Decommissioning a Gas Processing Offshore Hub Pipeline and Installing a Subsea Bypass System to Operate Without Platform in the Norwegian North Sea

Abstract Heimdal is a gas field in the central North Sea. Discovered in 1972, Heimdal facilities were originally designed to process hydrocarbons from Heimdal field and later became a hub for dry gas transport in addition to processing production from several fields. Production from fields was declining, therefore it was decided to decommission Heimdal and install a bypass on seabed linking the gas pipelines. This paper reviews the job sequence to make this operation successful. Heimdal Subsea Bypass (HSB) system allowed existing pipelines to operate without the Heimdal Main Platform (HMP) and Heimdal Riser Platform (HRP) being in place. Parts of the existing pipelines were decommissioned before cut/tie-in to the HSB system. A complex decommissioning operation which involved important pipelines in the Norwegian gas transport system, such as Statpipe (36"x155km), Oseberg Gas Transport (36"x109km), Grane (18"x48km) and Vesterled (32"x361km) was carried out. The operational sequence contained pumping, subsea pigging with radioactive isotopes (RAI) as one of the tracking systems, nitrogen and hydrate mitigation operations and receiving hydrocarbons onboard Draupner. All four pipeline systems: Statpipe (Connected to HMP), Oseberg Gas Transport, Grane and Vesterled (Those three connected to HRP) were cut from the platforms and tied into the HSB system, letting transportation of hydrocarbons to occur totally at subsea level instead of using the previous gas processing platforms center, improving operational efficiency. The operations involved pigging trains with RAI, isolation tool (IT) in some cases and chemical additives such as Mono Ethylene Glycol (MEG), oxygen scavenger, corrosion inhibitor and biocide; same as utilization of Pig Launcher-Receiver (PRL). The HSB system was effectively installed and the decommissioning of the pipelines tied-in to the HMP and HRP successfully completed, allowing the start of the dismantling and recycling of the platforms, which were used as a gas processing center from several fields such as Atla, Huldra, Vale, Valemon and Byggve / Skirne fields, until mid-2023. This paper will discuss how the intercompany collaboration, good communication and engineered solutions, enabled the safe and successful execution of a complex project with decommissioning and commissioning operations, to enable the risers / pipelines to be connected to the subsea bypass spool and continue Norwegian hydrocarbons transportation to deliver to Europe.