Predicting the ultimate bearing capacity of suction anchors in trenched seabeds for offshore mooring systems

The trenching effects induced by mooring chains significantly compromise the bearing capacity of suction anchors for offshore mooring systems. The unloading effects caused by trench formation lead to stress redistribution in the remaining soil surrounding the suction anchor, altering key soil parameters such as shear strength and thereby affecting the anchor’s ultimate bearing capacity. This study utilizes finite element modeling to evaluate the trench-induced unloading effects. By modifying soil parameters of the trenched seabed based on pre- and post-trenching stress states, the differences in bearing capacity are compared. The results reveal that the trench-induced unloading effect reduces soil strength of the remaining soil, with particular sensitivity to the thickness of the wall-attached soil layer between the anchor sidewall and the trench, the anchor aspect ratio, and the trench width. Furthermore, regardless of whether the unloading effects are considered, the wall-attached soil layer substantially affects the ultimate horizontal bearing capacity of suction anchors in trenched seabeds, contributing to over 10% of the total capacity. Finally, this study develops modification factor formulas (defined as the ratio of ultimate capacities with versus without unloading effects) derived from numerical results. These formulas enable corrected capacity predictions for trenched seabeds where unloading effects were initially neglected.