A Large Deformation Random Finite-Element Study on the Pull-Out Capacity of Two Typical Offshore Anchors

In deep to ultra-deep water developments, various types of offshore anchor provide technically feasible and economically attractive options for mooring systems. Amongst them are the novel self-penetrating torpedo anchor and conventional plate anchor. Subjected to environmental loading, both these types of anchors experience combined vertical (V) and horizontal (H) loading. Moreover, mobilizing their full potentials or maximum pull-out capacities in soft sediments often render soil in the close proximity severely deformed and distorted. It is thus implied that large deformation analyses are warranted to accurately estimate the VH pull-out capacities. Although finite element calculations have been widely reported in the literature to determine the pull-out capacity of torpedo and plate anchors, most of them are small-strain finite element studies. More significantly, the spatial variability or random heterogeneity is rarely taken into account, while there is increasing recognition that the inherent spatial variability of seabed soils can decrease the overall capacity of offshore foundations. This article reports a three-dimensional random finite element study on the pull-out capacities of torpedo and plate anchors, with a view to providing rational interpretation of pull-out mechanism in a spatially variable soil and probabilistic prediction of pull-out capacities. The analysis results show that the soil flow mechanisms over the course of torpedo and plate anchors extraction are apparently affected by the spatial variability of soil. The pull-out capacities in both vertical and horizontal directions are distributed in fairly wide range. Findings in this research may be helpful mainly in that it shed light on the significance of spatial variability on the offshore anchors.

Keywords: anchor, combined loading, spatial variability, Coupled Eulerian-Lagrangian method.