The paper presents the methodology adopted for the challenging geotechnical design of suction anchors with small aspect ratio of a FPSO installed in May/June 2016 in deepwater Gulf of Guinea. The design needed to cope with anomalous soil conditions characterized by the presence of shallow hard grounds in soft normally consolidated clay deposits. Pile length was limited to mitigate the risk of penetration refusal due to the presence of hard grounds. This required a large pile diameter leading to a small length to diameter ratio (6.5 m diameter with 10.5 m penetration) which is unusual if compared to typical aspect ratios adopted in deepwater environments.
The project site is located in Angolan Deep Offshore, approximately 350 km north-northwest of Luanda and 130 km west of Soyo. Water depth around the development area ranges between 500 m and 600 m. The development consists of a Subsea Production System (SPS) tied back through Subsea Umbilical, Flow Lines and Riser systems (SURF) to a Floating Production Storage and Offloading (FPSO) unit containing the process, storage and offloading facilities. The FPSO is located in about 450 m water depth and is anchored to the seabed by 9 anchors arranged in 3 clusters of 3 lines each, with an angle of approximately 120° between each cluster and approximately 5° between each leg of the individual clusters. The FPSO has a Design Operating Life of 20 years.
The subject of this paper is the geotechnical design of the FPSO suction anchors. The geotechnical design of the anchors meets the requirements of API-RP-2SK (2005). The analyses include advanced laboratory testing interpretation, buried chain configuration, optimization of padeye position, holding capacity calculation, installation and removal analyses and soil reactions for structural design.
The holding capacity is assessed first using CAISSON_VHM program (Kay, 2010; Palix et al., 2010) considering the effect of installation tolerances (tilt and misorientation). The ultimate vertical and horizontal holding capacities are then verified by means of 3D PLAXIS finite element analyses confirming the optimal padeye position under design inclined loading. Soil reactions against the anchor wall for the operational case are derived from the 3D numerical analyses and approximated with a combination of linear trends of shear and normal stresses in equilibrium with the external loads. Finally, the study presents the monitored installation data with an interesting comparison between the predicted and the recorded field results.
