Comparative study on the dynamic responses of monopile and jacket-supported offshore wind turbines considering the pile-soil interaction in transitional waters

In this study, simplified finite element models for monopile and jacket offshore wind turbines, referred to as MOWT and JOWT, respectively, were established for dynamic response analysis using the X-SEA program, based on the National Renewable Energy Laboratory's 5 MW OWT model. First, using five design constraints, an optimization design was conducted for the substructures and foundations of both MOWT and JOWT located in transitional waters to ensure a fair comparison of the dynamic performances of the two structures. Next, putting special attention to the natural frequency, tower top displacement and member stress, two kinds of OWT models with fixed and flexible bases subjected to combined wind, wave and current loads were analyzed using the pile-soil interaction module of X-SEA to further explore the influence of the foundation flexibility on dynamic responses. Subsequently, a parametric study was conducted to evaluate the sensitivity of the dynamic responses to the changes in environmental conditions, soil properties and pile embedded lengths. Finally, a comparison of the dynamic performances, parameter sensitivities, and total masses of the MOWT and JOWT was performed to determine the optimal fixed OWT support structure for the 50 m water depth of the southwest sea area of South Korea.