Concrete Breakwater for the Greater Tortue Ahmeyim Project for BP in Mauritania and Senegal

During summer 2017, EIFFAGE GENIE CIVIL MARINE was invited by BP to bid for the construction of a concrete caisson breakwater protecting an offshore LNG liquefaction floating terminal which will be located 10 km west from Saint Louis, at 33 m water depth on the Mauritanian / Senegalese maritime border. The basic design as originally proposed by BP was composed of 18 rectangular concrete caissons laying on an underwater rumble-mound foundation. Dimensions of the concrete boxes were approximatively 63,5 meters long, 32 meters wide and 30 meters high, on a design consisting of multi rectangular cells (128 units/caisson) EIFFAGE GENIE CIVIL MARINE answered to the ITB by proposing under an EPC basis an alternate caisson shape optimizing drastically the concrete quantities. Alternate caisson dimensions were 54,5 m long, 28 m width, 35m height including a 4 m high crest wall at the top against extreme waves overtopping. The geometry of the caisson has been changed to a 10 lobes caisson. The weight of each caisson is around 16 000 tons. As a result of the subsequent FEED studies performed from April 2018 to February 2019, including 3D model testing in basin, Eiffage was able to reduce the amount of concrete required by 40 % compared to the first design, leading to both financial as well as environmental benefits. Execution plan involving Mauritania and Senegal This infrastructure offers key local content components for this gas field development in each country, something Eiffage had key experience with thanks to an historical presence in the the region : Eiffage started its activities in Senegal more than 100 years ago. The original execution plan as proposed by Eiffage for the EPC phase signed in February 2019 was to build the 21 concrete caissons in Dakar-Senegal. A dedicated yard of 12 hectares has been reclaimed besides the actual port of Dakar providing a safe direct access to the sea and the required water depth for the caissons towing. As a result, at peak more than 600 jobs will be created in Senegal, with the reclaimed land for the fabrication yard left as a legacy for the port area. The underwater rubble mound requires 2 million tons of quarry material. EIFFAGE’s proposal was to produce, transport and load from Mauritania those materials. A specific logistics scheme between the quarry and vessels loading point has been developed including a bypass road around Nouakchott. The transport of the quarry materials requires the mobilization of many trucks. A special safety mitigation plan including dedicated training is being implemented in order to reduce identified risks linked to road transportation.