue life calculated as if the corrosion additions did not exist at all. If

this interpretation is adopted, it will require additional steel weight above the

latest developed IACS Common Structural Rules (CSR), mainly at the deck, at

the bottom and the primary support members.

As noted above, one of the main differences between the two interpretations is

how the average or simultaneous corrosion is handled for the longitudinal strength

evaluation. The IACS proposal is consistent with the current IMO 10% allowable

diminution of the hull girder section modulus as per Resolution MSC.105(73) and

Resolution MSC.145(77) for tankers and bulk carriers, respectively. It should be

noted that this 10% diminution is consistent with actual vessel corrosion patterns



and rarely do actual vessels exceed the allowable 10% limit. The “simple”

definition and interpretation seeks to increase this allowable up to 20%.

IV. Method

In order to obtain a general understanding of these two definitions, and to gain

and understanding of the impact they would have on steel weight, three typical

tanker designs of varying sizes were used, representing VLCC, Aframax and

Product sizes.

The main difference between the two definitions lies in the way the global

diminutions are handled for the longitudinal strength and the primary support

members. The IACS proposal uses half of the corrosion allowance to represent

the simultaneous or average corrosion mechanism, therefore to calculate the

impact of using the “simplified” interpretation, half of the IACS margins have to

be added back into the affected structural areas. The following method was used:

1) Deck area; add 0.5tcorr to the deck area including the longitudinal plating and

attached stiffeners.

2) Bottom area; add material as per the attached table below to the longitudinal

plating and attached stiffeners.

Actual vs offered bottom

section modulus Material to be added

Zbot < 1.2 Zbot-rq 0.5tcorr

1.2 Zbot-rq < Zbot < 1.3 Zbot-rq

0.25tcorr

1.3 Zbot-rq < Zbot No addition

Zbot is the actual calculated hull girder bottom section modulus of the vessel. Zbot-rq is the required hull girder bottom section modulus.

3) Primary support members; add 0.5tcorr to all primary support members

including web frames, floors and horizontal stringers of transverse bulkheads.

4) The longitudinal extent was taken as the cargo block area of the vessel. The

midship results of the steel weight differences were simply extracted for the

whole cargo block.

V. Results

The following is a summary of the impact on steel weight.

VLCC:

The resulting total added steel weight is 1383 tonnes, or a 6.21 percent increase.

Material was added to the longitudinal members as highlighted in red in the

sketches below as well as the primary support members including web frames and

floors in accordance with the method described above.



Midship tanks

Aft tank

Forward tank

The following table lists general information for the net scantling definition for

the “simplified” interpretation in GBS and the IACS proposal. The gross

scantling, the net (renewal) thickness and the corrosion allowances for selected

major areas of the vessel are indicated.



Gross Net Corrosion Allowance

GBS IACS GBS IACS Both GBS and IACS

Longitudinal Elements

Deck plate 20.5 18.5 16.5 14.5 4.0

Side shell plate 24.0 24.0 20.5 20.5 3.5

Inner side plate 19.5 19.5 16.5 16.5 3.0

Bottom plate 19.5 19.5 16.5 16.5 3.0

Inner-bottom plate 23.0 21.0 19.0 17.0 4.0

Long. bhd. CL 19.0 19.0 16.5 16.5 2.5

Bottom girder 17.0 17.0 14.0 14.0 3.0

Long. stringer 14.0 14.0 11.0 11.0 3.0

Deck longs. (W / F) 14/22 12/20 10/18 8/16 (4.0 / 4.0)

Side shell longs. (W / F) 12.5/20 12.5/20 9.5/17 9.5/17 (3.0 / 3.0)

Inner side longs. (W / F) 12/20 12/20 9/17 9/17 (3.0 / 3.0)

Bottom longs. (W / F) 13/25 13/25 10/22 10/22 (3.0 / 3.0)

Inner-bottom longs. (W / F) 14.0/26.5 12.5/25 11/23.5 9.5/22 (3.0 / 3.0)

Long. bhd. CL longs. (W / F) 12/20 12/20 9.5/17.5 9.5/17.5 (2.5 / 2.5)

Bottom girder longs. (W / F) 11.5/16.0 11.5/16.0 8.5/13 8.5/13 3.0

Long. stringer longs. (W / F) 13 13 10 10 3.0

Transverse elements

Deck web plate 17 15 14 11 4.0

CL web plate 21.25 20 18.75 17.5 2.5

Bottom floor and side plate 19.5 18 16.5 15.0 3.0

The following table contains a summary of the steel weight calculation. For

reference, the “CSR Effect” for the longitudinal elements is included which

indicates the amount of steel weight increase that resulted from the application of

the new IACS CSR for tankers, which was 485 tonnes or 2.22%. Transverse web

and bulkhead are not updated according to the CSR rules for tankers.

