The scope

As part of the decommissioning hydrocarbon free phase our client, a North Sea operator, planned to clean a 20” interfield gas/condensate pipeline and a 3” MEG pipeline to both be later cut subsea and left in situ on the seabed.

The pipelines ran from a normally unmanned installation and tied into a live platform that exported to an onshore terminal. The operation required careful logistical and technical planning to avoid any disruption to ongoing production.

The operator asked us to complete the detailed pipeline cleaning design and implementation and manage this scope alongside all other topside decommissioning activities, and HSE and DECC regulatory requirements.

The solution

To effectively clean the gas/condensate line a three phase pigging programme was designed.  First, a foam pig was run to remove hydrocarbon liquids from the pipeline, second a hard bodied cleaning pig to remove solid debris, and finally a train of cleaning pigs driven by seawater to flush the pipeline to below 30 ppm oil in water. The MEG from the 3” pipeline was removed with a gel pig and offloaded into the export line to onshore terminal.

Key activities 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.

The benefits

We have extensive experience managing, designing and providing engineering support to a wide variety of pigging campaigns. This, combined with our late life and decommissioning capabilities, allowed us to make a significant impact to the project; drawing on our key expertise where required.

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