Dynamics of Offshore Wind Turbines

The dynamic behavior of offshore wind turbines decides the design of several components, such as bearings, gear boxes, foundation platform and tower. In this paper, an analytical rotordynamic model of offshore wind turbine is developed and effects of several parameters, such as rotor blade size, rotational speed, distribution of mass imbalance, wind loading, wave loading, current speeds, and location of rotor-nacelle assembly are investigated on the dynamic behavior (response and stability) of an offshore wind turbine. It is particularly important to determine the rotor response for minimizing structural resonance and corresponding failure of wind turbine components. The dynamic response prediction is also desirable to address the fatigue design of wind turbine components. The rotordynamic model of the wind turbines is expressed as partial differential equations, with timedependent boundary conditions. These equations of motion are integrated with respect to time for a simple case of rotor imbalance to determine the closed-form solutions of the rotor response and its resonance frequencies, in the absence of wind or wave loadings. The effect of wave and wind loadings is described in the governing equations of motion. The predictions developed through the analytical model for the wind turbines free vibrations are compared with finite element modeling of the wind turbines. The results are compared and the relative merit of using either analytical or finite element modeling is discussed.