Dynamic soil-structure interaction issues of offshore wind turbines

One of the important design drivers for offshore wind turbine (OWT) structures is the fatigue life. In order for such structures to make worthwhile investments, they need to be in operation for 20-30 years after installation. The wind turbine and the foundation are subject to fatigue damage from environmental loading (wind, waves) as well as from cyclic loading imposed through the rotational frequency (1P) through mass and aerodynamic imbalances and from the blade passing frequency (3P) of the wind turbine. Through dynamic amplification and resonance, the fatigue damage suffered by the structure can severely increase if the natural frequency of the wind turbine gets close to the frequency of excitation, thereby reducing the service lifetime of the OWT. Therefore, predicting the first natural frequency is of paramount importance. In this paper a mechanical and mathematical model is presented, which provides a good initial estimate of the natural frequency of OWTs for conceptual design. The soil-structure interaction (SSI) is modelled through a set of springs, which also includes the crosscoupling between the lateral and rotational stiffness of the foundation. Approximate analytical formulae are given to approximate the natural frequency. The results are compared to measured data as well as results from similar software. The sensitivity of the natural frequency of the structure to the stiffness parameters of the foundation are analysed and discussed.