Assessment of Seafloor Trench for Suction Pile Moorings: a Field Case

Abstract

Abstract Seafloor trenching is recognized as potential threat for reduced holding capacity of suction piles for mooring facilities. Trench formation due to chain movements occurring in typical deep water soft soil conditions offshore West Africa has been identified by the industry. Trench heights approaching lug level have been recorded. This paper describes an assessment to account for chain trench development in front of a suction pile for in-place holding capacity. The paper identifies the change in failure mechanism due to presence of a trench. Consequently, a downwards shift of optimum lug level is recommended to account for possible trench formation. Trench formation may adversely influence reverse end bearing capacity, which is generally relied upon for moored configurations. Reduction of the horizontal capacity is found to range from 20% for "fixed head" conditions, to about 32% for "free head" conditions, depending on the selected lug level and presence of global scour. Based on finite element analyses and a yield envelope framework, an assessment of trenching effects on suction piles capacity is presented in this paper. Future suction pile designs could be optimized by considering the presented methodology.