Innovative Mooring Systems

Many concepts for floating offshore wind, as for instance spar buoys or semi-submersibles, relies on a catenary based mooring system. Typical catenary systems for the oil and gas industry has been developed for water depths ranging from about 100 meters to several thousand meters. Floating offshore wind has been proposed for water depths of 50 meters and below. The typical catenary systems consist of chain and possibly wire and clump weights. Using a catenary mooring for low water depths is challenging due to small catenary effect and a limited possibility for using clump weights. This implies a need for heavy chain especially when including offset criteria implied by power cable restrictions. The cost of the mooring system for a floating offshore wind unit is significant, and a reduction in this cost will be important for making floating offshore wind commercially viable. The current project has studied several novel designs for mooring of floating offshore wind structures. The study was based on the results from a previous research project studying the OO-Star Wind Floater, where some novel systems were proposed, but not studied in detail. Several new ideas have been identified after the OO-Star Wind Floater project was completed and this project has studied a number of alternative systems. The OO-Star Wind Floater, with a 6 MW turbine, will be used as a basis for the development of new systems, but the results will be valid for other types of semi-submersible substructures as well as spar buoys and other moored floating structures in relatively shallow water. Hence, this project will also be valuable for industries other than offshore wind. The project was executed with two phases. The first phase was a conceptual phase where many components and configurations was studied. In the second phase a small number of systems was studied in more detail. At the end of the second phase a cost efficient concept was proposed. The new concept shows a significant cost reduction potential compared to catenary systems. Examples of other possible applications of the new mooring systems is oil & gas, floating harbours and residential buildings and floating wave Power.

Bottom fixed wind turbines are today used in shallow waters, up to 40-50 m water depth. Suitable shallow water sites are limited in number and there is a need to advance into deeper waters with new solutions. For this reason there is a growing interest for floating offshore wind. The current study propose several novel designs for mooring systems suitable for floating offshore wind structures. The study will be based on the results from the OO Star Wind Floater development (ref. RCN Project No 225946), where some novel systems were proposed, but not studied in detail. Several new ideas have been identified after the OO Star project was completed and this current study will evaluate a number of alternative systems. The OO Star Wind Floater, with a 6 MW turbine, will be used as a basis for the development of new systems, but the results will be valid for other types of semi-submersible substructures as well as spar buoys. The proposed systems can also be relevant for other moored floating structures in relatively shallow water. Hence, this project will also be valuable for other industries than offshore wind. Other relevant applications could be oil & gas, floating wave power, offshore fish farms, floating harbours and residential buildings. The fully coupled software 3DFloat will be used for documenting and evaluating the proposed systems throughout the project. The software will also be further developed to ensure that all types of mooring systems can be modelled and analysed. This will make 3Dfloat suitable for analysing a greater number of mooring systems. It is, for instance, planned to include new elements capable of capturing the non-linear effects of synthetic lines, with varying stiffness and hysteresis effects. It is also planned to develop and implement devices for load cut-off and damping in the software.