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Cost Effectiveness of Hull Girder Safety

Cost Effectiveness of Hull Girder Safety. Rolf Skjong & EM Bitner-Gregersen Det Norske Veritas. OMAE, Oslo, June 24-28, 2002. Content. Acceptance criteria in Structural Reliability Analysis Acceptance Criteria in Formal Safety Assessment Common Criteria?

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Cost Effectiveness of Hull Girder Safety

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  1. Cost Effectiveness of Hull Girder Safety Rolf Skjong & EM Bitner-Gregersen Det Norske Veritas OMAE, Oslo, June 24-28, 2002

  2. Content • Acceptance criteria in Structural Reliability Analysis • Acceptance Criteria in Formal Safety Assessment • Common Criteria? • Application to Hull Girder Strength • Conclusion

  3. Traditional Approach - SRA • Typical Example:DNV Classification Note 30.6 (1992) on Structural Reliability Analysis of Maritime Structures • SRA is Bayesian theory • Explains why SRA does not produce Probabilities with a frequency interpretation • No gross error • Epistemic uncertainty & model uncertainties included • SRA talk of “notional” reliabilities

  4. Traditional Approach - SRA • Target should depend on consequence • Calibration against known cases (that are acceptable good/best practices in the industry) • Calibration against similar cases with similar consequences • Based on accepted decision analysis techniques • Based on tabular values (presented as a last resort)

  5. Traditional Approach - SRA • Based on tabular values (presented as a last resort)

  6. Traditional Approach - RA • Quantitative risk assessment is the basis for regulations in many industries • PSA/PRA - Nuclear • Hazardous Industries (Seveso I/II) • Offshore (Safety Case) • Shipping (FSA) • Etc.

  7. Traditional Approach - QRA • Present Risk Results in terms of • Individual Risk (Fatalities) • Individual Risk (Health and Injuries) • Societal Risk (Group Risk) • Environmental Risk • Economic risk (not necessarily a regulatory issue)

  8. Traditional Approach - RA • Example Individual risk

  9. Traditional Approach - QRA • Example Societal Risk

  10. Traditional Approach - QRA Not acceptable Intolerable High Risk Acceptable if made ALARP ALARP Negligible Acceptable Low Risk

  11. Traditional Approach - QRA • The As Low As Reasonably Practicable Area implies that cost effectiveness assessment may be used • Risk is made As Low As Reasonably Practicable, when all cost effective safety measures have been implemented • Implies that a decision criteria for cost effectiveness will be required • It seems to be accepted at IMO that most ship types are in the ALARP area but not ALARP • Cost Effectiveness will be the only criteria

  12. Common Criteria • Cost effectiveness criteria is easy to use, both for SRA and FSA studies • Advantage to SRA: Only change in risk is used in the decision process, not the absolute numbers • Criteria to use: $ 1.5 - $ 3.0 million • Decision at MSC 76 in December 2002

  13. Previous IMO decisions • Example from IMO: UN Organisation for maritime safety and environmental protection regulations

  14. Girder Collapse/Sagging

  15. PROCODE • Use limit states formulation with target beta • Optimisation of partial safety factors • Control Variables: Partial Safety Factors • Minimum Scatter around a target reliability by minimising the penalty function • Target to vary to produce cost effectiveness ratios

  16. PROCODE One design Case: Subjected to: With one of the inequalities turning into equality This is generalised to Multiple design cases in PROCODE

  17. PROCODE • Programmable functions • Limit States • Code Checks • Penalty functions • Defined by Data (additional to PROBAN) • Scope • Safety Factor • Design Parameter

  18. PROCODE RESULTS • Code Evaluation (before optimisation starts) • Optimised partial safety factors • Resulting reliabilities • Resulting design parameters (input to cost analysis)

  19. PROCODE Results

  20. PROCODE Results

  21. Resulting Steel Weight

  22. Costs

  23. Costs

  24. GCAF Calculation of GCAF (L Profiles) GCAF = (Cost of RCO)/( PLL)  -> 3.50 to 3.72 (Pf=2.41 10-4 to 10-4) 20 persons 20 years 50% survives  P  PLL = (2.41 10-4 - 1.0 10-4 ) •20 •20 •0.5 = 0.0302 GCAF = (4,309,760 -3,997,769)/0.0302 = $ 10.3 million

  25. NCAF Calculation of NCAF (L Profiles) NCAF = (Cost of RCO -Economic Benefits)/( PLL)  -> 3.50 to 3.72 (Pf=2.41 10-4 to 10-4) 20 years Old Ship  P Cargo Benefits = (2.41 10-4 - 1.0 10-4 ) •20• ($11 + $ 21 million ) = $ 90,240 NCAF = (4,309,760 -3,997,769-90,240)/0.0302 = $ 7.13 million

  26. Results

  27. Conclusion • In reliability based code calibration CEA (NCAF) may be used as an alternative to target  • Consistency with RA • The decision is based on the derivative of PF with respect to design variables only • Not reliant on probability (the absolute number) • Mid ship bending moment (ship deck/sagging) • NCAF at about $ 3 million corresponds to 10-4 target • Compares well to last line of defence, lifeboats at $ 1 million • More studies necessary for other limit states

  28. Evacuation Failure Flooding Collision Human error, navigation Fire One criteria HE, QRA, SRA Optimising risk control options according to their cost effectiveness:

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