Mooring systems for floating offshore wind turbines (FOWTs) are designed to resist extreme loads, employing heavy mooring lines and large 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 mooring lines 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 data from 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.
To maintain a 1.5°C global warming pathway, the International Energy Agency (IEA) is calling for a sevenfold increase in global offshore wind capacity by 2030 (GWEC, 2021). Up to 80% of worldwide offshore wind resources are located in water depths greater than 60 m (Burges Salmon, 2021) where traditional fixed-bottom wind turbines are not economically viable. In these deeper waters, offshore wind turbines must be deployed on floating platforms, which are connected to the seabed via mooring lines and anchors to maintain their position. Various fixed and floating offshore wind concepts are shown in Fig. 1.
