Design of floating wind parks with shared mooring

The question is no longer if floating wind parks will be developed, but how. There is already some floating wind parks commissioned today, but to be competitive with other energy sources, further development and cost reductions are needed. Traditionally, floating wind turbines are moored with individual lines, and individual- or shared anchors. The main function of a mooring system is to restrict the horizontal motion of the wind turbine and ensure that the power cable is intact. One alternative to conventional mooring systems is shared mooring systems, where the number of mooring lines and anchors can be reduced. This has the potential to reduce costs, material usage and seabed impact. The mooring lines can either be connected directly between the units, or through buoys and anchors. One example of a shared mooring system is from the CYBERLAB project2, with a grid of 3x3 wind turbines where each unit is connected to four mooring lines, the number of lines and anchors can be reduced by 1/3 and up to 2/3 respectively. Shared mooring systems are more complex than conventional systems. Opposed to conventional systems, loads can be transferred directly between units trough the shared line(s). Numerical analysis is an essential part of the design process of floating wind turbines. Normally, only one single unit is included in a numerical model of a conventionally moored wind turbine. For shared systems, several (or all) units in the grid must be included in the numerical model, making the model much heavier with longer simulation time. This places new demands on the efficiency of the analysis models to avoid excessive simulation time, while it is also important that the models are accurate enough. Consequences of line break is one of the most common arguments against shared mooring systems, and it is required that the mooring system is designed to maintain its system keeping functionality also after line break. For conventional systems, it is expected that power production is shut down until the line is replaced, as this reduces the load on the mooring system. As only one single unit is required to shut down, it is considered as economically feasible. For shared mooring systems, it would not be economical feasible to shut down all units until the line is replaced. As described, there are significant differences between conventional- and shared mooring systems, and the current rules and regulations are not proven to be adequate for shared systems. The aim of this thesis is to form a basis for updating- or propose new reliability models for shared mooring systems for floating wind turbines. Focus will be on intact mooring systems with extreme loading and line break cases with accidental loads. In summary, the aim of this doctoral thesis is to examine the following points: Evaluate if the current rules and regulations are adequate for shared mooring systems for floating wind turbines, the focus is on maximum line tensions and bending moments in power cables. Propose new- or updated reliability models for design of shared mooring systems for floating wind turbines. Propose a state-of-the-art numerical modelling- and analysis strategy for preliminary design of shared mooring systems. Propose guidelines for power production in shared systems after accidental line failure.