Monopiles subjected to uni- and multi-lateral cyclic loading

Offshore wind turbines are subjected to significant environmental loads from a combination of current, wind and wave action. Under such conditions, the directions of these environmental loads vary over the service life of the structure and therefore the cyclic lateral loading on the foundation also changes direction. The work reported in this paper examines the effects of multi-directional loading on the performance of offshore wind turbine monopile foundations. Tests were carried out in a model sand bed and a mobile loading platform was manufactured to apply loading on the pile in various directions. Tests were also carried out where the cyclic loading was applied under both one-way and two-way loading. The observations indicate significant differences in the stiffness of monopiles between uni-directional and multi-directional lateral cyclic loading. Multi-directional lateral cyclic loading generally results in higher displacements and lower stiffness compared to uni-directional loading, most likely due to shear deformation of a larger volume of soil mass adjacent to the pile.