A Numerical Method for Structural Dynamic Analysis of Fixed Offshore Wind Turbines Considering Multi-Field Interaction

With the development trends of large-scale and deep sea for offshore wind turbines, the structures (i.e. blades, tower and foundation) become more flexible increasingly, meanwhile, the wind loadand hydrodynamic loadare stronger. As a consequence, it is necessary to take the effect of multi-filed interaction on the structural dynamics into account when evaluating safety of the wind turbine, which may involve several nonlinear impacts such as the wake influence and the nonlinear deformation of the blade. In this paper, a numerical approach that considers the interactions among wind, structure, and offshore waves is proposed. A nonlinear beam model of the blade is established based on precise geometric beam theory. An unsteady aerodynamic model of the blade is constructed using a combination of nonlinear lift-line theory and free wake vortex methods. A nonlinear aeroelastic model of the wind turbine is developed using a partitioned weak coupling approach. The tower and monopile foundation models are modeled using the Timoshenko beam theory, and hydrodynamic force acted on the foundation is calculated by the Morison method. The proposed models are applied to the DTU 10 MW turbine. The natural frequencies, modes of the blade, tower, and the entire turbine unit, as well as the dynamic responses under uniform inflow and shear wind conditions are analyzed. The results are compared with the OpenFAST software. Good agreement demonstrates the accuracy of the proposed structural dynamics simulation method for large-scale offshore wind turbines.