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

Design and modelling of a constant tension mooring system for a 5-MW semi-submersible wind turbine
Yuefeng Chu1, Kai Wang1,2*, Shuo Huang1,2
1. School of Ocean Engineering and Technology, Sun Yat-Sen University;
2. Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai)
Zhuhai, China

accounts for 20% - 30% of the total investment cost of the project, as acomparison, for offshore oil and gas platforms, it accounts for only 2%(Fitzgerald, 2009). Therefore, optimizing the mooring system is of greatsignificance to promote the commercialization of offshore floating windpower.
The application of existing mooring design guidelines orspecifications for offshore oil and gas platforms to the offshore floating
wind power industry is not a simple process and can lead to overly
conservative and expensive designs (Weller et al., 2013). The maindifferences between the two are operating water depth, design natural
frequency, structure size, allowable range of motion, mooring systemtrajectory and number of mooring lines. Although failure of the mooring
system can lead to catastrophic loss of floating wind turbines, theconsequences are usually less severe than those for oil and gas platforms
because floating turbines are unattended and do not cause casualties(except during installation and maintenance) (Christensen et al., 2005;
Xu et al., 2019).
The mooring scheme commonly used in marine engineering is thecatenary mooring system. In extreme sea conditions, the suspension
mooring line of a conventional catenary mooring system is quicklystraightened. If the floating body is still retreating with the waves and themooring line has been straightened, the huge kinetic energy generated by
the motions of the floating body cannot be consumed, and theconsequence is that either anchor dragging or chain breaking.
Subsequently, the floating body loses its restraint and may cause
catastrophic losses. By increasing the chain diameter, increasing thenumber of mooring lines, and improving the anchor structure, thereby
increasing the mooring force capacity (Ma, 2013; D'Souza and Majhi,2013), such solutions increase the capacity of the mooring system at ahuge mooring cost, but cannot radically reduce the possibility of themooring system being destroyed in a storm or even a typhoon (Kvitrud,2014). Some of the non-self-propelled floating platforms alleviate thelack of elasticity by increasing the mooring lines with greater elasticity,or adding clump weight or float attachments in the mooring system(Huang et al., 2018). Nevertheless, as the distance of floating body
motion increases, the effective buffering capacity of its elastic link isABSTRACT
In order to improve the reliability of mooring system of floating
wind turbine under extreme sea conditions, a constant tension mooring
system was proposed in this paper. In extreme sea conditions, when themooring force reaches the set value, the mooring system automaticallyreleases the weights stored on the floating body, and the floating bodymoves along the waves with a basically constant mooring force. Throughcoupled time-domain simulations, the dynamic responses of 5-MW-CSC
semi-submersible wind turbine with the constant tension mooring systemwere studied under the extreme conditions with 50-year return period.

KEY WORDS: Offshore floating wind turbines; mooring system;semi-submersible platform; 5-MW wind turbines; extreme conditions;constant tension.
INTRODUCTION
Offshore floating wind power is maturing and has significantly
reduced costs over the past few years. Compared with fixed offshore
wind power, floating wind power is still in the stage of small-scale
development. With the rapid maturity of technology and the continuous
emergence of demonstration projects, it is expected to realize large-scalecommercial development by 2030. According to the prediction of theEuropean Wind Energy Association, the global installed capacity offloating wind power will reach 15 GW by the end of 2030. It isconservatively estimated that the cost of floating units will decrease by
38% by 2050. At that time, 150 GW of offshore floating wind power will
be developed in Europe, which means that one-third of offshore windpower will be floating wind power in deep sea.
Mooring and anchoring technology will be the key research anddevelopment field of floating wind power. A well-designed mooring
system can effectively maintain the safety and stability of wind turbine
in extreme environmental conditions such as typhoon and tsunami. In theactual project, the mooring system of offshore floating wind turbine344