OC6 Phase Ia: CFD Simulations of the Free-Decay Motion of the DeepCwind Semisubmersible

Abstract

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.

Validation of CFD determined hydrodynamic coefficients for a semisubmersible floating offshore wind turbine

“…Further, the snappyHexMesh tool refined the mesh closer to the floater. The refinement zones are consistent with the recommendations by Wang et al [13]. Figure 2 displays the very fine resolution near the corner of the heave plates and the bottom of the main column to capture the flow separation phenomenon.…”

“…Heave and pitch free decay simulations of the floater are carried out to determine the natural periods and damping. The results are validated with the experimental data, as a part of the OC6 project [13]. The details of heave and pitch free decay tests are specified in Table 3.…”

“…All the simulations are carried out at a 1:50 model scale. The dimensions of the rectangular fluid domain are defined according to the specifications mentioned by Wang et al [13]. The boundary conditions, along with the boundary locations specified in OpenFOAM, are described in Table 1.…”

“…Figure 2 displays the very fine resolution near the corner of the heave plates and the bottom of the main column to capture the flow separation phenomenon. In addition, prism layers were added to capture the wall boundary layer [13].…”

“…The details of heave and pitch free decay tests are specified in Table 3. Load cases 1.1 and 1.2 correspond to load cases 4.4 and 4.6 in [13]. The damping ratio ζ is calculated using logarithmic decrement δ from the response amplitude decay plot.…”