Vibration Control of Offshore Wind Turbine Towers

Abstract

In this study, the dynamic response of a Offshore Wind Turbine (OWT) tower under wind loading is investigated adopting the vibration absorption concepts. Application of vibration control systems (VCS) to OWT towers has the potential to significantly improve the dynamics of the tower and the nacelle, increasing its reliability. The first vibration mitigation approach is the classical tuned mass damped (TMD) and it is used as benchmark. In the TMD approach an additional mass is installed inside the nacelle at the top of the OWT tower. Usually, a large additional mass is required to achieve significant effective damping, which may lead to undesirable second order effects and ultimately increase the tower displacement. In the second approach, a passive vibration absorption concept is implemented, namely the Extended KDamper (EKD). The KDamper device is a special mechanical configuration consisting of an additional mass, an artificial damper and both positive and negative stiffness elements. The OWT is subjected to extreme environmental loading such as the wind and sea waves. These OWT loadings, are usually calculated based on stochastic interpolation of meteorological data or by artificial time-histories. In this study, the Blade Element Momentum (BEM) theory is taken into consideration in order to produce the wind load time histories. The improvement on the dynamic response of the OWT tower using vibration control systems is illustrated. Moreover, this research shown that the EKD presents a significant increase of the damping ratio with a minimal value of added mass at the top of the tower.