Proceedings of the Thirty-second (2022) International Ocean and Polar Engineering Conference
Shanghai, China, June 5–10, 2022
Copyright © 2022 by the International Society of Offshore and Polar Engineers (ISOPE)
ISBN 978-1-880653-81-4; ISSN 1098-6189

www.isope.org

New optimization method of multiple tuned mass dampers installed on offshore wind turbines
for seismic response control
Rui He*, Lijun Pan

College of Harbor, Coastal and Offshore Engineering, Hohai University.
Nanjing, Jiangsu, China

and the amplification effect on horizontal vibration, which provides anew idea for seismic vibration damping of submarine foundation.
However, for the superstructure, the tall and flexible tower is extremely
sensitive to the horizontal seismic input, and the resonance effect willcause up to tenfold acceleration amplification. Seismic control devices
need to be introduced for offshore wind turbines.

ABSTRACT
Large amount parameters make optimizing multiple tuned massdampers' (MTMD) performance difficult. A simplified method of
MTMD optimization is proposed. In this method, tuned mass dampers(TMDs) are used to suppress the peak responses in the frequency domain
by being tuned to the frequencies of the peaks. In the optimization
process, only the mass ratios of the TMDs need to be considered. Finally,
additional viscous dampers are arranged to further optimize MTMD'sperformance. The results show the optimized MTMD is excellent for
OWTs seismic response control. The research also shows the importanceof considering seismic wave's characteristics for MTMD optimization.

Tuned mass dampers (TMD) are widely used in vibration control of civil
construction (Sota and Adeli, 2013), especially suitable for windvibration control. Vibration can be reduced by transferring main
structural vibration energy to the TMD devices. However, its single andnarrow bands control ability limits its application range. For seismic
loads with rich frequencies, the control efficiency of the single tuned
mass damper (STMD) is significantly weakened. To improve its control
ability, plenty of additional viscous damping devices are required, which
violates the original advantages of high efficiency and low cost inpractical engineering. At present, the studies of offshore wind turbines
controlled by STMD are mostly limited to tower installation (Tian, et al.,2019; Sander, et al., 2020). STMD's performance on seismic vibration
control has not been verified by actual earthquakes. The multiple tuned
mass dampers (MTMD) accomplish the control of broadband vibration
by matching multiple TMDs near the resonance frequency, its averagedamping performance in the covered frequency domain is better than thatof STMD. The above feature makes MTMD more in line with theseismic response control. In the process of STMD's parameter
optimization, only three parameters need to be considered (damping ratio,tuning ratio, and mass ratio). In contrast, 3*n parameters need to beconsidered for n-MTMD composed of n TMDs. Most of the studies on
MTMD are aimed at the dynamic characteristics and controlperformance (Jangid, 1995; Kareen and Kline, 1995; Li, 2000; Park andReed, 2001; Yu, et al., 2008; Moon, 2010; Elias, et al., 2016). The current
studies of MTMD's parameter optimization often neglect to consider theMTMD's dynamic characteristics, leading to the enormous amount ofcomputation and difficulty in extending to practical engineering projects.
For OWTs, installation height is also an additional parameter that needs
to be considered. 4*n parameters make MTMD optimization harder toimplement. So far, the application of MTMD to OWTs is only in its
infancy (Dinh and Basu, 2015; Hussan, et al., 2017; Zuo, et al., 2017;
Chen, et al., 2021). There are fewer studies on MTMD's parameter
optimization for OWTs seismic control.

KEY WORDS: Offshore wind turbines; seismic excitation; vibration
control; multiple tuned mass dampers; single tuned mass damper.
INTRODUCTION
In the context of traditional energy's decreasing supply and demand ratio,
the mature offshore wind power industry will play a more critical role in
the future energy structure. Except for the design loads (wind, waves,and currents), offshore wind turbines near zones with frequent seismic
occurrence and high seismic intensity need additional consideration ofseismic dynamic safety. Low-frequency and high-intensity earthquakes
will cause severe resonance damage to the wind turbines (see Fig. 1),
which will result in a large-scale structural catastrophe of the entire windfarm.
The dynamic safety characteristics of offshore wind turbines underearthquakes have always been a hot research topic. The substructure
approach divides the structure response into the lower pile-soil
interaction part and the upper wind turbine dynamic response part(Richart et al., 1970; Clough and Penzien, 2003; Kausel, 2010), which
can conveniently and effectively solve the complex problems inearthquake engineering. Recent studies have shown that the structural
response to vertical seismic excitation cannot be ignored (Kjørlaug andKaynia (2015); De Risi et al., 2018; Kaynia, 2019; Ali et al., 2020),
especially in the pile-soil interaction part. He et al. (2020 2021) studied
the dynamic response of monopile and seabed to vertical and obliqueseismic waves through a rigorous semi-analytical method and found forthe first time that seawater has the damping effect on vertical vibration

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