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). The benefits of reliability-based methods include consistent safety levels, optimized solutions that make best use of available resources and flexibility in addressing non-standard problems. The key limitations of the methodology are that it requires specialized expertise, good data and a significant analysis effort.
One approach that has been successfully used to simplify the application of reliability-based methods is to develop simple design and assessment rules that are designed to meet specified safety levels. In this approach, which is referred to here as limit states design and assessment, the checking rules incorporate safety factors that are “calibrated” to meet pre-selected reliability targets, within a specified tolerance, over a wide range of possible design and assessment cases. Probabilistic analyses are performed as part of the development stage, but the resulting checks are deterministic.
The basic elements required to calibrate limit states design and assessment checks have been developed as part of the RBDA methodology, making the development of a limit states approach feasible. This paper provides an overview of an ongoing Joint Industry Project to develop a limit states design and assessment standard that addresses the key threats to the safety of onshore pipelines. The benefits and limitations of this approach are discussed in comparison to the full RBDA approach, and the expected outcomes of the project are described.