The additional effect of using the “simplified” interpretation in the GBS is

calculated as 1383 tonnes or 6.21%. Note that the percentage is taken as GBS Diff

(weight)/ CSR (weight) = 1383 / (21799+485).



Total Steel Weight CSR Effect GBS Net Scantling

Tank Element As-Built (Tonnes) Difference

(Tonnes) Difference

(%) GBS Diff (Tonnes)

GBS Diff (%)

Mid-ships


Plating 2885 77 2.7% 175 5.9%

Stiffeners 1329 63 4.8% 118 8.5%

Sub-total 4214 140 3.3% 293 6.7%

Transverse Elements

Web Frame 70 3 3.8% 8 11.0%

No. of webs 8 0.0%

Sub-total 563 21 3.8% 68 11.6%

Sub-total-Midships cargo tank area 14330 485 3.4% 1083 7.3%

Aft tank


Plating 2537 -66 -2.6% 29 1.2%

Stiffeners 1203 77 6.4% 109 8.5%

Sub-total 3740 12 0.3% 137 3.7%

Fwd Tank


Plating 2544 -32 -1.3% 80 3.2%

Stiffeners 1184 21 1.8% 83 6.9%

Sub-total 3728 -11 -0.3% 163 4.4%

TOTAL 21799 485 2.2% 1383 6.21%

Aframax Tanker:


Since the bottom as-built section modulus (net) is about 10% greater than the

required section modulus (net), 0.5tcorr material was added to the bottom in

accordance with the method mentioned above. Material was added to the

longitudinal members as highlighted in red in the sketch below as well as the

primary support members including web frames, floors and transverse bulkhead

including horizontal stringers. (Stiffeners on transverse webs and bulkheads are

not included in the weight estimates.)










Deck plate 21 19 17 15 4.0

Side shell plate 16.5 16.5 13 13 3.5

Inner side plate 15 15 12 12 3.0

Bottom plate 21.5 20 18.5 17 3.0

Inner-bottom plate 19 17 15 13 4.0

Long. bhd. CL 14.5 14.5 12 12 2.5

Bottom girder 17.5 16 14.5 13 3.0

Long. stringer 13 13 10 10 3.0

Deck longs. (W / F) 13 / 18 11 / 16 9 / 14 7 / 12 (4.0 / 4.0)

Side shell longs. (W / F) 13 / 18 13 / 18 10 / 15 10 / 15 (3.0 / 3.0)

Inner side longs. (W / F) 12 / 17 12 / 17 9 / 14 9 / 14 (3.0 / 3.0)

Bottom longs. (W / F) 13.5 / 16.5 12 / 15 10.5 / 13.5 9 / 12 (3.0 / 3.0)

Inner-bottom longs. (W / F) 12.5 / 15.5 11 / 14 9.5 / 12.5 8 / 11 (3.0 / 3.0)

Long. bhd. CL longs. (W / F) 13 / 17 13 / 17 10.5 / 14.5 10.5/14.5 (2.5 / 2.5)

Bottom girder longs. (W / F) 10.5 / 15.5 9 / 14 7.5 / 12.5 6 / 11 3.0

Long. stringer longs. (W / F) 12 / 12 12 / 12 9 / 9 9 / 9 3.0

Transverse elements

Bulkhead near deck 15 13 11 9 4.0

Bulkhead elsewhere 16.75 15.5 14.25 13 2.5

Bulkhead stringer (W / F) 14.5/31.5 13/30 11.5/28.5 10/27 3.0

Deck web plate (W / F) 14.5 / 27 12.5 / 25 10.5 / 23 8.5 / 21 4.0

CL web plate (W / F) 13.75/31.75 12.5/30 11.25/28.25 10/26.5 (2.5 / 3.5)

Bottom floor and side plate 14.5 13 11.5 10 3.0




reference, the “CSR Effect” for the longitudinal elements is included which

indicates the amount of steel weight increase that resulted from the application of

the new IACS CSR for tankers, which was 359 tonnes or 3.60%. Transverse web

and bulkhead are not updated according to the CSR rules for tankers.




