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GOAL-BASED NEW SHIP CONSTRUCTION STANDARDS

General principles for structural standards

MSC 80/6/6

Rolf Skjong, DrRolf.Skjong@dnv.com

IMO MSC 80, Lunch Presentation May 12th, 2005

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Goal Based Construction Standards

IMO work-plan: .1.1 methodology;.1.2 establishment of definitions

[common terminology];.2 the exploration of the possibility of a

linkage between FSA and GBS;

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Why are GBR risk based?

Answers:Because IMO is regulating risks (as opposed to e.g. commercial issues)Because what is common to collision, grounding, fire, equipment failures, human error, buckling, fatigue etc is that the events can be associated with probabilities and consequencesBecause risk is the only known concept with such general applicability Because risk is the yardstick in all safety work

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Goal Based Standard - IMO

IMO goal-based standards are (bullet 2/Def):

• the required level to be achieved by the requirements applied by class societies and other recognized organizations, Administrations and IMO;The discussion is about a META-STANDARDA STANDARD FOR STANDARDS (RULES)

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Goal Based Construction Standards

The basic theory for GBCS was developed in the period 1973 –1993.The theory and methods are normally referred to as Structural Reliability Analysis (SRA)The theory is probabilistic or risk basedSRA may be applied for

Probabilistic or Risk Based Design (develop basis for design)Calibration or Optimisation of Design Codes (Rules)

In this tradition the following is available:Theory with textbooksA very large number of well documented applicationsGeneral purpose and special purpose softwareStandard TerminologyStandard use of mathematical Symbols

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Goal Based Construction Standards

Example of existing such META STANDARDS:

EN 1990 ‘EUROCODE - BASIS FOR STRUCTURAL DESIGN’

ISO 2394 ‘General principles on reliability for structures’

Public Reading on the Internet – all basic methods explained:

http://research.dnv.com/skj/OffGuide/SRAatHOME.pdf

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Books on SRA

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Goal Based Construction Standards

The method is therefore not FSA, but the more specialised SRA methodology

Commonalities FSA and SRA:

Both are risk based or probabilistic

Both may be used as basis for design (Risk Based Design and Probabilistic Design)

Both are used for developing Rules (FSA and Code Calibration/Optimisation)

Both require use of risk acceptance criteria

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INTRODUCTION

The traditional presentation of the SRA problem is to state that a structure is safe if the load (S) does not exceed the resistance (R)g(x) = R - S g(x) ≤ 0 constitute failure. g(x) is the limit state function

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INTRODUCTION

Some analytical results exists for simple cases, for example for linear limit states and normal variables

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SRA General Description

In practice SRA rely on the use of software, as this is necessary for nonlinear limit states and non-normal random variables in an n-dimensional space (u-space of random variables)The software performs a mapping of the problem into a space (x-space) that preserve the probabilistic properties (the Normal Space), through the Rosenblatt TransformationThe challenge relates to estimating small probabilities (standard Monte Carlo not realistic for realistic limit states)

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Probability by FORM

PFORM=Φ(-βFORM)

β=⏐u*⏐

u1

u2

FORM

u*

u-space

safe set

failure set

PFORM

= Probability of linear approximation of the failure set, linearization at the design point

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Goal Based Construction Standards

A partial safety factor format – deterministic rule can be directly extracted from the FORM analysis by using the design point coordinate as design valuesFor a specific design the results are therefore written in the form of a standard rule (For a user there is no difference) For Rules with wider scope (environmental conditions, dimensions, materials, load ranges, etc.) the Code calibration may be formulated as an optimisation problemIn this case the designs will vary slightly in structural reliabilityThe optimisation consists of selecting the PSF that gives the lowest scatter around the target reliabilitySoftware is available (Since 1990)

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Goal Based Construction Standards

Characteristic LoadDesign LoadDesign Load = Characteristic Load ● γCharacteristic Value (Material)Design Value (Material)Design Value (Material) = Characteristic Value (Material) / γ

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Fatigue – Risk Based• SN-curve for new component )(log)(log)(log SmKN ⋅−=

• Experience from similar components :

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Deterministic vs. Probabilistic Approach

Critical crack size

Initi

al d

efec

t siz

e Deterministic crack growth

Probabilistic crack growth

Time until critical crack

Time

Cra

ck si

zea

2c

MP

t

P

M t

2c

a crack

Life time

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Fatigue Analysis MethodsS-N Fatigue Assessment

S S

Structural detailweld

PotentialCrack growth

• S-N data - experimental data

S-N DATA

1.80

1.90

2.00

2.10

2.20

2.30

2.40

2.50

2.60

2.70

2.80

4.5 5.0 5.5 6.0 6.5 7.0 7.5Log(N)

Log(

S)

S-N data

S-N DATA & S-N Curve

1.80

1.90

2.00

2.10

2.20

2.30

2.40

2.50

2.60

2.70

2.80

4.5 5.0 5.5 6.0 6.5 7.0 7.5Log(N)

Log(

S)

S-N data

S-N curve - Fit

S-N DATA & S-N Curve

1.80

1.90

2.00

2.10

2.20

2.30

2.40

2.50

2.60

2.70

2.80

4.5 5.0 5.5 6.0 6.5 7.0 7.5Log(N)

Log(

S)

S-N dataS-N curve - FitDesign S-N curve (mean-2*StD)

µLogN

µLogN − 2 σLogN

Log(N) = Log(ā) - m · Log S - 2 σLogN

• S-N Design Curve:

• S-N Mean Curve:Log(N) = Log(ā) - m · Log(S)

N = ā · S-m

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Integrated Fatigue analysis -Uncertainties

Environmental description

Load And Response analysis

Detailed Stress analysis (SCF)

Fatigue analysis

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Specify Design Life?

This should be specified by IMO

This should beS

pecified by owner

Design life

Maintenance free Ship?

Minimum acceptable: Annual ProbabilitiesThis applies to all limit states!

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High Level Goal – Tradition (NKB/DNV/ISO/CEN)

Calibration against well established codes that are judged acceptable or best practices for the same type of structuresCalibration against well established codes that are judged acceptable or best practices for similar type of structuresCalibration against tabulated values, using distribution assumptions that are judged to be (slightly) conservative

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PF = 10-6, βT = 4.75PF = 10-5, βT = 4.26III - No warning before the occurrence of failure in a non-redundant structure

PF = 10-5, βT = 4.26PF = 10-4, βT = 3.71II - Significant warning before the occurrence of failure in a non-redundant structure

PF = 10-4, βT = 3.71PF = 10-3, βT = 3.09I- Redundant Structure

SeriousLess serious

Consequence of FailureClass of Failure

Table 1: Annual Target Probabilities (and Target βT) from DNV Classification Note 30.6

High Level Goal – Tradition (NKB/DNV/ISO/CEN)

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CONCLUSIONS

Experience with GBS in other industries is worth studyingConcepts like fatigue life and design life are risk based – and have always been soFor structures the methodologies are available and ready for use – mentioned in a number of submissions to MSC 79The relation between ‘goal based’(performance based) and ‘risk based’ is as between style and method