Numerical investigations of the effect of strain softening on the behaviour of embedded mooring chains

Abstract

Mooring systems typically consist of an anchor and a mooring line and chain that connect the anchor to the floating infrastructure. When the anchor connection point (the ‘padeye’) is below the seabed surface, the interaction between the chain and the seabed will affect the amount of load transferred to the anchor and the load angle at the padeye. Reliable methods are needed therefore to assess these aspects in order to determine appropriate anchor design.Available solutions for the interaction between soil and chain generally ignore any reduction in the undrained shear strength of the soil as it is remoulded under the large strains associated with tensioning of the anchor chain. This is an unconservative assumption for anchor design, hence providing motivation for the study presented here. The system behaviour and the interaction of short chain segments with the seabed have been studied using a coupled Eulerian-Lagrangian (CEL) approach. The findings have led to two new design approaches that encapsulate how remoulding of the soil (which affects sliding resistance more than bearing resistance) affects the chain system response. Calculations using these methods captured the modelled chain system response well. Both the global chain analyses and the proposed design approaches suggest that approximately the entire chain load at the seabed surface (the ‘mudline’) is likely to be transferred to the anchor padeye, challenging conventional design practice.