Hydrostatic Test Establishes New MOP for 100-mile Pipeline
Project management and engineering for an integrity-driven effort that included:
- Nitrogen displacement of the pipeline
- Pipe cutouts and replacements for anomaly testing and analysis
- Water fill with dye and tracer gas injection
- Spike test on short heavy-wall section to ensure five-year reassessment interval
- Spike, strength, and leak tests on entire 100-mile pipeline to reconfirm MOP and achieve five-year reassessment interval
- Dry line with methanol, refill with product, and filter and discharge test water
- Re-established MOP after multiple anomaly-related pressure reductions
- Allowed return to higher operating pressures and flow rates
- Secured reassessment interval of five years for entire pipeline
- Anomaly cutouts will inform future integrity assessments
While in-line inspection (ILI ) results often provide clarity about a pipeline’s condition, they sometimes create more questions than answers. After an ILI vendor reported thousands of cracklike features on a 100-mile pipeline, our client needed help. With hundreds of validation digs not an option economically and the line’s capacity hobbled by pressure reductions, a way was needed to establish a new maximum operating pressure (MOP) for the line and close out the repair plan. As our client’s go-to project management, engineering, and integrity experts, we were on the job. Because industry models of crack growth and failure showed that true cracks at the dimensions called by the tool would be failing at current operating pressures, and no failures had occurred, we knew the tool was not able to discriminate between injurious, sharp-tipped cracks and cracklike but non-injurious features. The best method to prove the line’s integrity was a hydrotest.
In addition to a new MOP, the project had two more objectives: achieve a five-year reassessment interval and procure pipe feature samples for testing and analysis. We identified four locations with representative anomalies and easy access as good candidates for pipe cutouts and replacements. However, one section of pipe was standing in the way of a five-year reassessment interval. Counterintuitively, the problem pipe had heavier wall than the remainder of the line. The maximum allowable test pressure for the remainder of the line was not adequate to rule out the presence of existing cracks in the heavy-wall pipe that could quickly grow to failure with pressure cycling. While the original plan was to sleeve or replace this pipe, preliminary excavation work revealed that it was surrounded by a spaghetti of other pipelines and conduits. We suggested that this pipe be isolated and spike tested separately, then tied back in to the pipeline for the mainline test, and our suggestion was adopted into the plan.
The safe and successful execution of this project was the client’s management’s top priority for the year. HT Engineering’s team worked closely with the client’s operations team to develop a detailed schedule and resource plan, and to prepare comprehensive procedures – vetted by all the stakeholders – for each stage of the work. We worked with a partner to procure the long-lead environmental permits required. We developed scopes of work and procured services from 11 different contractors and conducted a series of meetings to orient and align the complete project team.
The team first purged the line with nitrogen and blew it down, allowing two groups to perform the cutouts and replacements. We then spike tested the heavy-wall pipe section to 3,500 psig. With that test a success, we tied the pipe back into the mainline and proceeded to fill 100-mile line with water, with dye and tracer gas included to ease leak locating in the case of a failure. Thanks to careful planning and good communication, the fill was solid. After allowing the water temperature to stabilize, we successfully completed the brief 1,700 psig spike test, the 4-hour proof test, and the 4-hour leak test. After the test, the team discharged the water through carbon filters, then dried the line with methanol and refilled it with product. Despite complications with the methanol injection that caused a two-day delay, the project finished ahead of schedule and under budget. As seasoned pipeline project managers working with an experienced operations team, we knew that even the best laid plans encounter hiccups. Building in the right contingencies from the start was key.
In the end, the client achieved all three project objectives. The pipeline is now operating safely with a new MOP and restored capacity, the next integrity assessment is due in five years, and the representative anomalies are at the lab for testing and analysis, ready to yield their insights.