Mechanistic model for dynamic soil-structure interaction and natural frequency assessment of monopile offshore wind turbine structures

Offshore wind turbines have a potential to be alternative sustainable sources of energy. The structures of offshore wind turbine are dynamically sensitive and must be designed so that the natural frequency of the structures is kept away from the frequencies of the excitation to avoid the dynamic amplification and fatigue damage. This paper presents a mechanistic model for offshore wind turbines with monopile foundations, including the effects of soil-structure interaction to evaluate the natural frequencies of wind turbines. The dynamic model of wind turbine structure is based on the Euler-Bernoulli beam theory with varying stiffness and elastic-end support. The varying stiffness beam model can be used to consider offshore wind turbines with non-uniform cross-sections such as a wind turbine with tapered tubular tower. The interaction of the wind turbine structure with the foundation is represented by a set of springs, i.e., the lateral, rotational, and the cross coupling stiffness springs. The proposed model is used to evaluate the natural frequencies of offshore wind turbines with parameters reported in the literature. The influence of soil-structure interaction on the natural frequency of the wind turbine system is investigated.