Tripod suction caisson foundations for offshore wind energy and their monotonic and cyclic responses in silty sand: Numerical predictions for centrifuge model tests

Bucket foundations have been increasingly used to support offshore wind turbines as alternatives to monopiles and can be classified into two types: monopod and tripod/jacket supported on multiple shallow foundations. Despite the recent research on the bearing capacity and stiffness of skirted foundations, knowledge regarding the monotonic and cyclic responses of multiple bucket foundation systems in saturated sand is scarce. In this study, the angular rotation of mono-bucket and tripod foundation systems in dense sand due to drained monotonic and cyclic loading was analyzed by performing a series of three-dimensional finite element (FE) analyses. The Hardening Soil Model with Small Strain Stiffness (HS small) and the UBC3D-PLM soil model were employed to analyze the moment response of offshore foundations subjected to wind and wave loading. The procedures were validated against a database of well-documented centrifuge tests. Calibration was carried out based on the monotonic and cyclic model tests, in-situ shear wave velocity measurements, and empirical relationships for shear moduli. Long-term deformations and the resulting decreasing trend in accumulated rotation, which is a unique feature of tripod suction caisson foundations (the so-called “self-healing’’ mechanism), uncertainties, and nonlinearities in centrifuge tests and numerical predictions involving different alternative calibration scenarios of the models are discussed. Based on the numerical results, a closed-form expression is proposed that enables the prediction of drained bearing capacity of multiple caissons under combined loading. This expression was validated for a range of embedment ratios.