Seismic response of offshore foundations with emphasis in liquefiable ground conditions

Abstract

Following the rapid expansion of offshore wind farms in seismic areas, this study examines the hurdles encountered when applying conventional seismic evaluation methods, originally devised for onshore structures, to offshore installations. This includes the assessment of liquefaction offshore at large depths and its consequences on the response of offshore wind turbines supported by monopile foundations. With the aid of 3D dynamic finite element analysis of the entire SSI system (tower, monopile foundation and soil domain), it is shown that the resonant frequencies of the examined 5MW turbine were excited for the considered ground motion, inducing significant nonlinearity in the soil surrounding the monopile foundation. The vertical seismic motion, often overlooked in seismic design, is also discussed as it bears significance for the response of offshore wind turbines. Simple site response analysis for vertical ground motion emphasizes the need to consider the entire water column and soil profile depth to the bedrock for an accurate representation of the soil-water system's compression natural frequency in offshore environments.