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

A Novel Design of Hybrid Mooring Systems for Floating Structure in Shallow Water
Huiqu Fan a,b, Xiaolei Liu b,c, Sheng Ding b, Quanming Miao a,daChina Ship Scientific Research Center, Jiangsu, China, bOswind Energy (Shanghai) Technology Co.,Ltd. Shanghai, China
cShanghai Maritime University, Shanghai, China, dJiangsu Ocean University, Jiangsu, China

2050, then the annual market opportunity for manufacturers of windturbines with other clean energy type grows tenfold to USD 1.2 trillion
by 2050(IEA,2021).Wind energy, as the most commercialized renewableenergy source, has correspondingly shown great appeal.

ABSTRACT
Floating offshore wind turbine (FOWT) has the characteristics of easy
installation and disassembly, more stable and stronger wind resources,
and less visual and noise pollution. Compared with the traditional
offshore engineering floating platform in oil& gas industry, the lower
limit of the possible water depth for FOWT operation is much smaller,
and there is FOWT application case with a water depth of around 30
meters in China. When designing a FOWT in shallow water, its mooring
system will encounter significant safety and economic challenges. For
example, the catenary mooring system requires a huge mooring footprint
radius, while the tensioned mooring will withstand great dynamic tension.
The above two mooring methods will greatly increase the mooring costand seriously hinder the commercial development of shallow-water
floating wind. In this paper, a "low-tension hybrid mooring (LTHM)"
system is proposed. By arranging buoys and weight at intervals on themooring line, a flexible mooring system with load bufferingcharacteristics is formed, so that it has a designable horizontal stiffness.
The natural frequency of the system is far away from the wave frequency,
which greatly reduces the dynamic loads from the order of wave
frequency (WF) magnitude to the low frequency (LF) load, thereby
reducing the whole mooring system cost. It should be pointed out that
the LTHM system proposed in this paper can also be applied to otherfloating structures, especially in shallow water conditions. Two
demonstration cases of different floater types are presented in the paper
to show its favorable feature in shallow water.

At this stage, the main use of wind energy is still on land. However, thedevelopment of onshore wind turbine is subject to more and more
restrictions. For example, the distance between the energy development
center and the consumption center is too far. With the development ofhigh-quality wind energy resources, the visual and noise pollution of
onshore wind turbine has gradually attracted the attention of residents
and the government. Therefore, wind turbine development is graduallyshifting from onshore to offshore. Offshore wind turbine can be generally
divided into fixed and Floating offshore wind turbine. Compared with
fixed offshore wind turbine, in China, Floating offshore wind turbine has
the following advantages: 1) At present, fixed wind turbine can only be
used within a range of less than 50 meters, and FOWT is suitable for awide range of water depths, which can be used in both shallow water anddeep water; 2) Fixed wind turbine is mainly formed by piling as a fixed
foundation. When the offshore distance is large, the installation cost
increases sharply, while the continental shelf along the coast of China isrelatively smooth, and the water depth is still not large in places dozens
of kilometers offshore. The installation of wind turbine has become an
important factor limiting its development, and the Floating offshore windturbine can be hoisted at the yard and towed to the target sea area to be
attached to the mooring system. 3) With the continuous development ofdomestic fixed offshore wind turbine in the past two years, there arefewer and fewer available sea areas suitable for piling operations. In
addition, the overall offshore distance of China deep-water seas isrelatively long, the cost of power transmission is extremely high, and theeffective power conversion technology is not yet mature. Therefore, it isurgent to carry out fully research on shallow-water FOWT with shortoffshore distance in order to meet the increasing energy demand causedby social development.

KEY WORDS: Shallow water mooring; Floating offshore wind turbine;
Low tension hybrid mooring.

INTRODUCTION
As the source of power for the industrial and economic development of
human society, the development and utilization of energy has always
been the most important research section. In recent decades, theexcessive development and use of fossil energy that cannot beregenerated by human beings has caused significant environmental
pollution and climate change problems, which are becoming more andmore serious. Therefore, the development and utilization technology ofclean renewable energy has attracted more and more attention fromdomestic and foreign industry and academia. The International Energy
Agency predicts that if the world gets on track for net zero emissions by

Challenge of shallow water mooring
For moderate or deep-water mooring systems, typical mooring line types
are catenary, semi-tensioned, or tensioned moorings(Ma et al., 2019).
Deepwater mooring system design is relatively straightforward. For
shallow water mooring systems, especially long-term or permanentmoorings, when the mooring system needs be able to withstand large
environmental conditions, its design faces great technical challenges.
The difficulty in the design of shallow water mooring is that if the159