Evaluation of Motion-Vibration Couples for Semi-Submersible Offshore Floating Wind Turbines

The structural reliability design of floating offshore wind turbines (FOWT) relies on the analyzable and integrated coupled dynamics mechanisms. As the FOWT consisting of complex rigid-flexible coupled substructures is subjected to severe wind, wave and current loads, both accurate and quantitative evaluations of the coupled dynamic performance are challenging. Taking into account these factors, the model of the aero-hydro-servo-elastic FOWT system including a tower with a torsional degree-of-freedom (DOF) has been developed to quantitatively analyze the coupled dynamics. As the Kane method has the ability to efficiently analyze the performances of complex rigid-flexible systems, it has been implemented in the established model. To evaluate the motion-vibration effect of such model, the comparison responses between the rigid and rigid-flexible FOWT structures have been investigated. Results have shown that the sway response of the rigid-flexible FOWT, also called the motion-vibration coupled model, has been reduced by 12.63% as compared with that of the fully rigid FOWT under the steady state wind of 5 m/s and wave height of 1.2 m. Moreover, the discrepancy of the coupled responses for the heave and yaw calculated between two models has almost varied linearly with the environmental loads. Summarily, the present work provides analytical formations for the accurate modeling and quantitative evaluation of coupling effects for the FOWTs subject to harsh marine environments.