Integration of mooring system design with floating wind farm layout optimization

Abstract

In this thesis, the integration of mooring design with the layout optimisation of floating offshore wind farms is investigated. The objective of the research is to devise a method to unite the two design processes and then evaluate the benefits of this approach. Further, it will be investigated how to model mooring systems in an optimisation problem, the stage of the wind farm design process at which mooring design can be integrated, the couplings between mooring design, turbine placement and cable routing that exist and must be used to arrive at an improved design, and to what extent the approach improves the design of the floating wind farm.
Floating wind farms are still in a developmental phase, but will be key to meet the energy goals of the future. However, floating wind farm design brings with it challenges that do not exist for bottom-fixed offshore wind farms. Chief among these challenges is the mooring system, which not only has a significant impact on the performance of the turbine, but also due to its potentially large footprint can force alterations to the layout of the inter-array electrical cables with which turbines are connected to each other and the substations. Particular focus in this regard is on catenary mooring systems, given they are common in existing demonstrator projects and have a large footprint.
In this thesis, mooring design will be performed through a multi-objective optimisation using the NSGA-II algorithm, where the objectives will be to minimise the anchor radius as well as the system cost, with constraints to ensure adequate performance in terms of handling motions and loads. The mooring system will be described by 5 design variables, the line length ratio, synthetic fraction, anchor radius, synthetic line diameter, and chain line diameter. The Pareto-optimal solutions from this optimization will then be used to optimize the layout and cable routing using an algorithm based on the work of Cazzaro and property of Vattenfall AB.
It is found that the Pareto-optimal designs have anchor radii ranging from 394 to 494 m, with system costs ranging from \$3.352 million to \$6.441 million. However, with current layout design methods, the turbine placement is done independently of mooring system parameters, and the cable routing is not affected by a change in mooring system. Thus, using the cheapest mooring system regardless of footprint is optimal on a farm level, using current layout design methods.