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3 rd European-American Workshop on Reliability of NDE and Demining, September 10-13, 2002, Berlin Germany Use of POD Data in Inspection Planning and Prediction of Residual Flaw Distributions in the Offshore Industry – Do I need 90% POD

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  1. 3rd European-American Workshop on Reliability of NDE and Demining, September 10-13, 2002, Berlin Germany Use of POD Data in Inspection Planning and Prediction of Residual Flaw Distributions in the Offshore Industry – Do I need 90% POD Martin Wall and John LilleyNational NDT Centre, Culham, United Kingdom3rd European-American Workshop on Reliability of NDE and Demining, September 10-13, 2002, Berlin Germany Paper 9 Wednesday 11 September 2002 www.solutionsinengineering.com

  2. Scope of Talk • Offshore v Aerospace • POD Trials • Examples of use of POD • Probabilistic inspection planning & RBI • Procurement, as a comparator • Economic assessment (IVM) • Inspection qualification • To aid Introduction of new NDE technology • Prediction of residual flaw population • Strategy for ageing platform structures • Issues • What level of POD do I need (80%,90%, 90/95, 90/50) • POD or coverage • Peak value or transition in POD important • Multiple and Repeat Inspections

  3. Drivers for Quantifying NDE Performance • Nuclear • To improve poor NDE reliability (PISC III) • Complementary to probabilistic methods • Aerospace • Use of probabilistic methods in design • Low critical flaw size in airframes (<6mm) • Quantitative comparison of techniques • To assist introduction of improved NDE methods • Quantitative basis for inspection planning • Offshore • Use of probabilistic methods • Necessary for inspection planning

  4. Oil & Gas v Aerospace • Oil and Gas(Offshore/Petrochemical) • Large areas of pipework and structure. Access issues. • Generally large acceptable flaw size • Distinction structure versus process • For process localised corrosion main issue. • For structures weld-cracking • Redundancy in structures • Need to ensure sufficient time for intervention. • Aircraft • Small critical flaw size (few mm) • High consequences • Need very reliable inspection (High POD) • Cracks main defects • Defect locations well characterised • Not class based • Well defined procedures for reliability measurement e.g. MIL-HDBK-1823

  5. Probabilistic Inspection Planning • Used in North Sea oil and gas industry since 1980s for inspection scheduling • Mainly structural applications • Valuable to limit or optimise frequency of in-service inspection (ISI) • POD curves important input • POD data needs to be relevant. Best to use upper and lower bound curves and assess sensitivity.

  6. Integrity of Ageing Platform Structures • Older North Sea oil platforms reaching design life • Cracking not always at expected locations • Access difficulties (ROV/Diver) can give high inspection cost. • Incentive to inspect from inside or use newer remote access, global or screening methods

  7. Ageing Platform Structures • POD approach used to evaluate new versus current inspection technologies. • Upper and lower bound POD curves for key technologies • Combined with Probabilistic or deterministic crack growth analysis. • Included screening and remote-access methods • Basis for improved inspection strategy

  8. Prediction of Residual Flaw Distribution • Invert POD curve N1 = No * (1-POD) • Rules for repeat or multiple inspections • Fall off at small defect size in actual measured flaw distributions relates to POD curve effects (Wall, Silk 1995) • Silk 1994 Petrochemical significant crack 10-5/ km of weld • Estimate population at start of life based on POD for manufacturing inspections • UK National NDT Centre (NNDTC) has database of flaw distributions - - - - Typical Inspection data

  9. Cost benefit AssessmentInspection Value Method (IVM) • Recent IVM applications: • Refurbishment strategy for UK Pipeline network • Improved inspection of rail drive shafts • Non-intrusive v internal inspection of oil process vessels • Ultrasonics versus radiography inspection pipelaying vessel • Inspection technology for live water mains, tanks and reservoirs • Inspection of rail axles • Inspection of water pipelines • Threshold setting for oil storage tank

  10. Risk Based Inspection (RBI) • Where to inspect • Treatment of NDE performance limited – uses effectiveness rankings • More quantitative treatment of NDT reliability (POD) valuable to evaluate options

