Effects of higher wave harmonics on the response of monopile type offshore wind turbines



Offshore wind turbines (OWTs) are exposed to vibration-induced forces throughout their operational lives that may cause a catastrophic failure unless resonance is avoided by proper stiffness design. The standard design procedure for the OWTs is such that the first structural frequency should be far away from the first wave frequency to eliminate resonance. In this study, a three-bladed 5 MW monopile type OWT was first designed according to guidelines at a site located south of Massachusetts with a water depth of 25 m. Then, the effects of the higher wave harmonics on dynamic response of OWTs were investigated. Along this line, different combinations of structural and wave frequencies were considered, and it was found out that overturning moment, which is the most important design parameter, may increase as much as 45% as a result resonance of structure with higher wave harmonics (i.e., the second and third harmonics). Copyright © 2012 John Wiley & Sons, Ltd.