Phase one of the Greater Tortue Ahmeyim project includes four ultra-deepwater subsea production wells. Credit: Vismar UK/
The Greater Tortue Ahmeyim FPSO is equipped with eight processing and production modules. Credit: BP.
Gas processed at the FPSO will be transferred by pipelines to a nearshore FLNG facility. Credit: BP.
First gas from the Greater Tortue Ahmeyim LNG project is expected in 2023. Credit: BP.

The Greater Tortue Ahmeyim (GTA) is an offshore liquefied natural gas (LNG) project on the maritime border of Mauritania and Senegal. It involves one of the deepest subsea developments in Africa.

The project is being jointly developed by BP, Societe des Petroles du Senegal (Petrosen), Societe Mauritanienne des Hydrocarbures (SMH), and Kosmos Energy, with BP as the operator.

Mauritania and Senegal signed an inter-government co-operation agreement (ICA) in February 2018, which provided for the development of the cross-border gas project with each country sharing resources and revenues on a 50/50 basis. The presidents of both countries granted GTA the status of ‘National Project of Strategic Importance’ in July 2021.

The final investment decision (FID) for phase one development of the project was reached in December 2018, with the start of production expected in the last quarter of 2023.

The project is expected to produce 2.3 million tonnes per annum (mtpa) of LNG in phase one. BP and partners also announced the development concept for phase two of the offshore LNG project in February 2023, which is expected to produce up to 3mtpa of LNG upon commissioning.

Greater Tortue Ahmeyim field location and reserves

The GTA field is spread across the C-8 block off the shore of Mauritania and the Saint-Louis Profond offshore block in Senegal. The field is located 120km offshore at a water depth of 2,850m.

The project is estimated to contain up to 15 trillion cubic feet (Tcf) of recoverable gas resources.

Tortue field discovery and appraisal details

The Tortue gas field was discovered by drilling the Tortue-1 exploration well in 2015. The GTA field complex was subsequently appraised by drilling the Guembeul-1 and Ahmeyim-2 wells in 2016.

The Greater Tortue Ahmeyim-1 (GTA-1) appraisal well was drilled on the eastern anticline of the Greater Tortue unit in 2019. Drilled to a total depth of 4,884m, the appraisal well encountered 30m of net gas pay in the high-quality Albian reservoir.

GTA phase one development details

Phase one of the offshore LNG project will include an ultra-deepwater subsea system with four gas production wells, a mid-water floating production, storage and offloading (FPSO) vessel, and a nearshore floating liquefied natural gas (FLNG) facility.

Each of the four subsea wells will produce up to 200 million metric standard cubic feet per day (Mmscfd) of gas, which will be transported via an 80km subsea tieback to the FPSO for processing. The condensate separated from the gas will be offloaded from the FPSO, with the processed gas transferred to the FLNG facility through a 35km export pipeline system.

The LNG output will be exported to international markets by tankers, with a portion of the processed gas planned to be allocated to the two host countries for domestic consumption.

Greater Tortue Ahmeyim FPSO and FLNG details

The FPSO for the project set sail from the COSCO Qidong Shipyard, China, in January 2023. Scheduled to arrive at the project site in the second quarter of 2023, the FPSO will be moored 80km offshore at a water depth of 120m. It will accommodate up to 140 onboard during normal operations.

The FPSO is made of more than 81,000t of steel, 37km of pipe spools, and 1,520km of cable. Equipped with eight processing and production modules, it will have the capacity to process up to 500Mmscfd of gas. The main purpose of the FPSO is to remove condensate, water, and other impurities from the field’s gas stream.

The FLNG facility will be located approximately 10km offshore on the Mauritania-Senegal maritime border in 30m-deep waters. It will be designed to produce 2.3mtpa of LNG.

A 125,000m³ Gimi LNG carrier has been converted into the FLNG facility for the project at Keppel Shipyard in Singapore. It is expected to set sail to the project site in the second quarter of 2023.

The FLNG will be used for the project under a 20-year lease and operation agreement (LOA) signed between BP and Gimi MS, a subsidiary of Golar LNG, in February 2019.

Contractors involved

Technip Energies was awarded the engineering, procurement, construction, installation, and commissioning (EPCIC) contract for the GTA FPSO in 2019.

Eiffage Genie Civil Marine and Saipem were awarded the engineering, procurement, construction, and installation (EPCI) contract for the nearshore FLNG terminal facility in the same year.

McDermott and Baker Hughes were awarded the subsea umbilicals, risers and flowlines (SURF) and subsea production system (SPS) equipment contracts for the project in November 2019.

Keppel was engaged by Golar LNG to perform the FLNG conversion works.