p-y based approach to predicting the response of monopile embedded in soft clay under long-term cyclic loading

Offshore wind turbines (OWTs) are subjected to environmental loadings such as those induced by waves or wind typically comprising up to millions of cycles (i.e., high-cycle loading). To evaluate the performance of OWTs during a lifetime cycle, this study proposed a p-y based approach to predict monopile response subjected to a long-term cyclic loading. A cyclic loading may cause an accumulation of strain and a built-up of excess pore water pressure under undrained conditions, accompanied by a corresponding reduction of stiffness and shear strength in soft marine clay. In the proposed procedure, the cyclic degradation and strain accumulation of soft clay is considered by a generalized empirical model given the soil reaction calculated from p-y analysis for a prescribed loading condition on the pile. Afterward, the response of pile subjected to a long-term cyclic loading is predicted by modifying the preselected p-y curve by p-multiplier and y-multiplier to take account for the cyclic degradation and strain accumulation, respectively. The proposed approach is validated by three centrifuge tests on a monopile in soft clay. The simulation results are in good accordance with the measurements in terms of pile displacement and bending moment versus number of applied cycles. Therefore, the proposed approach is feasible to predict the long-term pile response subjected to high-cycle loading. Moreover, it is easily implemented in the widely used p-y approach.