NUMERICAL METHOD FOR CALCULATING SYSTEM FUNDAMENTAL FREQUENCIES OF OFFSHORE WIND TURBINES WITH MONOPILE FOUNDATIONS

The restriction of system frequencies due to the "soft-stiff" design criterion is one of the key points for structure and foundation design of offshore wind turbines. This study proposed a practical numerical method for analyzing system fundamental frequencies of offshore wind turbines with monopile foundations. The method was based on the kinematic equation of the damped system, taking into account the pile-soil interaction and the variation of cross-sectional geometry of the wind turbine tower. The method was validated against finite element solutions of an actual wind turbine. Parametric studies were conducted using the proposed method, and showed that the soil elastic modulus, the seabed height, and the pile diameter were major factors affecting the system's fundamental frequencies. However, the fundamental frequencies showed little sensitivity to soil damping and non-uniform distributing of the soil modulus. Furthermore, the method can be used to analyze the system frequency deviation during the operating period, allowing for an effective evaluation of the foundation design, and suggesting engineering measures.