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2010 EFCOG SAWG Meeting Using Quantitative Analysis to Make Risk-based Decisions

Outline. When to use the quantitative risk analysis (QRA) techniques How to determine the level of analysis that is neededExample 1: Aircraft maintenance procedureExample 2: Offshore oil and gas production facilityExample 3: Nuclear facility automatic HVAC fan controllerSummary. Definition of R

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2010 EFCOG SAWG Meeting Using Quantitative Analysis to Make Risk-based Decisions

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    1. 2010 EFCOG SAWG Meeting Using Quantitative Analysis to Make Risk-based Decisions John A. Farquharson

    2. Outline When to use the quantitative risk analysis (QRA) techniques How to determine the level of analysis that is needed Example 1: Aircraft maintenance procedure Example 2: Offshore oil and gas production facility Example 3: Nuclear facility automatic HVAC fan controller Summary

    3. Definition of Risk Terms Risk = Frequency x Consequence Risk assessment: A formal process of increasing one’s understanding of the risk by answering three questions: What can go wrong? How likely is it? What are the impacts?

    4. The Process of Risk Assessment

    5. Definition of QRA QRA is an art and a science Numerical estimates of: expected frequency projected consequences Most effectively used after design characteristics have already been specified

    6. Efficient Use of QRA Techniques Always perform the qualitative risk assessment study first Focus on the risk-based decision Perform the analysis with barely enough detail to answer the question Use a phased approach for both frequency analysis and consequence analysis

    7. Frequency Analysis Phased Project Phase 1 – Perform qualitative study (typically failure modes and effects analysis [FMEA]) Phase 2 – Prepare event trees to display accident scenarios of interest Phase 3 – Use branch point estimates to develop a frequency estimate for the accident scenarios Phase 4 – Use fault trees to solve the initiating event frequency and branch point probabilities

    8. Consequence Analysis Phased Project Phase 1 – Identify consequence types and screening thresholds Phase 2 – Perform subjective binning of consequence categories Phase 3 – If necessary, develop a detailed quantitative estimate of the impacts of the accident scenarios

    9. Portray the Risk Combine the appropriate phases of both the frequency analysis and the consequence analysis

    10. Sample QRA Projects Airline maintenance policy (required installation of the nose landing gear [NLG] pin for aircraft) Offshore oil production facility (internal vs. external riser) Nuclear research and development facility HVAC fan interlock study (automatic vs. manual fan shutoff capability)

    11. Aircraft Maintenance Policy Evaluate the policy regarding the installation of the NLG pin for Air Canada aircraft Risk tradeoff inadvertently leaving pin in place after takeoff versus NLG collapse

    12. Boeing 767 (wheels down, pilot wants them up for flight)

    13. Nose Landing Gear (should we always use a safety pin on ground?)

    14. Nose Landing Gear Collapse (if we don’t, this could happen)

    15. Technical Approach Discussed potential accident scenarios associated with NLG pin maintenance procedure Used existing risk matrix for display/judgment of risk Developed event scenarios using event trees Collected data to allow estimation of scenario frequencies

    16. Technical Approach (cont.) Calculated risk results frequency of each scenario major risk contributors to each scenario and overall risk Identified and evaluated risk management measures

    17. Maintenance Policy Options Case A - Mandatory NLG Safety Pin Installation During Towing to Hangar and Maintenance Case B - NLG Pin Installation Only During NLG Testing or Maintenance Case B* - Locking NLG Pin Installation Only During NLG Testing or Maintenance

    18. Example Event Tree

    19. Results

    20. Observations and Conclusions Risk tradeoff expected reduction in NLG collapse (Catastrophic) for Case A versus expected increase in planes taking off with the NLG safety pin in place (Moderate) Conclusion Case A policy (mandatory pin) represents a lower total risk than Case B policy

    21. Offshore Oil Production Facility QRA Produces oil and gas from predrilled offshore wells A square centerwell runs through the hull, which can contain individual oil and gas export risers and other flow lines

    22. Offshore Oil Facility (hazard of falling objects and impact from stray vessels)

    23. Floating to its New Home (should oil/gas risers be inside or outside?)

    24. Offshore Oil Production Facility QRA (cont.) Two designs considered for the location of the risers Option 1 – Through the centerwell protects the risers from external impact however, the centerwell interior is tightly confined; an explosion or fire could be far more damaging

    25. Offshore Oil Production Facility QRA (cont.) Option 2 – Outside the hull more likely to have external impact potential release not contained; lower consequence associated with fire or explosion

    26. Analysis Steps 1. Identify initiating events 2. Identify accident mitigation factors 3. Model accident sequences using event trees 4. Estimate frequency of initiating events and branch point probabilities 5. Estimate consequences using index 6. Summarize results

    27. Example Event Tree (Option 1)

    28. Example Event Tree (Option 2)

    29. Consequence Index

    30. Relative Risk Index

    31. Nuclear Research and Development Facility HVAC Fan Interlock Study Automatic vs. manual fan shutoff capability Direct Digital Control (DDC) system; failures of some fans will shut off others to maintain negative pressure in radioactive areas DDC could shut off exhaust to stand-alone fumehood while operator is working with toxic chemicals

    32. Fumehood (for chemical hazard)……

    33. ……..versus Fan (for nuclear hazard)

    34. Analysis Steps 1. Identify initiating events 2. Identify accident mitigation factors 3. Model accident sequences using event trees 4. Develop fault tree models for each initiating event and branch point 5. Qualitatively describe potential consequences 6. Develop failure data 7. Summarize results

    35. Fan Interlock Event Tree

    36. Fan Interlock Fault Tree

    37. Results Scenario 1 – positive pressure in rad area that does not result in an exposure: 2E-4/year (once per 5,000 years) Scenario 2 – positive pressure in rad area that does result in an exposure: <1E-8/year (less than once per 100,000,000 years)

    38. Results (cont.) Scenario 3 – severe chemical exposure suffered by onsite personnel due to shutdown of exhaust fumehood: 4E-6/year (once per 250,000 years)

    39. Discussion If DDC interlock system is not used: scenario 1 would increase by approximately two orders of magnitude (~once per 50 years) scenario 2 would also increase to approximately once per 1,000,000 years Warning placards and training needed to limit effects of scenario 3

    40. Conclusions QRA should focus on answering the risk-based question Do only enough analysis to answer the question Beware of dependent failures (common cause failures) where a failure causes initiating event and safeguard failure simultaneously

    41. ABSG Consulting Inc. 10301 Technology Drive Knoxville, TN 37932-3392 USA Telephone (865) 966-5232 Fax (865) 966-5287 www.absconsulting.com

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