ABERDEEN, UK – STATS Group has completed an isolation program for Apache on the Forties pipeline system (FPS) in the UK central North Sea.
The aim was to safely isolate the FPS so that subsea isolation valves (SSIV) and piping spools could be safely installed and leak tested, avoiding the need to depressurize the entire pipeline system.
The 36-in., 169-km (105-mi) FPS transports 40%of the UK’s oil, and runs from Apache’s Forties Charlie platform to the Cruden Bay terminal on the Scottish east coast, with daily throughput of around 700,000 bbl.
STATS provided two remotely controlledTecno Plugs, the first designed to provide double block isolation from pipeline pressure, and the second to provide a local leak test boundary.
The Tecno Plugs were deployed and pigged in a train with two bi-directional batching pigs positioned ahead of the plugs. The entire train was pigged to the set-location using inhibited seawater to ensure the section of pipeline being removed was fully de-oiled prior to breaking of containment, thereby preventing any unintended oil release.
Once the first Tecno Plug was in position, it was hydraulically set to activate the locks and dual seals. Positioning and tracking was ensured via a subsea extremely low frequency (ELF) control system which also provided continuous monitoring throughout the isolation. The second Tecno Plug was not hydraulically set until the construction scope was completed.
Independent testing of each seal with full pipeline pressure confirmed leak-tight isolation. The pipeline was then bled down to subsea ambient from the platform launcher to the rear of the Tecno Plug, with the annulus between the Tecno Plug seals vented to ambient.
After an eight-hour isolation stability hold period, an “Isolation Certificate” was signed and the double block isolation of the FPS was confirmed, allowing the subsea construction phase to commence. Two pipeline spool pieces were safely removed to allow the installation of the two metrology spools required to tie-in the pre-installed SSIV arrangement to the pipeline and the Forties Charlie platform.