Seismic Behaviour Of Monopile Supported Offshore Wind Turbines In Liquefiable Soil

Abstract

Offshore wind turbine (OWT) produces reliable quantities of renewable energy because of more substantial and stable wind conditions at offshore sites. Monopiles are widely used foundations for OWT structures due to the rapid installation process and cost-effectiveness. The power generation capacity of the wind turbine is proportional to the wind speed and rotor diameter. Hence various multi-megawatt offshore wind turbines i.e., 5 MW, 10 MW and 15 MW are constructed that are having large rotor diameter and slender tower to capture more wind energy. These structures are constructed with lightweight and high-strength material, which results in flexible structure. Owing to growing energy demand, OWT structures are constructed in seismically active areas, subject to seismic loading during its operational period. Several countries, such as the USA, China, India, and South East Asia are in high seismic zones, where the magnitude of M9-class earthquakes may occur. Hence these structures are under probable risk of earthquake. Seismic liquefaction due to strong seismic events caused the failure of various offshore structures. Many offshore sites consist of loose silty sands and sandy silts, which is susceptible to liquefaction during earthquakes. However, investigations on the mechanism and characteristics of liquefaction of seabed and its implication on the offshore structure due to strong seismic motion are limited. OWT is subjected to various loads having a wide frequency band from wind load, wave load, dynamic loads from the rotor, out-of-balance mass of blades, and the load due to blade shielding effect. Hence, it is essential to predict long-term natural frequency to estimate the structure s dynamic amplification factor for the non-liquefied condition, liquefied soil condition, and post-liquefaction condition. However, less attention has been given in this direction.