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Page 15

By Nagaraj.S.Anavekar

INTRODUCTION TO OFFSHORE STRUCTURES & DESIGN

(FOR OIL & GAS - EXPLORATION & PRODUCTION)

1.0 TYPES OF OFFSHORE STUCTURS

FIXED OFFSHORE STRUCTURES

a) Jacket or Template structures ( up to 300m)

b) Compliant Towers ( up to 600m)

c) Concrete gravity Platforms ( up to 100m)

d) Jack-up Rigs ( float during towing) ( up to 80m )

FLOATING OFFSHORE STRUCTURES (up to 2500m)

a) Tension Leg Platforms (TLP)

b) Semi-submersibles rigs (FPU)

c) Spar platforms (SPAR)

d) Ship shaped (FPSO)

2.0 OIL & GAS PROD. PROCESS OVERVIEW

3.0 DESIGN OF FIXED OFFSORE STRUCTURS

3.1 DESIGN CODES (Related to all types of offshore structures )

a) AMERICAN PETROLEUM INSTITUTE

1. API RP2A-WSD (21st edition) Recommended practice for Planning, Designing and constructing Fixed Offshore Platforms-Working Stress Design.

2. API RP2A-LRFD (21st edition) Recommended practice for Planning, Designing and constructing Fixed Offshore Platforms-Load &Resistance Factor Design.

3. API RP2L (4th edition) - Recommended practice for Planning, Designing and constructing Heliports for Fixed Offshore Platforms

b) AISC WSD 9th Edition.

Manual of steel construction Allowable stress design (AISC American Institute of steel construction, Inc. Specification for Structural steel buildings, Chicago, Illinois-USA)

c) CAP 437 (Civil Aviation Publication 437)

Offshore Helicopter Landing Areas- Guidance on Standards - Published by Safety Regulation group of CIVIL AVIATION AUTHORITY, United Kingdom (UK).

d) DNV Det Norske Veritas (Norway)

Following are the DNV offshore codes:

Recommended Practices

1DNV RP C2012002Buckling Strength of Plates

2DNV RP C2022002Buckling Strength of Shells

3DNV RP C2032001Fatigue Strength Analysis of Offshore Steel Structures

4DNV RP C2042004Design Against Accidental Loads

5DNV RP B4012005Cathodic Protection Design

Offshore Service Specifications

1DNV OSS 1012001 Rules for Classification of Offshore Drilling and Support units

2DNV OSS 1022001 Rules for Classification of Floating Production and Storage Units

3DNV OSS 103 Draft 2001 Rules for Classification of LNG/LPG Floating Production

and Storage Units

Offshore Standards

1DNV OS C1012000Design of Offshore Structures, General

2DNV OS C1022000Structural Design of Offshore Ships

3DNV OS C1032000Structural Design of Column-stabilised Units (LRFD Method)

4DNV OS C1042001Structural design of self-elevating Units (LRFD-method)

5DNV OS C1052001Structural Design of Tension-leg units (LRFD Method)

6DNV OS C1062001Structural Design of Deep-draught Units (LRFD Method)

7DNV OS C2012002Structural Design of Offshore Units (WSD method)

8DNV OS C4012001Fabrication of Offshore Structures

9DNV OS E4012001Helicopter Decks

Classification Notes

1Note No. 30-1July 1995Buckling Strength Analysis

2Note No. 30-2Aug 1984Fatigue Strength Analysis for Mobile Offshore Units

3Note No. 30-3Oct 1987Spherical Shells subjected to Compressive Stresses

4Note No. 30-4Feb 1992Foundations

5Note No. 30-5Mar 2000Environmental Conditions and Environmental Loads

7Note No. 30-7June 2001Fatigue Assessment of Ship Structures

9Note No. 31-1June 1999Strength Analysis of Hull Structures in Bulk carriers

10Note No. 31-2May 1980Strength Analysis of Hull Structures in Roll on / Roll off Ships

11Note No. 31-3Jan 1999 Strength Analysis of Hull Structures in Tankers

12Note No. 31-4Sep 1987Column stabilised Units (Semi submersible Platforms)

13Note No. 31-5Feb 1992Strength Analysis of Main Structures of Self Elevating Units

14Note No. 20-2Feb 1990Ships - Inclining Test & Lightweight Survey

e) British codes (BSI) ( Mainly for British offshore sector).

1. BSI BS EN ISO 19900 01/01/2002 - Petroleum and natural gas industries General requirements for offshore structures-

2. BSI BS EN ISO 19901-1 01/25/2006 Petroleum and natural gas industries Specific requirements for offshore structures Part 1: Metocean design and operating considerations.

