Pressure cycling is one of the many operational factors a liquids pipeline company has to contend with in their pipeline integrity cracking program. The management of pressure cycling is important because of the potential development and growth of cracks in the pipe wall by fatigue mechanism where pressure cycling acts as the driving force. The operational source of these cycles can be complex but often include planned start/stops, batch pigs passing pump stations, mid-point injections or deliveries, viscosity changes due to commodity transitions, flow rate changes, and unplanned line outages. The first step to understanding pressure cycling is the development of a methodology which defines pressure cycling targets and monitors cycling on the line. Enbridge Pipelines Inc. (Enbridge) has developed two processes to manage pressure cycling on existing and future assets. These procedures help define the path to limiting pressure cycling but also steer the cultural change required to mitigate this risk within an established operating environment.
All operational lines within Enbridge Pipelines Inc. are monitored monthly for pressure cycling risks. Understanding the impact of pressure cycling on these lines can be very complex. To determine the risks associated with an operating pipeline the line’s susceptibility to cracking and its pressure cycling severity must be understood. Once the risks are identified, a pressure cycling mitigation plan, to ensure continued safe operation of the asset can be developed. In order to complete a mitigation plan a detailed operational review needs to be conducted and a company-wide team engaged. The team will determine how the existing operational philosophy can change or what physical modifications are required to improve the current operation and limit or reduce pressure cycling.
All new projects within Enbridge have to meet the “Fatigue Design Standard for New Pipelines” to ensure the new line has been designed to handle the estimated cycling. To estimate the cycling of a new line the operational philosophy needs to be well understood; this includes: injection/delivery points, planned maintenance outages, estimated unplanned outages, commodity transitions, transient mitigation, and pressure profiles for each known event.
This paper will focus on the processes Enbridge uses to manage pressure cycling on new and existing lines. A separate paper from Enbridge titled “IPC2014-33566: Allowable Pressure Cycling Limits for Pipelines” focuses on the fatigue science and how the pressure cycling targets are determined for the pipelines.