Effect of Seabed Trenching on Holding Capacity of Suction Anchors in Deepwater Gulf of Guinea Clays: A Numerical Study

In recent years, there has been an increasing issue of seabed trenches developing around mooring lines attached to suction anchors in West Africa, likely related to relatively large motions of the mooring lines. These trenches are recognised by means of numerical modelling and of centrifuge testing program as significant concern to the in-place holding capacity, the integrity of the anchor and the safety of the floating facility. Conservative trench geometries and complete absence of soil and of soil resistance in front of the anchor above the padeye depth (in the loading direction) were essential assumptions due to lack of observation. Results suggested that a reduction in holding capacity of 20% to 40% for typical mooring line load angles of 30° to 45° can be expected and that the presence of the trench does not affect the portion of holding capacity developed by passive suction at the pile tip. Nevertheless, recent survey data has evidenced a wedge of soil remaining within trenches in front of suction anchors leading to a new problem-solving approach. Consequently, this paper describes a Plaxis 3D Finite Element modelling in a parametric study as a reliable tool for the assessment of the holding capacity of the suction anchors in presence of seabed trench taking into account the presence of residual soil within the trench in front of the anchor pile. Analyses were performed under 30° inclined loading (actual case of taut deep water mooring systems) with various scenarios of trench shapes, padeye depths and anchor geometries in very soft highly plastic Gulf of Guinea clays. Thus, the consideration of the residual wedge of soil in front of the anchor leads to a much less reduction in holding capacity compared to initial studies done with too pessimistic assumptions on the final trench shape. In addition, this study aims at looking at changes in the pile geometry, including the position of the attachment point, to compensate the unavoidable formation of a trench for taut moorings.