Assessment of Near-Field Strong Ground Motion Effects on Offshore Wind Turbines Resting on Liquefiable Soils Using Fully Coupled Nonlinear Dynamic Analysis

Design and construction of offshore wind turbines (OWT) in near-fault sites is more in demand worldwide. However, very limited studies are available that assess the effects of near-field ground motions on the response of OWTs. This paper describes a numerical study on the response of OWTs to pulse-like near-field earthquakes in liquefiable soils by performing a series of advanced fully coupled nonlinear dynamic analyses using FLAC-3D. The simple anisotropic sand (SANISAND) constitutive model was adopted for soil to consider liquefaction. Two types of foundations commonly used for OWTs, namely the monopile and suction bucket were modeled. One of the characteristics of near-field ground motions is containing a significant vertical component, which is mostly in the range of high frequencies. Meanwhile, OWTs can be sensitive to vertical ground motion in terms of having a high natural frequency in the vertical direction. Therefore, focusing on the frequency content of input seismic motion, a comparison was made between the response of OWT structures to horizontal and vertical components of earthquakes in the far-field and near-field conditions. The mechanisms behind the obtained results were elaborated, and some suggestions were made for the design of OWTs under near-field conditions. According to the results, liquefaction and tilt are the major potential risks associated with the near-field earthquakes in the horizontal direction. However, in the vertical direction, the excessive amplification of acceleration and upthrow of the rotor-nacelle-assembly (RNA), are the major issues to be considered. Under near-field conditions, a suction bucket foundation can be a better option in terms of lower amplification factor of vertical acceleration, although its aspect ratio should be selected with special care to the rotation response.