ABSTRACT

As part of the 1st FOWT Comparative Study the hydrodynamic response of a floating offshore wind turbine (FOWT) has been investigated (Ransley et al., 2022). The present work describes the detailed methodology and results obtained using a recently developed solver for the accurate simulation of such problems (Aliyar et al., 2022). The numerical model is based on the coupling of a nonlinear potential flow solver for the incident wave (HOS), a lumped-mass mooring dynamics model (Moordyn) and a CFD code named foamStar, based on OpenFOAM and developed by Centrale Nantes and Bureau Veritas.

INTRODUCTION

Offshore floating wind energy is an important element of the strategy to decarbonise our electricity production. Large wind farms are planned in a number of countries, in a variety of environmental conditions. Engineers need reliable tools for their design in order to lower the levelized cost of energy (LCOE). Offshore engineering has a long tradition and processes are in place to model the hydrodynamic features of large platforms in mild and extreme conditions. These methodologies are presently adapted to the needs of floating offshore wind turbines (FOWT). The first environmental condition to account for is the wave field. The floater needs to withstand different sea conditions to ensure a correct and stable positioning of the wind turbine and maximize its performance. There are two complementary solutions to investigate the response of a FOWT: perform model scale experiments in a wave tank facility or perform numerical simulations. Most of the time, both experimental and numerical approaches are used to characterize floater behaviour. Nowadays, the challenge lies in the numerical modelling of the floater with a high level of accuracy. This problem can be divided into three sub-problems: i) the mooring, ii) the waves, and iii) the interaction of the waves with the floating structure.

Various designs of floaters and turbines were developed in the past 20 years, with the size of the turbine increasing up to 15 or 20 MW to decrease the LCOE. Among the designs, the IEA 15MW reference wind turbine (IEA-15-240-RWT) (Gaertner et al., 2020) and the UMaine VolturnUS-S semi-submersible platform (Allen et al., 2020) were selected as a reference test case for the validation of numerical models in a comparative study proposed by CCP-WSI (Collaborative Computational Project in Wave Structure Interaction). Experiments were performed with a 1:70 scale model in the COAST Laboratory Ocean Basin (35 m long × 15.5 m width) at the University of Plymouth, UK (Ransley et al., 2022). The input data necessary to reproduce the experiments were then distributed to the research groups willing to participate. Low-fidelity methods based on potential flow theory or Morison elements are usually used for FOWT design (Zhang et al., 2020). Indeed, their low computational cost allows the possibility of different parametric study and the coverage of the thousands of needed load cases. However, the use of high-fidelity codes is expanding with the objective of enhanced accuracy in numerical simulations. In the present study, the CFD code foamStar (Li et al., 2019) based on OpenFOAM and co-developed by Centrale Nantes and Bureau Veritas, is used to reproduce the experimental tests. This solver is based on the multi-phase flow solver interDymFoam (Schmode, Bertram, and Tenzer, 2009). Among the methodologies available to generate waves in the OpenFOAM framework, foamStar uses a methodology similar to waves2foam (Jacobsen, Fuhrman, and Fredsøe, 2012), but in our case, we have used a specific wrapper named Grid2Grid (Choi et al., 2023) to ensure the link between the wave model based on the High-Order Spectral (HOS) method (Ducrozet, 2007) and foamStar, the OpenFOAM-based CFD solver.

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