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Predicting the Probability Of Failure of Gas Pipelines Including Inspection and Repair Procedures

Predicting the Probability Of Failure of Gas Pipelines Including Inspection and Repair Procedures. L.Zhang, R.Adey Computational Mechanics International San Sebastian 12.09.2007. Contents. Why Probability of Failure (POF) analysis An introduction to the POF calculation method

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Predicting the Probability Of Failure of Gas Pipelines Including Inspection and Repair Procedures

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  1. Predicting the Probability Of Failure of Gas Pipelines Including Inspection and Repair Procedures L.Zhang, R.Adey Computational Mechanics International San Sebastian 12.09.2007

  2. Contents • Why Probability of Failure (POF) analysis • An introduction to the POF calculation method • Overview of POF calculation tool- PipeSafety developed for NATURALHY project • A sample problem • Summary

  3. Probability Of Failure • What is Probability of Failure? • A number which indicates the probability that a pipeline could fail within a given timeframe • Failure criterion: leakage including breakage (zero tolerance to gas/hydrogen leakage) • The Probability of Failure can be used to assess the impact on the safety of a pipeline of changes in the design and management and the type of gas carried by the pipeline • The POF also provides a link to economics type models

  4. Probability Of Failure • The POF can include the impact on the pipeline safety of: • The type of gas being carried and its impact on the materials or defects present in the pipeline • The operational loads on the pipeline • The inspection strategy and the performance of the inspection tools • The repair strategy • The quality of the pipeline • e.g. The number and type of defects present

  5. Hydrogen • Hydrogen has been shown to change the behaviour of materials subject to low cycle fatigue loads • These changes are associated with plastic deformation of the material • Hydrogen is therefore expected to change the behaviour of sharp crack like defects • The POF calculation is therefore focused on crack-like defects

  6. Probability Of Failure • A probabilistic fracture mechanics approach has been used to determine the behaviour of crack like defects • There are three main ways of obtaining the POF • Analytical solution • First/Second Order Reliability Method (FORM) • Monte-Carlo simulation with/without variance reduction technique

  7. POF Calculation Method • The current failure criteria used in the model is based on BS7910 level 2 FAD • Other failure models could be added to the POF tool

  8. POF Calculation Method • Monte-Carlo simulation

  9. POF Calculation Method

  10. POF Calculation Method • All the inputs into the model can be expressed probabilistically • Defect distribution, material behaviour etc. • Common probability density functions • Lognormal distribution • Weibull distribution • Exponential distribution, normal distribution…etc.

  11. POF Calculation Method • The POF of a whole pipeline can be calculated as follows:

  12. POF Calculation Tool PipeSafety • Main page

  13. POF Calculation Tool PipeSafety • Pipe geometry

  14. POF Calculation Tool PipeSafety • Defect definitions

  15. POF Calculation Tool PipeSafety • Loads definitions

  16. POF Calculation Tool PipeSafety • Residual stresses

  17. POF Calculation Tool PipeSafety • Material properties Data is being obtained by experimental testing as part of the NATURALHY project

  18. POF Calculation Tool PipeSafety • Inspection tool characteristics

  19. POF Calculation Tool PipeSafety • Repair procedure

  20. POF Calculation Tool PipeSafety • Analysis and other options

  21. Sample Application Basic input data

  22. Sample Application Fatigue crack growth data from TNO, Netherlands for natural gas and 100% hydrogen

  23. X52 Base Material In 100% Natural Gas • Repair criteria • Pre-inspected and then inspected at every 10 years • If a detected crack will lead to failure in 12 years it is repaired • A pipeline with 10 cracks/km • Fracture and fatigue properties • c=5.2e-13 • m=3 • KIC=4743.416MPa*mm1/2 • Result • Cumulative POF of a single defect (60 years): 3.0e-5 • Total POF: 3.0e-4 per km

  24. POF Of A DefectBase Metal

  25. POF Of A Defectin welds

  26. Improve Inspection What happens if we improve the sensitivity of the inspection tool? New inspection tool Probability of Detection Curve

  27. POF Of A DefectBase Metal Higher sensitivity of inspection tool reduces the POF

  28. Summary • Probability of Failure analysis enables the impact of hydrogen on the safety of pipelines to be assessed • The analysis does not only consider the impact of hydrogen on the micro mechanical behaviour of the materials but also the way the pipeline is managed • Inspection strategy • Repair strategy • Operational loads

  29. Summary • It can be used as a sensitivity tool to assess management options and can link to more economic models • The initial crack size distribution is an important input parameter and further work is required to validate this and other data • Material properties such as crack growth threshold value, fatigue properties and toughness are key parameters

  30. The End Question time

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