Greater Tortue Ahmeyim project

Greater Tortue Ahmeyim is an offshore liquefied natural gas (LNG) project based on upstream gas production in two km-deep waters on the maritime border of Mauritania and Senegal. It is the deepest offshore project in Africa to date. The LNG project is being jointly developed by BP, Kosmos Energy, Societe des Petroles du Senegal (Petrosen), and Societe Mauritanienne des Hydrocarbures (SMHPM) with BP as the operator. The Greater Tortue Ahmeyim LNG project will produce up to 10 million tonnes of LNG a year (Mtpa).

Artist impression Greater Tortue Ahmeyim project. Picture: BP

The near-shore hub and terminal marine infrastructure are encompassing berthing facilities for a permanently moored FLNG, the loading of LNG Carriers, and an offshore breakwater to shelter those facilities. BigLift Shipping was awarded to provide transportation services for the project by Saipem and by Mc Dermott.

Early 2020 we were awarded by Saipem the shipment of 255 Piles from Taicang, China. The piles, 1.8 and 2.1 m in diameter, varying between 68 – 75 m length and weighing between 120 and 180 mton, are the basis for the foundation of the offshore HUB of the Tortue loading facility. The tonnage to be used had to be the P-8 or P-14 types. With their large hold depth and length, these vessels could carry up to 70 piles per shipment. Some piles were fitted with protrusions, so all piles had to be stored with 200 mm clearance, which also allowed removal and re-installation of the soft slings for lifting. To gain sufficient spreading of the loads, a timber hurdle developed by our engineering team was strapped to the top of the piles with a lashing belt, which also meant the hurdles could be pre-installed before loading.

Safe access was needed to remove the timber hurdles prior to discharge and to re-install the lifting slings. After internal brainstorming sessions with many departments within our group and a Dutch company specialized in safe access at heights, a system of a horizontal zipline, fitted with 2 SRLs (Self Retractable Lifelines) was developed. This line was suspended from towers for all works on deck, and from the coaming edges for all works in the tweendeck and lowerhold. The piles were loaded in tandem lifts over starboard side and each vessel needed about 9 to 10 days to load all piles. All vessels sailed from Taicang, via Singapore for bunkers, around the Cape of Good Hope to the Tortue HUB, a distance of 11,292 nm, covered in abt 36 days. The HUB lies behind a 1000 m long breakwater. Despite swell conditions, discharge could often be achieved as planned. Mooring lines needed constant attention and with the aid of protecting sleeves and exchanging worn out lines, it was possible to maintain safely alongside. A pile was discharged roughly about every 12 hours to a DP2 PSV (Pipe supply vessel). This PSV delivered the piles to a Jackup Barge that upended the piles and drilled them into the seabed. So, every vessel spent between 40 and 55 days at the HUB until all piles had been discharged. A Designated Safety Officer remained at the hub to transfer the knowledge and experience from one vessel to the next.

BigLift has also been involved with another part of the Tortue HUB development: the subsea production system, awarded to McDermott International. The production system ties back approximately 70 km to a spread-moored FPSO positioned at the shelf edge in 100-120 m deep water. The FPSO undertakes gas pre-treatment before exporting the gas 35 km to the above-mentioned nearshore hub where the gas is liquefied and offloaded to LNG carriers. The pipelines coming from the production location are led over a Riser Support Structure (RSS) before going up to the pre-treatment FPSO. The riser support structure consists of two main components: the tower – bottom part, abt 800 mt, 50*27*20 m – and the arch – the top part, abt 400 mt, 35*20*12 m. BigLift was contracted to transport these structures from the McDermott yard in Batam, ID, to Nouakchott, MR, with the Happy Star and to transfer the cargo to two barges going to the offshore location. The shipment further contained four piles of 62 m long to secure the structure to the seabed and about thirty other loose components. The main challenge on this transport was to safely transfer the main structure components to the barges at Nouakchott. The port is exposed and known for a strong, incoming swell. Lifting these large and heavy objects onto a significantly heaving barge is an exacting operation. The lift was executed successfully and safely due to the skill and experience of our personnel on site. With a lead time of three years of preparation in the office, this has been one of the longer running projects executed by BigLift.