The 5m-diameter driven anchor piles of Site U FPSO platform in 750m of water off Nigeria are currently the largest piles ever driven at a deepwater site. The use of driven piles was dictated by the soil conditions, with dense sand units interbedded within soft clays and into which suction piles could not penetrate. In the same area, suction piles were used for the Site K FPSO in 1300m of water, thus allowing a relevant comparison of the two anchor pile options. For both cases, the paper provides a summary of the general site characteristics and design methodology, together with comparison of the installation results to predictions.
Best for soft sediments, suction piles have become the preferred option for the mooring systems of floating production storage and offloading (FPSO), or offloading terminal (OLT), structures at deepwater sites offshore west Africa (Colliat et al., 2010). However, they are not suitable for a site where soil layers with high-penetration resistance are present near the seafloor. In relation with geological features, such as pockmarks, gas hydrates, carbonates or mass flow deposits, penetration of suction piles might prove impossible. In this case, driven anchor piles may become the best option of anchoring solutions for this type of site. In the deepwater Nigerian delta, Total Exploration & Production (E&P) Nigeria currently produces oil and gas from two fields located in about 750m (Site U) and 1300m (Site K) of water, with other fields being in their development or exploration phase. The Site K FPSO mooring, with 5m-diameter suction piles, was installed by Saipem in 2008. For Site U FPSO, because of the risk of penetration refusal of suction piles into medium dense sand units present at shallow depth, driven piles with the same diameter were adopted for this mooring.