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Subsea 7 was awarded the EPCI contract for the subsea tie-back of the Alligin field in April 2018. Image courtesy of SUBSEA 7.
The Alligin oil field was discovered in 1995. Image courtesy of BP p.l.c.

Alligin is a satellite oil field being developed within the Greater Schiehallion Area of the North Sea, approximately 140km west of Shetland, UK.

BP holds 50% ownership and is the operator of the field, while the remaining 50% stake is held by Shell.

The UK Oil and Gas Authority (OGA) granted approval for the Alligin oil field development in October 2018. Estimated to cost £230 ($329m), the project targets 20 million barrels of oil-equivalent (Mboe) of recoverable resources in place.

Schedule to come on stream in 2020, the Alligin oil field is expected to produce 12,000 barrels of oil per day (bpd) at peak.

Alligin is one of the two satellite fields BP is currently developing in the UK North Sea. The other field is Vorlich, which is being developed in partnership with Ithaca, in the Central North Sea.

Alligin oil field location and discovery

The Alligin oil field is located in blocks 204/19a and 204/20a under the production license P556,  approximately 20km away from the UK/Faroe median line.

Situated in 475m-deep waters, the field was discovered by the exploration well 204/19-6 in 1995, which encountered oil and gas in the Alligin North fault block. It was followed by appraisal drilling, which further confirmed the presence of oil and gas in the Alligin South fault block.

Alligin oil field development details

The Alligin oil field development plan includes the drilling of one production well and one water injection well both of which will be tied-back to the Glen Lyon floating, production, storage, and offloading (FPSO) vessel located in the Schiehallion and Loyal fields.

New subsea infrastructure to be developed as part of the project includes a 9km-long production flow line from a pipeline end manifold (PLEM) at the Alligin drill center to a new splitter manifold at the existing Loyal drill center. It will also include a 5.5km-long injection water flow-line and a 5.5km lift gas flow line from an existing PLEM at North West drill center of the Schiehallion field to the Alligin field.

A 5.7km-long flexible control umbilical from a new umbilical end termination (UET) at the Alligin Field to an UET near the DUTA D30 drill center of the Schiehallion field will also be installed as part of the project.

The subsea tie-back of the Alligin field will involve six jumpers, nine electrical flying leads (EFL), three fly to place connectors (FTP), and associated Xmas trees.

The development wells of the Alligin field will be drilled with the help of Odfjell Drilling’s ultra-deep-water and harsh environment mobile offshore drilling rig Deepsea Aberdeen.

Glen Lyon FPSO details

The new-build Glen Lyon FPSO, which replaced the aged Schiehallion FPSO, was commissioned in the second half of 2017.

With 21,000t topsides, the harsh water FPSO has a production capacity of 320,000 barrels per day (bpd) of oil and 220 million standard cubic feet per day (Mscfd) of gas, along with 380,000bpd of water injection capacity and one million barrels of crude storage capacity.

The FPSO features one oil processing train, two off-gas processing trains, along with power generation, water treatment, chemical injection, flare gas recovery, and accommodation facilities.

Contractors involved

Subsea 7 was awarded the engineering, procurement, construction, and installation (EPCI) contract for the subsea pipelines for the Alligin development project in April 2018. Subsea 7 carries out offshore works for the project using its reel-lay vessel Seven Oceans.

Dril-Quip was contracted for the wellheads in April 2018, while OneSubsea was contracted for the subsea trees March 2018.

Aker Solutions provides the subsea control system for the project under a contract awarded in April 2018, while AF Global was contracted for the subsea connection system in May 2018.

TechnipFMC is responsible for the supply of flexible jumpers and umbilicals for the Alligin oil field development.