Seismic Design of Large Offshore Wind Turbine Considering Rocking Vibration

Abstract

Offshore wind turbines (OWT) are gaining popularity due to the generation of a reliable quantity of renewable energy. Jacket structures are generally used to support the large OWT structures at a water depth of 50–70 m. Offshore wind turbine structures are generally designed as a soft-stiff approach, where the fundamental frequency of OWT structure lies between the rotor frequency (1P) and blade passing frequency (3P). The first mode of vibration of jacket supported OWT structure is most likely to be the rocking mode of vibration due to the relatively lower vertical stiffness of shallow foundations. The natural frequency due to rocking modes of vibration may close to the 1P and wave frequency, which may cause resonance of the OWT structure. This study investigates the effect of both operational and seismic load considering horizontal and vertical seismic motions on the dynamic response of a four-legged jacket supported OWT structure on a pile, installed in a layered sand deposit. A three-dimensional finite element model of the soil-pile-jacket-tower is developed in ABAQUS. A parametric study is carried out to examine the amplification of response at the tower and maximum mudline rotation of monopile due to various aspect ratios of jacket and embedment of the pile.