Introduction
Our client, a North Sea operator, planned to cut a 20” interfield gas/condensate pipeline and a 3” Monoethylene Glycol (MEG) pipeline subsea to be left in situ on the seabed as part of a decommissioning operation. This hydrocarbon free phase of the decommissioning required that both lines be cleaned prior to cutting.
Scope
The operator asked us to design and implement the pipeline cleaning operation and manage the scope of this project phase alongside all other topside decommissioning activities, and in-line with HSE and DECC regulatory requirements.
The two pipelines ran from a normally unmanned installation and tied into a live platform that exported to an onshore terminal. The operation therefore required careful logistical and technical planning to avoid any disruption to ongoing production during the cleaning operation.
Solution
To effectively clean the gas/condensate line we designed a three-phase pigging programme.
For the first phase a foam pig was run to remove hydrocarbon liquids from the pipeline. The second phase used a hard bodied cleaning pig to remove solid debris. For the third phase a train of cleaning pigs was used, driven by seawater, to flush the pipeline to below 30 ppm oil in water.
For the 3” MEG pipeline a gel pig was used, removing the MEG, then being offloaded into the export line to onshore terminal.
Key activities of this project included:
- Designing the pipeline cleaning programme
- Designing interface with live plant
- Pig selection
- Leading and organising risk assessments
- Decommissioning safety case
- Pig tracking design
- Chemical selection (cleaning agent, seawater treatment, scale inhibitor)
Vendor selection
- Cost analysis
- Contingency planning
- Stuck pig guidance note.
Conclusion
The successful outcome of this project was effectively cleaning the two lines in accordance with the relevant regulatory requirements – ready for the next stage of decommissioning.
Jee has extensive experience managing, designing, and providing engineering support to a wide variety of pigging campaigns. This, combined with our late life and decommissioning capabilities, ensured this project was completed to our client’s satisfaction.
