Safety Levels Associated With the Ultimate Limit State Reliability Targets in Annex O of CSA Z662

Abstract

In 2005, guidelines for the application of reliability-based design and assessment (RBDA) to natural gas pipelines were developed under PRCI sponsorship. The methodology underlying these guidelines has since been adopted as a non-mandatory Annex in the CSA Z662 standard (Annex O). Because the methodology is based on the concept of consistent risk, it is more restrictive than current design standards for some pipelines and less restrictive for other pipelines. Specifically, the RBDA reliability targets for ultimate limit states (ULS) are lower than the reliability levels achieved by current design standards for large diameter, high pressure pipelines in Classes 2 to 4 (referred to in this paper as high consequence pipelines) and for all pipelines in Class 1. This has naturally led to questions among users and reviewers of RBDA regarding the adequacy of the ULS reliability targets for these pipelines.

For high consequence pipelines, the safety levels associated with the ULS reliability targets are compared to risk tolerance criteria from three published European standards. The comparison demonstrates that the targets are generally more conservative than all three standards. For Class 1 pipelines, the average failure probability for pipelines designed to the RBDA methodology is compared to the average failure probabilities for pipelines designed to ASME B31.8 and Clause 4 of CSA Z662. This shows that there is no significant difference in overall failure probability because the wall thickness for most Class 1 pipelines with low ULS targets are governed by other criteria, such as minimum permitted wall thickness and maximum diameter-to-thickness ratio for standard line pipe. Recognizing the potential difficulties with reliability targets that do not govern any designs, a possible modification to align the targets with the actual reliability levels is proposed.

Author: Nessim, M., Stephens, M. and Adianto, R.

Publisher: International Pipeline Conference, Calgary, Alberta, September 24-28

Year: 2012

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