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

Proxy model for the design of extensible floating offshore wind turbine mooring systems
Oscar Festa1 , Susan Gourvenec1 and Adam Sobey2
1

Department of Civil, Maritime and Environmental Engineering, University of Southampton, Southampton, UK
Marine and Maritime Group, Data-Centric Engineering, The Alan Turing Institute, The British Library, London, UK2ABSTRACT

seabed via mooring lines and anchors to maintain their position. Various
fixed and floating offshore wind concepts are shown in Fig. 1.

Mooring systems for floating offshore wind turbines (FOWTs) aredesigned to resist extreme loads, employing heavy mooring lines andlarge anchors which contribute to high capital expenditure. Incorporating extensible sections to mooring lines reduces peak dynamic
loading, enabling the use of smaller, easier to install and therefore
cheaper anchors. Finite element (F.E.) modelling of extensible mooringlines is time consuming and is not well suited to optimization since
all the mooring line variables must be defined a priori. This paper
presents a quick and versatile approach to exploring the effect of linear
and non-linear stiffness extensible moorings on the FOWT system,
which can be used for automated optimization of one variable for
given constraints in the others. The proxy model was based on datafrom fully-coupled F.E. analyses of the DeepCwind semi-submersible
FOWT. The proxy model is formed of numerical relationships between
environmental loads, fairlead tension, platform surge, and extensible
section design parameters (length L, axial stiffness EA). From these
relationships, the model can determine optimal extensible section
parameters which minimise fairlead tension for a given surge criterion,
or minimise surge for a given maximum tension criterion. When applied
to an example design scenario, results from the proxy model show good
agreement with F.E. results, with a maximum error of 1.2% for linear
stiffness results and 4.3% for non-linear stiffness section properties.

Fig. 1 Fixed and floating offshore wind concepts

Significance of FOWT mooring design

Mooring system design involves striking a balance between stiffness
and compliance. Stiff mooring systems provide high restoring forces
to the platform, while compliant designs reduce loads transferredto the mooring line and anchor at the cost of additional platform
motions. Current FOWT mooring designs are derived from oil and gas
practices and typically employ heavy, inextensible chain catenary lines(QuestFWE, 2020) which provide high stiffness to keep the platform
on station in extreme conditions. FOWT station-keeping requirements
are often more lenient than those of oil and gas installations (Dumont,2016), meaning that more compliant designs could be employed to
avoid over-designed mooring lines and anchors. Avoiding over-design isimportant from an economical perspective, as mooring and anchoring
systems can account for over 15% of overall FOWT costs (Reuters,2021).

To maintain a 1.5°C global warming pathway, the International Energy
Agency (IEA) is calling for a sevenfold increase in global offshore windcapacity by 2030 (GWEC, 2021). Up to 80% of worldwide offshore
wind resources are located in water depths greater than 60 m (BurgesSalmon, 2021) where traditional fixed-bottom wind turbines are noteconomically viable. In these deeper waters, offshore wind turbinesmust be deployed on floating platforms, which are connected to theThis paper presents a method of tailoring mooring designs to projectspecific station-keeping requirements, ensuring an optimal level of compliance in the mooring system. This will reduce the loads transferredto the anchor, enabling smaller anchoring systems that are more costeffective. Such reductions in costs are necessary to improve the scalability of FOWTs, in turn facilitating commercialization and allowing
floating wind energy to play a key role in meeting net zero targets.

KEY WORDS: Floating offshore wind turbines (FOWT); Extensible
moorings; FEA; Proxy modelling; System optimisation.

INTRODUCTION

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