Development and verification of a dynamic analysis model for floating offshore contra-rotating vertical-axis wind turbine

Vertical axis wind turbines (VAWT) have a lower center of mass because the mechanical systems are placed at the bottom of the tower. This leads to more stable motions in the offshore environment However, the effect of torque leads to continuous loading of the tower, constant platform motions, and increased mooring tension. For this reason, a contra-rotating VAWT is adopted to mitigate the structural loads and platform motions induced by the counterbalanced torque produced by the two rotors. Initially we developed and validated an integrated analysis tool for floating VAWTs and extended to account for the contra-rotating rotor. Finally, the loads and motions of the contra-rotating VAWT are assessed using the developed tool. The results indicated that the contra-rotating VAWT had better structural loads and, in case of the floating model, the platform motions were smaller, and the mooring tension was lower. A land-based contra-rotating VAWT with blade length and rotor radius greater than 48.5 m and 54 m can achieve better performance compared to conventional VAWT with 80 m and 39 m respectively. Consequently, notwithstanding the reduced aerodynamic power, the decreased structural responses of the contra-rotating VAWT imply that its fatigue loads, would be lower compared to those of the conventional VAWT.