Numerical investigation on rotation accumulation and natural frequency degradation for offshore wind turbine in clays

Prolonged lateral cyclic loading leads to soil stiffness degradation around offshore wind turbine (OWT) foundations, which reduces the system's natural frequency and increases the accumulation of foundation rotation angle. Proper evaluation of natural frequency and rotation angle is crucial for the design of OWT foundations. This study develops a two-stages numerical approach to calculate the fundamental frequency of OWT systems considering the foundation stiffness degradation by integrating a stiffness degradation model of soft clays with a simplified three-spring model. Subsequently, it investigates the evolution of accumulated rotation and natural frequency for three foundation types—monopile, monopod bucket, and hybrid monopile-bucket—throughout their service life. It is observed that the hybrid foundation shows the smallest rotation in the first cycle, attributable to its relatively high initial stiffness compared to the other two foundations with the same steel consumption. However, it also exhibits the highest rate of cumulative rotation growth. At the same load level, the monopod bucket foundation exhibits the largest cumulative rotation angle due to its lower bearing capacity, and the degradation of natural frequency is most pronounced for monopod bucket OWT. For all three foundation configurations, increasing either the pile diameter or the bucket diameter is the most effective approach to reduce the cumulative rotation angle and improve natural frequency degradation, while maintaining the same steel consumption. These findings should be considered in the design of OWT foundations.