3. BSI BS EN ISO 19901-2 04/07/2005 Petroleum and natural gas industries - Specific requirements for offshore structures - Part 2: Seismic design procedures and criteria.

4. BSI BS EN ISO 19901-4 08/26/2003 Petroleum and natural gas industries Specific requirements for offshore structures Part 4: Geotechnical and foundation design considerations.

5. BSI BS EN ISO 19901-5 08/11/2003 Petroleum and natural gas industries Specific requirements for offshore structures Part 5: Weight control during engineering and construction.

6. BSI BS EN ISO 19901-7 01/23/2006 Petroleum and natural gas industries Specific requirements for offshore structures Part 7: Station keeping systems for floating offshore structures and mobile offshore units-AMD 16988: March 30, 2007.

7. BSI BS EN ISO 19902 01/31/2008 Petroleum and natural gas industries Fixed steel offshore structures ACTV-CURR

8. BSI BS EN ISO 19903 01/31/2007 Petroleum and natural gas industries Fixed concrete offshore structures-AMD 16926: March 30, 2007.

9. BSI BS EN ISO 19904-1 12/29/2006 Petroleum and natural gas industries Floating offshore structures Part 1: Monohulls, semi-submersibles and spars-AMD 16927: March 30, 2007.

3.2 TYPES OF ANALYSIS & DESIGN REQIURED

INPLACE CONDITION

a) Global In-place analysis (Operating & Extreme storm case). (3.2.1)

b) Corrosion protection (anode design for jacket structure/piles) (3.2.2)

c) Miscellaneous member local analysis. (3.2.3)

d) Fatigue analysis. (3.2.4)

e) Pushover or collapse analysis (Intact & Damage). (3.2.5)

f) Earthquake (Seismic) analysis. ( 3.2.6 )

INSTALLATION CONDITION

a) Lift analysis

b) Load-out analysis (including set-down)

c) Jacket launching & upending analysis

d) Barge transportation analysis.

3.2.1 GLOBAL INPLACE ANALYSIS (OPERATING & EXTREME STORM)

The purpose of this analysis is to check jacket & Deck structure strength adequacy for operating and extreme storm waves. The SACS analysis & design includes following steps.

a) Structural modelling of jacket & Deck members.

b) Specifying LDOPT, OPTIONS, LCSEL, Section name, Group name (Ky,Kz) etc.

c) Specifying support conditions at mud-line (Pile head , fixed, pinned etc)

d) Specifying wind areas / drag areas, wind shielding etc.

e) Specifying Drag coefficient (Cd) & Inertia coefficient (Cm).

f) Specifying Marine growth

g) Specifying Group/Member overrides

h) Specifying Dummy members ( Non structural members - Risers, Riser guards, Boat landing, Fenders etc)

i) Gravity Loading (Basic or primary ):

i) Generated self weight with buoyancy

ii) Generated self weight no buoyancy

iii) Jacket non Generated loads (pile sleeves, Anodes, Boat landing /Fender walkway loads, Pile grout etc.)

iv) Non modelled dead load of Deck ( Stair, ladder, handrail, non modelled members, joint/node stiffeners, Safety net & plating of helideck ,

v) Riser content weight

vi) Conductor content weight.

vii) Bridge loads (on Deck)

viii) Bridge Friction loads (on Deck)

ix) Equipment Operating loads (on Deck)

x) Piping operating loads (on Deck)

xi) Wireline rig operating loads (on Deck)

xii) Open area live loads (define separate load case for each deck).

xiii) Open area loads on Helideck (Storage).

j) Environmental loads :

i) Wave loads. (act from direction as per environmental survey)

ii) Current loads. (act to direction as per environmental survey)

iii) Wind loads. (act from direction as per environmental survey)

Operating & Extreme environmental loads to be applied in 8 or 12 directions depending on shape of the Jacket.

k) Load combinations (sample case , rectangular jacket ) :

