Offshore wind turbine monopiles under real wind and wave conditions: Considering cyclic softening of clays

In marine environments, monopiles of offshore wind turbines (OWTs) are often subjected to horizontal cyclic loads caused by wind and wave. Due to cyclic loading, the stiffness and strength of clays degrades, further affecting the serviceability limit state of OWTs. This study developed a 3D finite difference model for the OWTmonopile-seabed system under real wind and wave conditions. A nonlinear cyclic constitutive model was demonstrated to be effective in describing the degradation of strength and stiffness of the soft clay under irregular cyclic loading. The monopile-clay interaction was validated against a centrifuge model test in term of the load-displacement relationship. The aerodynamic wind loading is calculated using the Kaimal spectrum, and the wave loading is computed using the JONSWAP wave spectrum and Morison equation. It was found that the peak the horizontal displacement and rotation angle of the monopile at the mudline were primarily influenced by the amplitude and combination of wind and wave loads and were also significantly affected by the pile-soil interaction related to the cyclic softening of clays. The cumulative peak horizontal displacement of monopiles caused by wind and wave underscores the importance of continuous structural health monitoring and risk assessment of OWTs.