Effect of monopile-soil interaction on performance of negative stiffness-assisted compliant liquid dampers-inerter designed for offshore wind turbine towers

Previous research has shown that compliant liquid damper-inerter (CLDI) systems, especially with a negative stiffness (NS) element, effectively reduce vibrations of monopile-supported offshore wind turbine towers (MOWTTs) exposed to multiple hazards, including earthquakes, wind, and waves. These studies assumed the monopile to be fixed at the base. However, in reality, dynamic interaction occurs between the monopile and soil under dynamic loading, which affects the damper system's performance. This study explores the impact of monopile-soil interaction (MSI) on the control effectiveness of a negative stiffness-assisted compliant liquid damper-inerter (NS-CLDI) designed for an MOWTT. The equations of motion are developed for the combined MOWTT-damper system. A 5-MW NREL MOWTT is selected as the case structure for the numerical analysis. Soil parameters are obtained from relevant literature. One hundred earthquake ground motion records, scaled to 0.1 g, and five wind-wave loadings are used to create a multi-hazard scenario. The coupling between the monopile and soil is modelled using distributed springs representing the soil stiffness. Results reveal that MSI increases both displacement and acceleration responses of the uncontrolled tower. During the damper design, the peak seismic responses and root mean square wind-wave responses serve as objective functions (OFs). These OFs are minimized using a genetic algorithm-based multi-objective optimization framework. Two sets of optimal NS-CLDI parameters are identified: one for controlling side-side (SS) vibrations, and another for fore-aft (FA) vibrations. The control effectiveness of NS-CLDI is evaluated considering MSI under multi-hazard conditions in both time and frequency domains. While NS-CLDI can significantly reduce vibrations in both SS and FA directions, neglecting MSI during damper design leads to a serious underestimation of MOWTT responses with NS-CLDI.