ABSTRACT

The content of the present paper is the comparison between three different numerical models with the results of the experimental campaign conducted by the University of Plymouth, in the context of the 1st FOWT Comparative Study. Experimental tests were conducted to evaluate the hydrodynamic properties of a 1:70 scaled Volturn US semisubmersible floating platform, supporting a 15 MW offshore reference wind turbine coupled with the mooring system. This paper uses three different software with increasing numerical accuracy: MOST, Orcaflex, and STAR-CCM+. MOST is a numerical model developed by the Politecnico di Torino and implemented in a Matlab-Simscape environment; Orcaflex is a commercial software dedicated to offshore structure analysis, especially used for the design of mooring systems; STAR-CCM+ is commercial software for CFD analysis based on Unsteady Reynolds-Averaged Navier-Stokes equations (URANS). The tests considered for the validation of the experimental tests include free decay tests and focused wave tests. All the experiments are performed considering the 1:70 scaled Volturn US semisubmersible floating platform in its moored configuration. The results show how linear models, if properly calibrated, can provide more than satisfactory results with a low computational cost.

INTRODUCTION

The interest in Floating Offshore Wind Turbines (FOWTs) is growing worldwide as the energy demand is increasing rapidly and the traditional energy sources are both environmentally and economically unsustainable. The floating technology would allow the exploitation of large sea areas at great distances from the coast, where the wind potential is greater and the seabed is too deep for bottom-fixed installations.

Today, many floating offshore wind foundations are available, some more consolidated, such as the spar-buoy or the semi-submersible, while others are still at an earlier level of design and experimentation (Ghigo et al. 2020). The first operative floating wind farm, Hywind Scotland was concluded in 2017, consisting of 5 spar-buoys supporting 6 MW devices for a nominal capacity of 30 MW. Currently, there are several floating wind farm projects, especially in the North Sea, along the Atlantic Ocean coasts and the Mediterranean Sea, but still in the preliminary stage. Among the main obstacles to the diffusion of floating offshore wind is the lack of public data, which can be used to verify and validate numerical models essential for assessing the performance of the various technologies. The most known open-source software is Open-Fast (Jonkman et al. 2005), developed by National Renewable Energy Laboratories (NREL), which is considered the reference software for the simulation of floating offshore wind turbines. The main drawback of OpenFast is the poor readability of the source code, making it difficult to change the code and adapt to the user's different needs. As a result, numerous models have been developed over the last few years, as reported by Farraggiana et al. (2022), for different applications: to investigate the dynamic response of floating wind turbines (Liu et al. 2023), to increase the wind turbine productivity (Cottura et al. 2022), for the optimization of floating foundations (Ojo et al. 2022) and for the implementation of new control systems (Zhang et al. 2022).

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