  11. Decision Guidance on Non-IntrusiveInspection (NII) of Pressure Vessels HOIS2000 JIP Develop decision guidance documents for • Deciding on whether NII is appropriate in principle. Replace Internal Visual Inspection (IVI). IVI is accepted by industry/regulators. • Defining NII requirements • Selection of a technique that meets the requirements - POD, Efficiency. • Issue 2 of the User Guidebook has been distributed to HOIS members (May 2002). Approval for public issue • To be published on HSE web page together with Mitsui Babcock Recommended Practice GSP-235 • http://www.aeat.co.uk/ndt/hois/

  12. When can NII be used? • Many factors play a role but key is • Can the inspection detect the defects of concern? • IVI is typically good for detecting a wide variety of defect types • NII techniques typically restricted to specific defect types • Because of this fundamental difference, Justification of use of NII requires a much greater confidence in knowing what to look for • Confidence based on • Historical evidence, experience and knowledge • Effectiveness of previous inspections on vessel • When can NII be used • Knowing what to look for is primary factor but also • important are • Severity and rate of degradation • Influences efficiency (POD) required • Confidence in knowing where to look • Influences coverage required

  13. a Overall process 1. Determine if NII is acceptable in principle 2. Define the requirements for NII technique efficiency (Pod, Coverage) 3. Determine if techniques are available meeting the requirements

  14. HOIS Interactive Knowledge Base (IKB) • Comprehensive source of Knowledge on NDE • Accessible via Internet at company offices, offshore platforms and refineries • Data on most oil industry and relevant POD trials • ‘Industry accepted ‘ values for POD – From expert panel

  15. HOIS IKB ‘Expert Panel’ POD Estimates Defect definition

  16. HOIS IKB ‘Expert Panel’ POD Estimates • ‘Industry accepted ‘ values for POD • Conservative • Important to have a quantitive basis for comparison of techniques • 50%,75% and 90% POD Values defined • http://www.aeat.co.uk/ndt/hois/

  17. Issues • What level of POD do I need (90%, 90/95, 90/50 Other) • POD or coverage • Peak value or transition in POD important • Multiple and Repeat Inspections • Treatment of False Calls • POD Trials Analysis method • Application in modern inspection strategies: screening, Permanent monitoring (PIMS)

  18. Oil Industry NDE POD NIL POD Trials Manual v Automated UT • POD in oil industry applications often low for a single inspection • Human and Environmental factors significant • 90/95 may not be realistic • Lower POD inspections still valuable • Strategy is screening, multiple or repeat inspections

  19. POD v Other Performance Factors A compromise is often needed between POD and other performance factors, Particularly coverage • Sensitivity What will a procedure find? • Speed How long will it take? • Coverage How much of the component is or can be inspected? • Reliability What mistakes are expected? (Probability of Detection POD, false calls)

  20. Transition or ‘Peak’ value • Is Transition in POD or Peak Value more important?

  21. Multiple or Repeat Inspection • Large benefit for small defects in transition region • Not unexpected. This is where human and environmentaal effects are most important (e.g. PISC III).

  22. Oil Industry Guidance on POD • Nordtest – Guidelines for replacement of NDE Methods • HSE Guidance on RBI Methods, Giorgiou Report 2001 • DNV Guidelines on Pipeline Weld Inspection • API 581 Procedures for RBI Assessment • US MIL Standard on Reliability Measurement MIL-HDBK-1823. Guidance on repeat inspections. • Varied guidance. (90% POD often a target (90/95, 90/50).Trend to use average not lower bound POD (e.g. PANI) • Conclusion: Not POD value that is important but justification. Need to quantify in POD and reliability terms. Look at whole inspection strategy.

  23. Grouping or Hit/Miss? • Most oil industry POD trials have used Grouping of data not Hit/Miss. This choice is questionable?

  24. Conclusions • POD widely accepted in the oil industry with variety of applications.Main basis for NDT system procurement. • Often need to balance POD with other performance factors (Coverage, Speed, false calls) • Distinction process and structural applications. • Guidance should take account of full inspection strategy including multiple and repeat inspections. • There is no ‘correct POD’ value. Transition in POD may be as important as the peak value.

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