Tern

Location and Geology

The Tern field is situated in the North Sea, approximately 167 meters below the water surface. The installation consists of a four-legged, steel jacket substructure anchored by piles to the seabed. An 8-legged Modular Support Frame (MSF) is attached to the jacket, supporting two levels of modules.

Production and Reserves

The Tern field has produced 288 million barrels of oil (MMbbls) out of the estimated 604 MMbbls of Stock Tank Oil Initially In Place (STOIIP), achieving a recovery factor of about 48%. There are estimated to be 6 MMbbls still to be produced, which would add a further 1% to the recovery factor. The primary drive mechanism for oil recovery in the Tern field is water injection, although there is also some aquifer support.

Infrastructure and Operations

The Tern installation serves as a central hub for several nearby fields, including Hudson, Kestrel, Falcon, and Cladhan. Oil from these fields is produced and exported from the Tern installation through a 16-inch subsea pipeline to North Cormorant and then via the Brent Oil Pipeline System to Sullom Voe in the Shetland Islands. Separated gas is distributed among these facilities for use as fuel gas and lift gas, with excess gas exported via subsea pipelines to other facilities.

Decommissioning Plan

The Tern Topside Decommissioning Programme is currently underway, with early planning having commenced and an execution window starting as early as 2024 and expected to be completed by 2028. The programme is supported by an Environmental Appraisal. The early removal of the Tern Topside is designed to minimize the period between the cessation of production and the removal of the topside, enhancing safety and reducing maintenance requirements. The decommissioning will not prejudice any future decommissioning options for the remaining substructure.

Asset Reuse Considerations

TAQA, the operator, has reviewed the Tern asset for potential reuse. Alternatives considered include converting the platform into an offshore renewable energy generation station (wind, wave, or tidal), a marine research station, a training center, or a fish farming site.

Environmental and Safety Aspects

The decommissioning process and ongoing operations are subject to stringent environmental and safety standards. The programme ensures alignment with the Petroleum Act 1998 and involves discussions with relevant stakeholders, including the Western Isles Operator, to manage the disconnection of the Western Leg Gas Pipeline (WLGP) and other pipeline configurations.

Connected Fields and Pipeline Management

The Tern field is connected to several other fields and pipelines. The decommissioning of Tern, along with other TAQA-operated platforms connected to the WLGP, will impact the current pipeline configuration. Discussions are ongoing to determine feasible disconnection options and future arrangements for these services.