Failure envelope of suction caisson anchors subjected to combined loadings in sand

This paper reports the behaviour of caisson anchors under vertical-horizontal (V-H) loadings in medium dense to dense sand. The three dimensional finite element (3D-FE) analyses are carried out using a modified Mohr-Coulomb (MMC) soil model to capture the stress dependent hardening – softening behaviour of sands. The results are validated against centrifuge test data prior to undertaking a detailed parametric study, exploring the relevant range of parameters in terms of caisson aspect ratio, padeye location, mooring angle at the padeye and sand relative density. It is found that the caisson anchor failure is governed by the sand relative density, mooring angle and the normalised padeye position. The padeye position not only affects the anchor capacity, but also influences the rotation angle of the caisson at failure. An optimal padeye position inducing minimal rotation at caisson failure is found to lie within the range of 0.6-0.7L, where L is the caisson length. To assess capacity under inclined loading, normalised failure envelopes are presented in the V-H space. A design framework is established to estimate the optimal padeye location and caisson capacity for sands with strain hardening – strain softening characteristics.