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A Multiline Anchor Concept for Floating Offshore Wind Turbines

Floating offshore wind turbines (FOWTs) hold great potential for the renewable energy industry, but capital costs remain high. In efforts to increase FOWT substructure efficiency and reduce costs, this thesis investigates a novel multiline anchor concept in which FOWTs share anchors instead of being moored separately. The goal of this thesis is to evaluate the force dynamics, design, and potential cost reduction of the system. Anchor forces are simulated using the NREL 5 MW reference turbine and OC4-DeepCwind semisubmersible platform, and multiline anchor force is computed as the vector sum of the contributing mooring line tensions. The use of a multiline anchor configuration in large scale farms would result in reductions in the number of anchors approaching 67% and 83% for 3-line and 6-line anchor systems, respectively. Maximum anchor force is up to 16% smaller in 3-line anchor systems and up to 20% larger in 6-line anchor systems, compared to conventional single-line anchor systems. Direction of the multiline anchor force generally aligns with the wind, wave, and current (WWC) direction, and direction reversal within a single force cycle occurs in extreme load cases. Spatial coherence of the wave fields is considered due to the interconnectedness of the system, revealing that spatial correlation of the waves decays to insignificant levels within several hundred meters. Given that FOWT spacing is greater than 500 m, it is asserted that the assumption of independent wave fields at different FOWT locations is sufficient for obtaining multiline anchor load characterizations. The site characteristics of the first and only floating offshore wind farm as of writing this thesis are used to evaluate the multiline anchor concept in the context of a real farm. Average anchor size is similar between the installed single-line and novel multiline anchor layouts, but the multiline system results in a 41% reduction in the total anchor weight required. A capital cost analysis of the mooring lines, anchors, anchor installation, and geotechnical site investigation is carried out over a range of water depths, turbine spacings, and farm sizes, resulting in cost reductions for the multiline anchor system of 8-16% for a 100-turbine wind farm.
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