Total Steel Weight

CSR Effect GBS Net Scanting Definition Effect

As-Built (Tonnes)

Difference (Tonnes)

Difference ( % )

GBS Diff (Tonnes)

GBS Diff (%)


Plating 4994 174 3.49% 344 6.66%

Long'l bhd 421 0 0.00% 9 2.05%

Stiffeners 1943 185 9.54% 166 7.81%

Sub-total 7358 359 4.89% 519 6.73%

Transverse Elements

Bulkhead incl. hor. stringers 1000 0 0.00% 106 10.57%

Web Frame 1616 0 0.00% 181 11.20%

Sub-total 2615 0 0.00% 287 10.96%

TOTAL 9974 359 3.60% 806 7.80%

Product Tanker:


Since the bottom as-built section modulus (net) is about 50% greater than the

required section modulus (net), no material was added to the bottom in

accordance with the method mentioned above. Therefore material was added to

the longitudinal members as highlighted in red in the sketch below as well as the

primary support members, e.g. the floors and webs.









Deck plate 15.5 13.5 11.5 9.5 4.0

Side shell plate 13.0 13.0 9.5 9.5 3.5

Inner-hull plate 13.0 13.0 9.0 9.0 4.0

Bottom plate 16.0 16.0 13.0 13.0 3.0

Inner-bottom plate 17.5 17.5 13.5 13.5 4.0

Deck longs. (W / F) 13 / 18 11 / 16 9 / 14 7 / 12 (4.0 / 4.0)

Side shell longs. (W / F) 10 / 16 10 / 16 7 / 13 7 / 13 (3.0 / 3.0)

Inner-hull longs. (W / F) 10 / 15 10 / 15 7 / 12 7 / 12 (3.0 / 3.0)

Bottom longs. (W / F) 11.5 / 16 11.5 / 16 8.5 / 13 8.5 / 13 (3.0 / 3.0)

Inner-bottom longs. (W / F) 12 / 17 12 / 17 9 / 14 9 / 14 (3.0 / 3.0)

Deck transverse web plate 13.5 12.0 11.0 9.5 2.5

Side transverse plate 12.5 11.0 9.5 8.0 3.0

Bottom transverse floor plate 12.5 11.0 9.5 8.0 3.0


reference, the “CSR Effect” is included which indicates the amount of steel

weight increase that resulted from the application of the new IACS CSR for

tankers, which was 258 tonnes or 5.94%. The result of using the IACS proposal

in included in these values. The additional effect of using the “simplified”

interpretation in the GBS is calculated as 168 tonnes or 3.65%..






Total Steel Weight

CSR Effect GBS Net Scanting Definition Effect

As-Built (Tonnes)

Difference (Tonnes)

Difference ( % )

GBS Diff (Tonnes)

GBS Diff (%)


Plating 2345 103 4.41% 80 3.27%

Long'l bhd 259 0 0.00% 0 0.00%

Stiffeners 711 134 18.85% 29 3.43%

Sub-total 3315 237 7.16% 109 3.07%

Transverse Elements

Bulkhead 583 20 3.43% 0 0.00%

Web Frame 449 1 0.22% 59 13.11%

Sub-total 1032 21 2.03% 59 5.60%

TOTAL 4347 258 5.94% 168 3.65%

VI. Conclusions

If the “simplified” definition and interpretation for net scantling is used, which

calls for all strength calculations including hull girder strength assuming full

simultaneous corrosion of the structure, it would add steel weight to the structure

mainly at the deck and bottom areas and the primary support members. The

outcome is the steel weight of tankers will generally be increased by 3.65% to

7.8% over that of the IACS proposal. This increase in steel weight will only

increase the magnitude of the required net and associated gross scantlings and will

not affect the magnitude of the wastage allowances used in service to assess

thickness measurements, the wastage allowances will remain the same between

the two definitions.

The main concern is that the proposed “simplified” definition and interpretation

of net scantling is not technically justified and does not reflect the actual

corrosion mechanisms seen in service. Also the “simplified” definition and

interpretation may not be shared by the majority of the Industry.