201) Total Gravity load with contingency

202) Total Deck load

203) Total Live load on Decks

204) Total Bridge load

301) 201+203+204+ (Operating Wave + Current + Wind) 0 deg , Min W.D

to

308) 201+203+204+ (Operating Wave + Current + Wind) 330 deg , Min W.D

311) 201+203+204+ (Operating Wave + Current + Wind) 0 deg , Max W.D

to

318) 201+203+204+ (Operating Wave + Current + Wind) 330 deg , Max W.D

401) 201+203+204+ (Extreme Wave + Current + Wind) 0 deg , Min W.D

to

408) 201+203+204+ (Extreme Wave + Current + Wind) 330 deg , Min W.D

411) 201+203+204+ (Extreme Wave + Current + Wind) 0 deg , Max W.D

to

418) 201+203+204+ (Extreme Wave + Current + Wind) 330 deg , Max W.D

l) Platform pile foundation properties: Pile structural & soil properties are specified in a separate file and is called PSI input file in SACS. Pile Structure Interaction (PSI) module of SACS analyze the behaviour of a pile supported structure subject to one or more static load conditions. Finite deflection of the pile (P-Delta effect) and non-linear soil behaviour both along and transverse to the pile axis are accounted for. The program uses a finite difference solution to solve the pile model which is represented by a beam column on non-linear elastic foundation. The structure resting on the pile is represented as a linear elastic model.

PSI first obtains the pile axial solution, then uses the resulting internal axial forces to obtain the lateral solution of the piles. In general, soil exhibits non-linear behaviour for both axial and transverse loads, therefore an iterative procedure is used to find the pile influence on the deflection of the structure.

Features of PSI module are

i) Tubular & H pile cross sections with variable properties.

ii) Soil axial behaviour may be represented by adhesion data, non-linear T-Z data, or as a linear spring.

iii) End bearing effects may be accounted for. Represented by Q-z curves.

iv) Soil lateral behaviour represented by non-linear P-Y curves.

v) Basic soil properties may be used to generate the soil axial properties in the form of T-Z curves or adhesion data, end bearing T-Z data and/ or lateral soil properties in the form of P-Y curves, based on API-RP2A recommendations.

vi) Creates up to two equivalent linearized foundation super-elements to be used by dynamic analysis in lieu of pile stub (equivalent pile stub that yields the same deflections and rotations as the soil/pile system).

vii) Creates foundation solution file containing pile stresses to be used for fatigue analysis.

3.2.1.1 UNDERSTANDING CALCULATION OF WAVE & CURRENT FORCES

Wave load on structure is computed based on following methods;

1) Morrison equation if L/D >5 ( L- wave length , D-member dia.)

2) Diffraction theory if L/D 0.10, earthquake analysis shall be performed as per API RP 2A Cl. 2.3.6d.

Accordingly, the response spectrum analysis is carried out using API RP 2A-WSD response spectrum based on soil type. This response spectrum is applied equally along both principal horizontal axes of the platform and 50% of horizontal ground accelerations applied in the vertical direction simultaneously. An overall damping ratio of 5% is generally considered.

Response spectrum analysis is carried out using SACS-DYNAMIC RESPONSE module. The CQC (Combined Quadratic Combination) method has been used for combining the responses from the different modes in each direction while SRSS (Square Root of the Sum of the Squares) method has been used for combining the responses from different directions as per API RP 2A-WSD.

Combination of static & Earthquake results:

The results of response spectrum analysis needs to be combined with the results of static analysis assuming the earthquake axial stresses are either tensile or compressive using COMBINE module. In either case, all other stresses are assumed to have the same sign as the corresponding static stresses.

EMBED Excel.Sheet.8

Pile Foundation

PILE

SEASTATE

single load case

Super element

ANALYSIS

ANALYSIS

generation

generation

(zk95-sac-dyn.inp)

(For center of damage wave)

DYNAMIC

vibration modes

SEASTATE

transfer functions

ANALYSIS

calculation

ANALYSIS

calculation

(dyninp.inp)

(wave frequencies

dynmas

to be investigated)

dynmod

(zk95-sac-stf.inp)

WAVE

RESPONSE

ANALYSES

FATIGUE

( zk95-ftginp )

ANALYSES

ftglst

( saccsf.tf1 to saccsf.tf8 )

zk95-wrs-315.inp)

8 analyses for

8 directions

wvrinp.csp

(zk95-wrs-0.inp to

_1178625719.xls

Sheet1Pile FoundationPILESEASTATEsingle load caseSuper elementANALYSISANALYSISgenerationgeneration(zk95-sac-dyn.inp)(For center of damage wave)DYNAMICvibration modesSEASTATEtransfer functionsANALYSIScalculationANALYSIScalculation(dyninp.inp)(wave frequenciesdynmasto be investigated)dynmod(zk95-sac-stf.inp)wvrinp.cspWAVE(zk95-wrs-0.inp toRESPONSE8 analyses forzk95-wrs-315.inp)ANALYSES8 directions( saccsf.tf1 to saccsf.tf8 )FATIGUE( zk95-ftginp )ANALYSESftglst

Sheet2

Sheet3