GOAL-BASED NEW SHIP CONSTRUCTION STANDARDS General principles for structural standards MSC 80/6/6

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Stewart Chambers
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1 GOAL-BASED NEW SHIP CONSTRUCTION STANDARDS General principles for structural standards MSC 80/6/6 Rolf Skjong, Dr IMO MSC 80, Lunch Presentation May 12th,

2 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; 2

3 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 consequences Because risk is the only known concept with such general applicability Because risk is the yardstick in all safety work 3

4 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- STANDARD A STANDARD FOR STANDARDS (RULES) 4

5 Goal Based Construction Standards The basic theory for GBCS was developed in the period The theory and methods are normally referred to as Structural Reliability Analysis (SRA) The theory is probabilistic or risk based SRA 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 textbooks A very large number of well documented applications General purpose and special purpose software Standard Terminology Standard use of mathematical Symbols 5

6 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: 6

7 Books on SRA 7

8 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 8

9 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 9

10 INTRODUCTION Some analytical results exists for simple cases, for example for linear limit states and normal variables 10

11 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 Transformation The challenge relates to estimating small probabilities (standard Monte Carlo not realistic for realistic limit states) 11

12 Probability by FORM P = Probability of linear approximation of the failure set, FORM linearization at the design point u 2 P FORM =Φ(-β FORM ) safe set FORM failure set u* β= u* u-space u 1 12

13 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 values For 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 problem In this case the designs will vary slightly in structural reliability The optimisation consists of selecting the PSF that gives the lowest scatter around the target reliability Software is available (Since 1990) 13

14 Goal Based Construction Standards Characteristic Design Load Value = (Material) Characteristic = Characteristic Design Load Value γ Value (Material) (Material) / γ 14

15 Fatigue Risk Based SN-curve for new component log( N) = log( K) m log( S) Experience from similar components : 15

16 Deterministic vs. Probabilistic P Approach M a crack t Crack size a 2c t M P Critical crack size 2c Time until critical crack Initial defect size Probabilistic crack growth Life time Deterministic crack growth 16 Time

17 Fatigue Analysis Methods S-N Fatigue Assessment S-N data - experimental data weld Structural detail Potential Crack growth S S Log(S) S-N DATA S-N DATA & S-N Curve µ LogN 2 σ LogN 2.00 S-N S-N data data 1.90 S-N S-N data curve - Fit Design S-N curve S-N curve - Fit (mean-2*std) µ 5.5 LogN Log(N) S-N Mean Curve: Log(N) = Log(ā) - m Log(S) N = ā S -m S-N Design Curve: Log(N) = Log(ā) - m Log S - 2 σ LogN 17

18 Integrated Fatigue analysis - Uncertainties Environmental description Load And Response analysis Detailed Stress analysis (SCF) Fatigue analysis 18

19 Specify Design Life? Maintenance free Ship? Design life This should be specified by IMO Minimum acceptable: Annual Probabilities This applies to all limit states! 19 This should be Specified by owner

20 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 structures Calibration against well established codes that are judged acceptable or best practices for similar type of structures Calibration against tabulated values, using distribution assumptions that are judged to be (slightly) conservative 20

21 High Level Goal Tradition (NKB/DNV/ISO/CEN) Table 1: Annual Target Probabilities (and Target β T ) from DNV Classification Note 30.6 Class of Failure Consequence of Failure I- Redundant Structure II - Significant warning before the occurrence of failure in a nonredundant structure Less serious P F = 10-3, β T = 3.09 P F = 10-4, β T = 3.71 Serious P F = 10-4, β T = 3.71 P F = 10-5, β T = 4.26 III - No warning before the occurrence of failure in a nonredundant structure P F = 10-5, β T = 4.26 P F = 10-6, β T =

22 CONCLUSIONS Experience with GBS in other industries is worth studying Concepts like fatigue life and design life are risk based and have always been so For structures the methodologies are available and ready for use mentioned in a number of submissions to MSC 79 The relation between goal based (performance based) and risk based is as between style and method 22

Robustness - Offshore Wind Energy Converters

Robustness of Structures - February 4-5, 2008, Zurich 1-14 Robustness - Offshore Wind Energy Converters Sebastian Thöns Risk and Safety, Institute of Structural Engineering (IBK) ETH Zurich Division VII.2: