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.

Numerical Investigations into Development of Seabed Trenching in Semitaut Moorings

“…For simplicity, the 'deep' ultimate resistance was adopted for the normalization even for the shallow part of the chain. In softening soil with sensitivity St = 5, the post-peak steady-state normal resistance (for monotonic chain movement in deep conditions) was 0.68Qult (Sun et al 2019b) and this will theoretically reduce to 0.2Qult for fully remolded conditions. Lower resistance will be obtained near the seabed surface or as gapping develops.…”

“…The approach has been shown (e.g. Kim et al 2015;Sun et al 2019a;Zhao and Liu 2016) to capture chain-soil interaction during tensioning of an embedded chain, including incorporation of strain softening to capture soil remolding during chain movement (Sun et al 2019b). In the analyses presented here, only in-plane cyclic motions of chain in undrained conditions are considered, without consideration of lateral excursions of the mooring line.…”

“…During pretensioning, the chain tension at the anchor padeye, Ta, is only slightly smaller than the mudline tension Tm. The small difference (Ta/Tm ≈ 0.97) reflects the low mobilization of frictional chain resistance because of the combined effects of high mobilization of normal resistance over most of the chain(Sun et al 2019a) and the greater reduction in frictional resistance than normal resistance during remolding(Sun et al 2019b). For the subsequent cyclic loading, Ta is within 1% of Tm.…”