This contribution to the CCP-WSI FOWT comparative study presents numerical results from three popular potential-flow-based models, including OpenFAST, Orcaflex and WEC-Sim. The subject of the study is the VolturnUS-S semi-submersible platform with the IEA 15MW reference wind turbine. Decay tests and focused waves are simulated in the numerical models, and these are compared to experimental results, using a 1:70 scale model of the system. Comparisons between the numerical models and physical experiments are made. Results of surge motion response and line tension are presented for the decay tests and focused wave tests. The numerical models are seen to underpredict surge response in the low frequency regions of the extreme focused wave test for all three numerical models and of the operational focused wave test for WEC-Sim. Adding an empirically tuned coefficient proportional to the relative velocity between the platform and the water particles in WEC-Sim is seen to improve results in surge motion response significantly, resulting in better agreement than any of the other numerical models. Mooring line tensions are also compared in this study, and an underprediction is observed during the extreme focused wave test for the fore and aft mooring lines, including in the wave-frequency region.


Development of offshore wind continues to increase, due to increasing energy demand, increasing awareness of energy security issues, financial opportunity, and efforts to reduce dependence on fossil fuels. Until recent years, offshore wind farms have only consisted of wind turbines fixed to the seabed via permanent structures such as jackets or monopiles. However, fixed offshore wind turbines become uneconomic in waters deeper than 60m, therefore limiting the available site locations, and ruling out deeper potential site locations with a better wind resource and higher possible capacity factor. Floating offshore wind turbine platforms are being developed to access these sites and capitalise on this opportunity. A myriad of different floating platform designs exist, the majority of which can be categorised into spars, semi-submersibles, tension-leg platforms (TLPs) and barge types. At the point of writing, only two platforms have reached array-scale deployment; the Hywind spar (Equinor, 2023) and the WindFloat platform, which is a semi-submersible (Principle Power, 2023). The UMaine VolturnUS-S platform (Allen et al., 2020), is the subject of the presented study and is an example of a semi-submersible platform design, with three vertical columns providing ballast and stability.

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