Pressure cycle fatigue has been shown in industry to be a contributing factor to pipeline failure. There are methods for pressure cycle fatigue monitoring that can be used as a leading indicator for the risk of the pipeline to fatigue related failure. Once lines with high cycling are identified, the risk of the cycling to the asset and the mitigation strategies for the cycling can be discussed within the organization. By mitigating the driving force of crack initiation and grow to failure in-service, the pipeline community is safer.
Shell Pipeline Company, LP. (SPLC) experienced two in-service failures on the same pipeline in under a year where fatigue was a common root cause. Following the investigation of these failures, management requested communication of the risk of pressure cycle fatigue throughout the organization with the intent to mitigate the levels of pressure cycling across the system. All pipelines were put on a monthly dashboard of pressure cycling and sent to all staff for awareness and action.
The company measures pressure cycling on all pipelines by normalizing the number of cycles to 25% of the specified minimum yield strength (SMYS). From January 2016 to December 2019, the number of monthly cycles on the top ten highest cycled segments were reduced from 45,000 cycles per month, to 18,970 cycles. This is a reduction of 58%. The number of Very Aggressively cycled pipelines was reduced from 2 to 0. The number of Aggressively cycled pipelines were reduced from 13 to as low as 3. This paper will share the strategies and methodologies used to achieve these results.
The paper will share how the list of highly cycled pipelines and the monthly status reports were developed. The paper will also share how pressure cycling mitigation strategies for pipeline systems were developed in collaboration with facility engineering, business unit leads, controllers, schedulers, and integrity staff. The effectiveness of mitigation methods such as pressure reduction, installation of back-pressure control valves, changing of valve timing on startup and shutdown, changes to the scheduling on the pipeline, utilization of flying switch between tankage, etc. will be discussed.
By reducing pressure cycling, the risk of fatigue related failures can be reduced. This program is continuously being improved because there is both management commitment and ownership of the issue throughout the organization.