An Investigation into the Effect of Long-Term Cyclic Loading on the Natural Frequency of Offshore Wind Turbines

This paper presents a numerical study of the natural frequency of a monopile-supported offshore wind turbine in sand subjected to long-term cyclic loading. The numerical model is based on the characteristics of the soil-monopile interaction under long-term cyclic lateral loading observed in well-designed laboratory model tests, and the free vibration method was adopted to estimate the natural frequency. The study shows that the simulation, without taking into account the effect of long-term cyclic loading, yielded a natural frequency that is 8.39% greater than the measured value. A parametric study was then conducted to evaluate the effects of several key design parameters on the natural frequency. Results indicate that Young's modulus and the depth of subsidence of the soil around the monopile have a significant influence on the natural frequency of the wind turbine. An important implication of the parametric study is that the current practice of accounting for +/- 10% variation in the natural frequency may not be suitable for offshore wind turbines.