Response of Monopile Supported Offshore Wind Turbine in Liquefied Soil

Abstract

Offshore wind turbines (OWT) are the potential source of renewable energy. Monopile is a common choice as foundation for OWT, since this type of foundation has proved to be economical at shallow water depth and is designed for 25–30 years. These structures are designed as soft–stiff approach, where the fundamental frequency of soil–monopile–tower system is placed between the rotor frequency (1P) and blade passing frequency (3P for 3 bladed turbines). Design guidelines suggest that the fundamental frequency of the OWT system shall be at least 10% away from the 1P and 3P frequencies. Previous studies mostly focus on the behaviour of these structures under long-term cyclic loads. Limited studies are available on the behaviour of OWT structures in liquefied soil when subjected to earthquake loading. Soil surrounding the monopile may liquefy during earthquake which affects the dynamic response of OWT structures. In this study, dynamic behaviour of OWT structure in a liquefied soil is examined using numerical model in open-source code, OpenSees with aid of OpenSeePL. In this study, monopile is embedded in saturated silty sand with 60 m depth soil profile is considered. Spectrum consistent strong motion accelerogram is generated for various earthquake moment magnitudes. The dynamic analysis is carried out in time domain, and response of the OWT system is examined for various diameter and length of monopile, and magnitude of earthquake. Finally, design implications are suggested.