Trends in Time-Delayed Failure Under Constant Pressure

It has long been recognized that a defect in a pipeline, such as a crack, or a gouge, or a dent and gouge, can fail at a constant pressure after some period of time has elapsed, which is commonly referred to as a time-delayed failure. Modern line-pipe steels are more defect tolerant than the earlier vintages, but can be more susceptible to time-delayed failures by virtue of their higher toughness and tolerance for larger defects. Nowadays many pipelines are subjected to in-line inspection such that many more defects that previously went undetected are now found. However, because metal-loss due to external interference could be inferred as external corrosion, whereas a feature associated with a dent could lead to more immediate response, it is not clear that ILI will expose this threat. Thus, the defects prone to time-delayed failures could remain in the pipeline even where ILI is used. To prevent time-delayed failures and minimize the risk to first-responders, it is important that the operator quantify the potential response defects relative to the line-pipe steel, the pressure history since detection or presumed date of contact, and the damage size, while initially making the conservative assumption that the metal-loss is a gouge. This paper describes work undertaken to develop these guidelines to direct first-responders, and highlights the remaining work to complete their validation.