Long-term deformations of monopile foundations for offshore wind turbines studied with a high-cycle accumulation model

A parametric study on the long-term deformations of monopile foundations for offshore wind turbines is presented. The finite element calculations of a monopile in fine sand were performed with a high-cycle accumulation (HCA) model. The results are analyzed with respect to the horizontal displacements, the tilting of the pile head, settlement of the ground surface and effective stress changes in the surrounding soil. The dimensions of the pile (diameter, length of embedding, wall thickness), the initial density of the sand and the cyclic loading (average value, amplitude, lever arm) have been varied. The influence of the drainage conditions (either fully drained or partially drained) is also discussed. The validity of Miner's rule is checked for both types of drainage conditions. Furthermore, the effect of changes of the direction of the cycles and of a multidimensional cyclic loading is addressed. The interesting phenomenon of a self-healing, i.e. a re-erection of the deformed monopile after a storm event, is demonstrated based on simulations with small cycles following a large preloading. Simulations with homogeneous and stochastically fluctuating fields of void ratio are compared.