Evaluation of Seabed Trenches Based on Floater Hydrodynamic Calculation

Abstract Inspections in the Gulf of Guinea indicated that a large number of seabed trenches appeared in front of suction anchors. The trenches had an adverse impact on the anchor capacity, which attracted wide attention from industry and academia. Currently, reliable methods need to be developed for accurately predicting the trench formation and development, for the benefit of both marine oil & gas resources and offshore wind. This paper proposes a procedure to evaluate trenches. The motion of semi-submersible floating structures is acquired under extreme environmental conditions to obtain the trajectory of platform fairlead. Then, three-dimensional (3-D) profile is evaluated according to the fairlead motion. Finally, some measures to reduce the trench dimensions are proposed by changing the mooring systems. The results indicate that the seabed trenches are easier to form in the soft clay due to its lower shear strength. The mooring line is hard to cut into sand, thus trench is unlikely to appear in sand according to the proposed method. Besides, trench formation is also related to mooring line tension and configuration. Increasing the mooring line length can reduce the disturbing soil regions, thus inhibiting the trench formation. Other methods, e.g., changing the mooring line components, can also change the disturbed soil region. This paper presents a design procedure from the hydrodynamic calculation of floater to the trench evaluation in sand and clay, which provides some useful guidance to taut and semi-taut mooring system design for marine resource exploitations.