The effect of strain softening on the development of anchor chain trenching

The development of trenches in the vicinity of mooring anchors can have a significant impact on the anchor performance. These trenches can develop due to the action of the anchor chain in the vicinity of and within the seabed, the properties of the soil and other processes that influence the interaction. This paper investigates the effect of strain softening of the soil on the seabed trenching due to cyclic chain movements through large deformation finite element (LDFE) analyses. The global chain system response during cyclic loading and the local behaviour of selected chain segments are compared between cases with various soil sensitivities. Soil remoulding (i.e., strength degradation with accumulated shear strain) is found as a driving factor that exacerbates the extent of a developing trench (including the tunnel portion) and its growth rate (per load cycle). The soil resistance profiles derived from these analyses provide good reflections of the chain kinematics during cycling and the chain-soil interaction. The resistance-transition depth for uplifting resistance profiles can be used as part of an interpretation of the potential extents that trenches could develop to.