We numerically simulate the hydrodynamic response of a floating offshore wind turbine (FOWT) using CFD. The FOWT under consideration is a slack-moored 1:70 scale model of the UMaine VolturnUS-S semi-submersible platform. This set-up has been experimentally tested in the COAST Laboratory Ocean Basin at the University of Plymouth, UK. The test cases under consideration are (i) static equilibrium load cases, (ii) free decay tests and (iii) two focused wave cases with different wave steepness. The FOWT is modelled using a two-phase Navier-Stokes solver inside the OpenFOAM-v2006 framework. The catenary mooring is computed by dynamically solving the equations of motion for an elastic cable using the MoodyCore solver. The results of the static and decay tests are compared to the experimental values with only minor differences in motions and mooring forces. The focused wave cases are also shown to be in good agreement with measurements. The use of a one-way fluid-mooring coupling results in slightly higher mooring forces, but does not influence the motion response of the FOWT significantly.


Offshore wind is a rapidly expanding industry. In 2019, 146 offshore wind farms with a total of 27.2 GW installed power were in operation globally (WFO 2020). Specifically, floating offshore wind turbines (FOWTs) have entered the commercialization phase. Outside Scotland the 30 MW Hywind farm has been in operation since 2017 using a spar-type design. The 27 MW Windfloat Atlantic was commissioned in 2020 18 km of the Portuguese coast, and the 50MW Kincardine floating offshore wind farm became operational in 2021 in Scottish waters. Both of the latter projects use Principle Power's Windfloat semi-submersible. Still, many new concepts of FOWT are under development, and, in addition, the ever growing turbine size is putting new demands on the floaters.

Numerical modelling of FOWT are typically done with aero-elasto-control-hydro-mooring software, e.g., OpenFAST (OpenFast 2023) and DeepLines Wind (Principia 2023). Focusing on the hydrodynamic modelling these models are based on standard linear potential flow (LPF) assumptions and approximations. The mooring modelling is usually of dynamic type using lumped masses or finite element methods. Models based on LPF are computationally efficient but loose accuracy for survival cases with highly nonlinear waves. Additionally, within the OC5 projects problems relating to low-frequency response were identified when using LPF models, see Wang et al. (2021). High-fidelity models overcome the above problems, but the computational cost is quite high.

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