***

IACS INTERNATIONAL ASSOCIATION

OF CLASSIFICATION SOCIETIES 36 Broadway London, SW1H 0BH, U.K. Tel: +44 (0)20 7976 0660 Email: [email protected]

IACS Study Impact of Applying the CSR

Corrosion Addition on the Hull Girder Section Modulus

3 June 2007

Submitted to:

INTERNATIONAL MARITIME ORGANIZATION Maritime Safety Committee

IMO Pilot Project


MSC 83/INF.5

ANNEX 7

IACS - International Association of Classification Societies ©

All rights reserved. Except as permitted under current legislation no part of this work may be photocopied, stored in a retrieval system, published, performed in public, adapted, broadcast, transmitted, recorded or reproduced in any form or by any means, without prior permission of the copyright owner. Where IACS has granted written permission for any part of this publication to be quoted such quotation must include acknowledgment to IACS. Enquiries should be addressed to The Permanent Secretary, International Association of Classification Societies, 36 Broadway, London, SW1H 0BH Telephone: +44-(0)207 976 0660 Fax: +44-(0)207-808 11007 E-mail: [email protected] TERMS AND CONDITIONS “The International Association of Classification Societies (IACS), its Member Societies and their officers, members, employees and agents (on behalf of whom this notice is issued) shall be under no liability or responsibility in negligence or otherwise to any person in respect of any information or advice expressly or impliedly given in this document, or in respect of any inaccuracy herein or omission herefrom or in respect of any act or omission which has caused or contributed to this document being issued with the information or advice it contains (if any).Without derogating from the generality of the foregoing, neither IACS nor its Member Societies and their officers, members, employees or agents shall be liable in negligence or otherwise howsoever for any indirect or consequential loss to any person caused by or arising from any information, advice, inaccuracy or omission being given or contained herein or any act or omission causing or contributing to any such information, advice, inaccuracy or omission being given or contained herein.” Produced in June 2007 for the International Association of Classification Societies.


IACS Study: Impact of Applying CSR Corrosion Addition on the Hull Girder Section Modulus 3 June 2007 Page 3

IACS Study

Impact of Applying the CSR Corrosion

Addition on the Hull Girder Section Modulus

I.

As a follow-up to the IACS documentation package dated 16 february 2007 and the

discussions on the Net Scantling definition, the IMO Pilot Panel asked IACS to report

on the actual effect on the hull girder section modulus (SM) of uniformly deducting

half of the corrosion addition from the longitudinal members as per the Common

Structural Rules for Tankers and Bulk Carriers. For further information please refer to

CSR Section 6/3.3.2 (0.5tcorr) for Tankers and Chapter 3, Section 3.2.1 (0.5tc) for bulk

carriers.

II.

Deducting half of the corrosion addition simultaneously from all the longitudinal

elements is used to represent the overall accumulation of corrosion to approximately

represent the point that the hull girder property reduction is similar to the existing 10

percent allowable degradation that is used during thickness measurement assessments.

For existing thickness measurement assessment, individual members are locally

permitted to waste to higher local levels, but the aggregate reduction of the hull girder

must not be more than a 10 percent degradation of the hull girder section modulus.

Therefore, as can be seen, there are two separate wastage criteria; global and local.

This report summarizes the effects on the global properties only.

III.

Another point associated with the CSR that should be noted with regard to the hull

girder properties is as follows, using mild steel levels for simplicity:

(a) the allowable longitudinal stress for gross scantlings used in the IACS pre-CSR

rules was 175 N/mm2. This was used in association with knowing that the hull girder

SM could reduce by 10 percent in service. In that case the associated allowable stress

in the corroded condition is 175 / 0.9 = 194 N/mm2.

(b) in the current CSR rules all members are simultaneously reduced using half the

corrosion addition as mentioned above, which results in degradation similar to the 10

percent reduction. Since actual corrosion could occur in millions of different patterns,

the 0.5 simultaneous reduction was used as a design representation. The CSR

requirements reflect hull girder net scantlings, therefore an allowable stress associated

with net scantlings was used, 175 / 0.9 = 194 but then rounded down to use an

allowable stress of 190 N/mm2 to be on the conservative side since there would be a

spread in the actual impact on the SM by deducting 0.5 of the corrosion allowance.

IV.

The following tables include results for representative tankers and bulk carriers for a

range of vessel sizes.



