Henderson, Andrew Raphael; (2000) Analysis tools for large floating offshore wind farms. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The location of multiple wind turbines on large floating structures offshore offers the obvious advantages of no land usage and a probably more reliable wind resource. However, there are potentially significant technical and cost drawbacks. This thesis describes the theory and results of research work aimed at developing analytical tools for evaluating the performance of offshore windfarms. The principal problems that have been addressed here include techniques for determining an optimum hull-form for the floating structure and the development of analytical tools for modelling the resultant turbine loads and fatigue damage. The vessel analysis focuses on developing numerically efficient methods to calculate the motion response of and structural loads on semi-submersible vessels suitable for supporting wind turbines. Therefore, it has been possible to evaluate a large number of candidate vessels and to identify optimised layouts. The vessel motion response is calculated using the Morison equation formulation. Structural loads are also calculated and a class of vessel designs capable of withstanding the extreme and fatigue loads are identified. The optimised design is found by assessing each candidate layout against the two objectives of low motion response (in order to minimise the fatigue loads on the turbine) and of acceptable structural loads (in order to minimise construction costs). The effect that the motion would have on the wind turbine was found by calculating the aerodynamic and inertial loads on the blades in a two-dimensional state-domain, representing the blade and the vessel motion respectively. Using a double Fourier transform, discrete deterministic frequency spectra of the loads are found and the fatigue damage evaluated. Undertaking the calculations for vessel motion in each degree of freedom allowed appropriate weightings to be developed to evaluate the vessel motion response in the vessel optimisation process. The analysis tools developed were tested against other methods and case studies are used to illustrate their application.

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