While pressure fluctuation is an inherent part of operating pipelines, review of historic operations of several pipelines in the years leading up to a failure shows a trend in the severity level of the pressure cycling. It was recognized that moderate to severe levels of pressure cycling are not acceptable on a pipeline for an extended period of time. As a result, pipelines that are operating at, or worse than this severity level have a higher likelihood of failure due to the potential fatigue crack growth induced by the pressure fluctuations.

Enbridge has developed a managed approach to addressing the ramification linked to reducing pressure cycling on its system to address both existing and new assets. This approach is presented in paper IPC2014-33485 “Pressure Cycle Management”. This paper discusses the particulars for establishing targets to help with driving and monitoring the success of pressure cycling mitigation activities.

Each pipeline is given different pressure cycling targets using a risk based approach taking into account the current condition and pipe properties of the pipeline, i.e. cracking susceptibility. Factors like feature population and distribution, vintage of pipe, longseam weld type, failure history, ILI technology used to inspect the pipeline, and when the last hydrostatic test took place are taken into account when determining the pressure cycling targets. The more susceptible to in-service crack growth that a pipeline represents, the more stringent the pressure cycling targets will be.

Pressure cycling severity is determined with a methodology based on TTO Number 5 and fatigue modeling. The challenge is in converting the pressure cycling severity target into meaningful cycling targets that can be used to reduce the cycling. The severity target is transformed into typical maneuvers performed on the pipeline to give operators and schedulers an idea of how many cycles and of what magnitude they have to work with to still meet the cycling severity target.

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