Seismic Response of Monopile-Supported OWT Structure Considering Effect of Long-Term Cyclic Loading

Abstract

Offshore wind turbine (OWT) structures are subject to wave, wind, and seismic loading. Due to the cyclic nature of these loads, OWT foundations can be vulnerable to cumulative deformation and liquefaction triggered by waves and earthquakes. The effects of cumulative deformation and liquefaction on the monopile-supported OWT are not fully appreciated. This paper develops a three-dimensional numerical model for analyzing the seismic performance of large monopile-supported OWT under the long-term effect of cyclic loading. The numerical model was established employing FLAC3D and utilizing SANISAND constitutive model to simulate the soil behavior. The numerical model was validated by comparing its predictions with the results of dynamic triaxial tests and centrifuge tests. A simplified densification and subsidence site model was integrated into the numerical model to facilitate considering the long-term effect of cyclic loading. The numerical model was then used to conduct a comprehensive study to evaluate the influence of long-term cyclic loading on the natural frequency and seismic response of OWT structure. The results demonstrated that the densified subsidence zone around monopile increased the liquefaction resistance. However, the horizontal displacement of pile and the response acceleration of tower-top increased due to soil subsidence around monopile.