Tankers

Type Lbp (m)

Breadth (m)

Depth (m)

SM deck reduction

(%)

SM btm reduction

(%)

Tanker 175.00 32.00 17.95 13.1% 10.7%

Tanker 175.00 40.00 17.90 12.6% 10.9%

Tanker 176.00 32.20 17.20 12.1% 10.8%

Tanker 179.54 32.20 18.30 12.6% 10.1%

Tanker 180.00 27.40 16.80 13.8% 11.8%

Tanker 200.20 32.20 17.35 9.2% 9.8%

Tanker 226.01 42.00 21.30 9.7% 9.1%

Tanker 234.00 42.00 21.00 12.0% 10.5%

Tanker 236.00 42.00 21.00 11.6% 10.4%

Tanker 244.00 46.00 22.20 9.4% 9.5%

Tanker 251.50 42.50 21.00 10.6% 9.5%

Tanker 256.50 42.50 22.40 9.4% 9.0%

Tanker 264.00 48.00 23.70 11.4% 10.1%

Tanker 264.00 48.00 24.00 11.3% 9.6%

Tanker 316.00 60.00 29.70 11.0% 9.5%

Tanker 320.00 58.00 31.00 10.1% 8.7%

Tanker 320.00 70.00 25.60 9.2% 8.1%

Ave. 11.1% 9.9%

Tankers

0.0%

2.0%

4.0%

6.0%

8.0%

10.0%

12.0%

14.0%

16.0%

150.00 200.00 250.00 300.00 350.00

LBP (m)

SM Reduction (%)

SM deck (%)

SM bottom (%)



Bulk Carriers

Type Lbp (m)

Breadth (m)

Depth (m)

SM deck reduction

(%)

SM btm reduction

(%)

Bulk Carrier 163.60 27.00 14.20 8.5% 10.7%

Bulk Carrier 170.00 28.00 14.00 9.3% 10.3%

Bulk Carrier 182.00 32.26 18.00 10.6% 10.8%

Bulk Carrier 183.25 32.26 17.50 9.0% 9.9%

Bulk Carrier 185.00 32.26 18.10 9.9% 10.0%

Bulk Carrier 215.86 32.26 20.05 11.6% 11.2%

Bulk Carrier 217.00 32.26 18.30 9.4% 10.6%

Bulk Carrier 220.00 32.26 19.39 11.0% 11.0%

Bulk Carrier 222.00 32.26 20.00 9.0% 10.1%

Bulk Carrier 222.00 32.26 20.00 10.1% 10.2%

Bulk Carrier 222.00 32.26 20.10 11.4% 11.6%

Bulk Carrier 222.00 38.00 20.70 10.0% 10.6%

Bulk Carrier 260.00 43.00 23.90 8.8% 9.5%

Bulk Carrier 278.00 44.98 24.00 8.3% 9.5%

Bulk Carrier 280.00 45.00 24.70 8.0% 9.9%

Bulk Carrier 281.50 45.00 24.10 7.1% 8.5%

Bulk Carrier 288.00 45.00 24.70 7.6% 8.9%

Ave. 9.4% 10.2%

Bulk Carriers

0.0%

2.0%

4.0%

6.0%

8.0%

10.0%

12.0%

14.0%

150.00 200.00 250.00 300.00 350.00

LBP (m)

SM Reduction (%)

SM deck (%)

SM bottom (%)



V.

Please note the following:

1) The percentages shown are the reduction of the section modulus deck and bottom

due to simultaneous reduction of 0.5 of the individual corrosion additions as

included in the CSR for tankers and bulk carriers, respectively. The percentage is

calculated as follows:

Percent = 100 x (SM gross – SM net) / SM gross

2) Uniformly deducting 0.5 of the corrosion addition from the longitudinal members

affects the global sectional properties in different and opposite ways for the two

ship types. For tankers it causes the neutral axis to be lower and for bulk carriers to

be higher, so the total effect (on the section modulus at deck) is a larger percentage

reduction for tankers because both the inertia and the height of the neutral axis are

reduced. Therefore, in general, the effect on bulk carriers will be less than on

tankers. This is inevitable given that the distribution of longitudinal material is

different for the two ship types.

2) Bulk carriers have lesser deck width than tankers since the former have hatch

openings. Accordingly it is required to have greater thickness in deck plate of bulk

carriers than tankers. Further, sometimes the deck plate thickness is determined by

the hull girder bending moment in flooded condition for bulk carriers and there is

not a similar hull girder strength requirement in flooded condition for tankers.

Thus deck plate thickness is generally greater in bulk carriers compared with

tankers. Since the deck corrosion addition is mostly 4.0 mm both for bulk carriers

and tankers, therefore it may be natural that the percent reduction of section

modulus at deck is generally smaller for bulk carriers than for tankers.

3) In general the gross offered bottom shell plate thickness of bulk carriers is similar

to or slightly smaller than that of tankers having similar vessel length. The

corrosion addition is mostly 3mm for both bulk carriers and tankers, therefore it

may be natural that the percent reduction of section modulus at bottom is generally

equal to or greater for bulk carriers than for tankers.


___________

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