Seismic vibration control of monopile supported offshore wind turbines by tuned mass dampers considering seabed liquefaction

Abstract

Seismic vibrations and site liquefaction are of great concern in offshore wind engineering, as many offshore wind turbines (OWTs) have been constructed in seismically active areas with sandy seabed. Previous studies have reported that tuned mass dampers (TMDs) with precise tuning are an effective means of controlling seismic excessive vibrations of OWTs. However, liquefaction can change the natural frequencies of OWTs, and the potential failure of tuning-sensitive TMDs is not fully appreciated. The aim of this study is to investigate the vibration control of OWTs by single/dual TMDs under the co-actions of earthquakes and seabed liquefaction. Firstly, a nonlinear model is established to simulate the seismic response of a monopile supported 5 MW OWT in sandy sites, and the effect of liquefaction on the soil-structure interaction (SSI) is considered. In addition to the observed significant threat of strong earthquakes to the safety service, liquefaction can result in a notable reduction in the first two natural frequencies of the OWT. TMDs are believed to mitigate the negative impacts of potential liquefaction. Secondly, a linear elastic two-degrees-of-freedom (2DOFs) analytical model of OWT-TMD is presented to optimize the TMDs, and the effect of liquefaction on the OWT is simplified as frequency drops of the first two eigenfrequencies. The 2DOFs linear theory is chosen due to its high efficiency and wide applicability. Finally, the feasibility of the proposed method is validated, and it is found that the dual TMDs optimized based on the proposed linear theory are effective and robust for OWT’s nonlinear seismic vibration with possible soil liquefaction.