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During Q1 of 2023 a 14” Emergency Shutdown Valve (ESDV) on a North Sea Operator’s gas export pipeline was found to be failing its leak off testing performance standard. A scope in Q3 2023 to replace the ESDV using existing subsea valves was unsuccessful, as the valves were unable to provide a suitable isolation for breaking containment topsides.
Prism Energy engineers completed a study of options to facilitate the changeout of the ESDV in 2024.
As a result of the design of the system, one of the key challenges was the lack of pigging facilities on the export line and a challenging pigging route due to the wall thickness, back-to-back 3D bends and multiple offtakes.
Due to passing valves in the system, the initial major challenge was providing an isolation to allow safe access to the pipeline.
The proposed methodology included the use of a DBB-SAVER provided by VALVETIGHT to facilitate a positive pipeline isolation. The DBB-SAVER provides an isolation by creating a vacuum through the bleed port of the ESDV, whilst mixing any passing hydrocarbons with N2 to create an inert gas before expelling to a safe location. The DBB-SAVER isolation allowed for the installation of a temporary PLR and pipework. With pigging facilities installed the project team were then able to pig the STATS HPIT to the set location outboard of the ESDV.
With the HPIT set and the isolation certificate issued the ESDV change out operation could take place. On completion of installation of the new valve, a line pressure test was carried out using N2 against the rear of the HPIT. The HPIT was then unset and recovered to the temporary PLR using pipeline inventory. The DBB-SAVER was then used in conjunction with the new ESDV to allow for removal of the temporary pipework before completing a 1.1x MAOP test on the new connections.
With the completion of the scope the following was achieved –
Prism Energy’s involvement in the scope was extensive, holding the overall responsibility of project managing the scope, while also providing planning support, construction work packs and HAZID / HAZOP sessions. We have a proven approach to these complex pipeline projects that is based on experience, best practise and lessons learned.
