Seabed trenching near the mooring anchor: History cases and numerical studies

Abstract

The floating platform with taut or semi-taut mooring systems is the main option for oil and gas exploitations in deep sea. However, it has been reported that the reciprocating motion of the mooring line tends to induce the seabed trench near the anchor, which consequently reduces the anchor capacity. In this paper, two parts of work have been carried out to enhance the understanding of the seabed trench. Firstly, the dimensions of reported trenches in the field as well as their formation conditions are summarized, and four feature points are proposed to describe the trench profile. Secondly, a numerical method considering mooring line dynamics to predict the final trench profiles is proposed, which contains two simulation modules, i.e. mooring line dynamics and seabed trenching. The method is verified using the data from the field and centrifuge tests. The influences of the mooring line tension and soil strength on trench profiles are comprehensively investigated further. The results show that obvious trenches are likely to form when the amplitude of mooring line tension variation is large and the soil shear strength is low. The main impact of the formed trenches is that the loading direction at the pad-eye tends to be horizontal.