ABSTRACT

This work forms a part of the contribution to the "1st FOWT comparative study" for modelling floating offshore wind turbine(FOWT) hydrodynamics based on the wave tank tests of a 1:70 scaled moored VolturnUS semi-submersible floater model. Two sets of test cases have been simulated - one is a static equilibrium test and the other is the heave decay test to determine the natural frequencies of the coupled floater-mooring system. The numerical modelling is carried out in a numerical wave tank using high-fidelity computational fluid dynamics based on the OpenFOAM toolbox and the results are generally in good agreement with the experimental measurements. In addition, it was found that the mooring lines have a significant influence on the pitch natural frequencies of the coupled system but their impact on the heave natural frequencies was minimal.

INTRODUCTION

Numerical tools have been widely adopted for predicting the hydrodynamic responses of floating offshore wind turbines (FOWTs) (Cheng et al. (2019); Otter et al. (2022)). Generally, the mid-fidelity potential flow method and high-fidelity Computational Fluid Dynamics (CFD) method are the most commonly used approaches for FOWTs analysis (Zhou et al. (2021); Wang et al. (2020)). However, the potential flow models would underestimate the responses of semi-submersible FOWTs under certain wave conditions (Wang et al. (2021)). On the other hand, the CFD models would allow us to solve the fluid flow governing equations directly, i.e., Navier-Stokes equations, and produce the required flow quantities in both the time and spatial domains so that the transient hydrodynamic loading on the FOWTs, can all be well resolved, including the effects from the extreme waves, fluid viscosity and wave-current interactions.

The Offshore Code Comparison, Collaboration, Continued, with Correlation (OC5) project led by National Renewable Energy Laboratory (NREL) validated and verified a wide range of mid-fidelity engineering-level models by using experimental data (AN Robertson et al. (2017)). However, it was shown that those mid-fidelity tools significantly underestimated the second-order difference-frequency hydrodynamic loads. This deficiency was mainly due to their limitations in capturing all the nonlinearities and viscous damping effects as demonstrated in the Offshore Code Comparison Collaboration, Continued, with Correlation, and Uncertainty (OC6) project (Wang et al. (2021)). In the OC6 project, work has also been conducted through a series of validation studies by using high-fidelity CFD models with tank test results. The outcome of the project showed that CFD models are able to provide more accurate results at low-frequency nonlinear excitation.

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