Currently, the design of floating offshore wind systems is primarily based on mid-fidelity models with empirical drag forces. The tuning of the model coefficients requires data from either experiments or high-fidelity simulations. As part of the OC6 (Offshore Code Comparison Collaboration, Continued, with Correlation, and unCertainty (OC6) is a project under the International Energy Agency Wind Task 30 framework) project, the present investigation explores the latter option. A verification and validation study of computational fluid dynamics (CFD) models of the DeepCwind semisubmersible undergoing free-decay motion is performed. Several institutions provided CFD results for validation against the OC6 experimental campaign. The objective is to evaluate whether the CFD setups of the participants can provide valid estimates of the hydrodynamic damping coefficients needed by mid-fidelity models. The linear and quadratic damping coefficients and the equivalent damping ratio are chosen as metrics for validation. Large numerical uncertainties are estimated for the linear and quadratic damping coefficients; however, the equivalent damping ratios are more consistently predicted with lower uncertainty. Some difference is observed between the experimental and CFD surge-decay motion, which is caused by mechanical damping not considered in the simulations that likely originated from the mooring setup, including a Coulomb-friction-type force. Overall, the simulations and the experiment show reasonable agreement, thus demonstrating the feasibility of using CFD simulations to tune mid-fidelity models.

The present work tests the performance of an efficient boundary condition called generating-absorbing boundary condition (GABC) for free-surface waves and studies its effect on computational efficiency. Numerical simulations of a semisubmersible substructure are performed with two different boundary conditions with low spurious reflection: relaxation zone and GABC. These simulations are conducted for regular sea states in OpenFOAM and have been validated using experimental data obtained from the OC6 campaign. The numerical results with relaxation zone boundary condition and GABC agreed well with the experimental results. Using GABC also resulted in an almost 15% reduction in computational time compared to the conventional approaches.

The hydrodynamic characteristics are crucial for accurately analysing floating offshore wind systems. In this paper, the added mass and damping coefficients of a semisubmersible floater are examined around the natural periods of the surge, heave, and pitch motion, using computational fluid dynamics (CFD). The OpenFOAM CFD setup is validated against experimental measurements from the free decay tests, and the same setup is used to determine the hydrodynamic coefficients of the platform subjected to forced motions with different amplitudes and periods. The added mass and quadratic damping coefficients obtained from forced oscillations are consistent with the free decay results. Moreover, the added mass coefficients obtained by CFD is significantly higher than the estimations of the potential flow theory: around 10% larger for surge and 22% larger for heave. The damping is almost independent of the frequency while it varies with the motion amplitude. The deviations in the CFD results from the potential flow theory are due to the viscous effects. Besides, viscous damping is dependent on the drag coefficient specified in the Morison’s equation.