Managing the Threat of SCC in Gas Transmission Pipelines

Abstract

A group of eight gas transmission pipeline operators, responsible collectively for operating over 160,000 miles of pipelines in North America, has participated in a Joint Industry Project (JIP) to examine the current status of Stress Corrosion Cracking (SCC) Threat Management. Many of these operators had previously participated in a JIP addressing the Integrity Management of SCC in High Consequence Areas. Completed in 2006, the JIP developed experience-based guidance for the conduct of hydrostatic testing and excavations, for the assessment of the severity of discovered defects, and for establishing the interval before the next assessment. The outcome was published in ASME STP-PT-011, and formed the basis for proposed revisions to ASME B31.8S. In this second phase of the work, the operational experiences and threat management experiences during the five years since 2006 have been reviewed. From an operational viewpoint, the situation has been very satisfactory; only three in-service failures (ruptures or leaks) due to SCC have been experienced during this period, a considerable reduction compared to the preceding years. However, there is still a legacy of SCC to be managed in older pipelines; for example, 80 near-critical cracks have been removed by hydrostatic testing, and around 100 cracks that would probably have failed a hydrostatic test have been discovered by crack detection ILI. From the threat management viewpoint, a consistent overall framework for addressing SCC is beginning to be established, within which the wide range of operational experience can be addressed using mitigation strategies that are appropriate, proportionate, and timely. Most operators, particularly those with a legacy of SCC in older pipelines, make use of hydrostatic testing. Several now make use of SCC Direct Assessment, following its acceptance as a formal process in around 2005, but mostly for addressing segments with low relative risk of SCC and/or no history of SCC. Many are exploring the application of crack detection ILI; among the JIP members around 45 runs totalling nearly 3000 miles have been completed using EMAT ILI vehicles, and more are scheduled. Almost all the JIP members are using two or more of these approaches in combination as part of their SCC Threat Management strategies. There are areas where the experiences of SCC Threat Management over the last five years point to opportunities for improvement. For SCC Direct Assessment, the use of feedback from excavations to refine the relative rankings for segment prioritisation and dig site selection will become an increasingly important aspect of process improvement. For crack detection ILI, the main issues are the accuracy and reliability of information determining the flaw size and shape for use in predictions of failure pressure and assessments of defect severity. As Threat Management moves from baseline assessment to regular re-assessment, issues that arise include determination of the re-assessment interval, particularly when using SCC Direct Assessment and crack detection ILI. There is also an issue about how best to actively monitor those segments where there is low relative risk and no experience of SCC.