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PRCI PR-185-9429
- Reliability-Based Fitness for Service Assessment of Welds
- Report / Survey by Pipeline Research Council International, 06/20/2000
- Publisher: PRCI
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L51779e
University of Texas at El Paso
Need: Requirements for the fabrication and inspection of transmission pipeline girth welds are given in national codes such as API 1104, CSA Z662, and BS 4515. Their objective is to ensure an acceptable level of quality, and implicitly, operational safety. They seek to achieve this by imposing workmanship standards which prescribe the allowable size and frequency of various discontinuities. These standards are largely arbitrary and thus highly conservative in most situations. However, the potential for economic benefit, an increasing demand for safety, and the development of fracture mechanics technology have motivated the inclusion of optional fitness-for-service (FFS)-based flaw acceptance criteria.
Benefit: Two major objectives of the present work were to educate those unfamiliar with structural reliability concepts, particularly as they apply to pipeline integrity, and review the worldwide codification of LRFD for industries that fabricate and maintain welded structures. The final objective was to develop a library of PSFs for pipeline girth weld flaw assessment thereby aligning girth weld fitness-for-service assessment procedures with emerging LRFD codes for transmission pipelines.
Result: With the application of partial safety factors such as those developed here, a reliability-based procedure rationalizes the assumptions made during an assessment, particularly in the treatment of input data, and allocates conservatism according to the influence each variable has on the overall reliability. It can thereby eliminate much of the arbitrariness of conventional deterministic approaches and provide a means to account for uncertainty in a repeatable, rational way. It thus provides a consistent, quantitative basis on which to base fitness-for-service decisions.
University of Texas at El Paso
Need: Requirements for the fabrication and inspection of transmission pipeline girth welds are given in national codes such as API 1104, CSA Z662, and BS 4515. Their objective is to ensure an acceptable level of quality, and implicitly, operational safety. They seek to achieve this by imposing workmanship standards which prescribe the allowable size and frequency of various discontinuities. These standards are largely arbitrary and thus highly conservative in most situations. However, the potential for economic benefit, an increasing demand for safety, and the development of fracture mechanics technology have motivated the inclusion of optional fitness-for-service (FFS)-based flaw acceptance criteria.
Benefit: Two major objectives of the present work were to educate those unfamiliar with structural reliability concepts, particularly as they apply to pipeline integrity, and review the worldwide codification of LRFD for industries that fabricate and maintain welded structures. The final objective was to develop a library of PSFs for pipeline girth weld flaw assessment thereby aligning girth weld fitness-for-service assessment procedures with emerging LRFD codes for transmission pipelines.
Result: With the application of partial safety factors such as those developed here, a reliability-based procedure rationalizes the assumptions made during an assessment, particularly in the treatment of input data, and allocates conservatism according to the influence each variable has on the overall reliability. It can thereby eliminate much of the arbitrariness of conventional deterministic approaches and provide a means to account for uncertainty in a repeatable, rational way. It thus provides a consistent, quantitative basis on which to base fitness-for-service decisions.
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