Analysis Of Wind And Wave Loads On Offshore Wind Turbine Towers

Wind and wave loads are the main external effects on offshore wind turbine towers. Offshore wind turbine towers are in general of great height and small stiffness. The wind load applied on the wind turbine tower may not only lead to a great overturning moment at its foundation, but also cause vibration of the upper structure. On the other hand, wind and wave loads are cycling loads. Long period cycling application of the loads may induce fatigue effect on the tower bottom and its foundation. Study of the computational method, effects and combination form of wind and wave loads on offshore wind turbine towers is absolutely necessary.In this paper, the finite element model of a certain 1.5MW offshore wind turbine tower is established. The wind load time history is obtained from the harmonic superposition method. Based on the structural dynamics and finite element analysis theories, wind induced responses of the upper tower structure are analyzed, and then the wind-induced vibration coefficients and equivalent wind loads of the structure are determined by using the MGLF (Moment Gust Loading Factor) method. Utilizing the linear wave superposition method and the Morison equation, the formula for determination of wave forces on the tower are derived. The total load and its dynamic responses of offshore wind turbine tower under the combination of wind and wave effects are examined by comprehensive analysis of wind and wave combination mechanism. Finally, fatigue load spectrum of the tower is computed with the time-domain rainflow method by taking into account different wind velocities. Then the equivalent fatigue loads are determined.The analysis results indicate that the displacement at the tower top and the responses at the tower bottom increase as the wind speed increases. The analysis results indicate that the displacement at the tower top and the responses at the tower bottom increase as the wind speed increases. The tower bottom bending moments and the tower top acceleration under wind-induced dynamic loads should be used as dominating parameter for the design of the tower. With the joint effect of wind and wave, the superposition of maximum wave loads and random wind loads can be generally considered as the critical load case of load effect combination.