The aerodynamic and hydrodynamic performance of floating offshore wind turbines interact more than that of traditional fixed ones under the influence of structure oscillation and unsteady environments. The details of the aerodynamic performance still remain to be discussed and are challenging to predict accurately. Here included in this paper, the aerodynamic performances of floating offshore wind turbines affected by the hydrodynamic terms in simulation are studied and discussed for more accurate simulation and prediction results. The quasi-steady BEM theory is chosen as the key theory to discuss the effect on the aerodynamic performance of floating offshore wind turbines by their hydrodynamic terms. A series of different formulas for characterizing the calculation of local velocity in the axial and tangential direction are tested to summarize the inherent law of quasi-steady BEM theory. The formulas derived from low-frequency motion seem more reasonable than the classic ones in the simulation based on the quasi-steady assumption.
Wind energy has been occupying an important position of renewable energy around the world. Meanwhile, offshore wind energy was experiencing sustainable growth with an average annual increasing rate of 30% during the last five years. Now, most offshore wind turbines have been installed in shallow waters (less than 50 m). Compared with shallow waters, deep waters may supply much more sites for wind turbines and greater resource with stronger and more consistent wind, also with less turbulence intensity (Jonkman, 2007). However, fixed offshore wind turbines applied in shallow waters may not be suitable for deep waters out of economic consideration. Inspired by the development progression from fixed substructures to floating ones in offshore petroleum industry, floating offshore wind turbines were developed and expected as the future of offshore wind energy.
Although floating offshore wind turbines are evolved from fixed offshore wind turbines, their dynamic performance is quite different from that of fixed offshore wind turbines. In addition to the structure oscillation of tower and blades, also found in fixed offshore wind turbines, floating offshore wind turbines always run with irregular motion of floating substructures attributing to wind, waves and currents. As a result, the aerodynamic and hydrodynamic performance of floating offshore wind turbines interact more than that of fixed offshore wind turbines. On one hand, the motion of a floating substructure induced by waves and currents will change the relative wind velocity on the rotor which affects its aerodynamic performance. On the other hand, the thrust and torque from a rotor induced by wind will influence the motion of the floating substructure which affects its hydrodynamic performance. The interaction brings challenges to the study of floating offshore wind turbines, especially in simulation and prediction of their aerodynamic performance.