Abstract
Current mid-fidelity modeling approaches for floating offshore wind turbines (FOWTs) have been found to underpredict the nonlinear, low-frequency wave excitation and the response of semisubmersible FOWTs. To examine the cause of this underprediction, the OC6 project is using computational fluid dynamics (CFD) tools to investigate the wave loads on the OC5-DeepCwind semisubmersible, with a focus on the nonlinear difference-frequency excitation. This paper focuses on assessing the uncertainty of the CFD predictions from simulations of the semisubmersible in a fixed condition under bichromatic wave loading and on establishing confidence in the results for use in improving mid-fidelity models. The uncertainty for the nonlinear wave excitation is found to be acceptable but larger than that for the wave-frequency excitation, with the spatial discretization error being the dominant contributor. Further, unwanted free waves at the difference frequency have been identified in the CFD solution. A wave-splitting and wave load-correction procedure are presented to remove the contamination from the free waves in the results. A preliminary comparison to second-order potential-flow theory shows that the CFD model predicted significantly higher difference-frequency wave excitations, especially in surge, suggesting that the CFD results can be used to better calibrate the mid-fidelity tools.
Highlights
Recent years have seen a rapid advancement in offshore wind technologies
It is still not feasible to perform extensive parametric studies and optimization exclusively with high-fidelity simulations, such as computational fluid dynamics (CFD), because of their prohibitive computational cost; the success of the offshore wind industry relies on researchers identifying and addressing the limitations of mid-fidelity, engineering-level tools, such as OpenFAST, developed by the National Renewable Energy Laboratory (NREL) [1]
One major limitation of engineering-level tools commonly used by floating offshore wind turbines (FOWTs) designers identified in OC5 and OC6 is the significant underprediction of the low-frequency, nonlinear wave excitation and response of semisubmersible FOWTs [2,3]
Summary
Recent years have seen a rapid advancement in offshore wind technologies. fixed-bottom systems are still dominant, floating offshore wind turbines (FOWTs) are being trialed, with several precommercial projects recently deployed or under development. The present investigation, differs from prior CFD efforts by placing a special focus on the nonlinear difference-frequency wave excitation This leads to a unique set of challenges resulting from the smallness of the nonlinear loads, which can be two orders of magnitude lower than the direct (linear) wave excitation. Preliminary comparison based on the bichromatic-wave case studied shows that the nonlinear, 4 of 25 difference-frequency wave excitation predicted by the present CFD model is significantly higher than that from the second-order potential-flow theory, especially in surge. The uncertainties will be especially important performing CFD simulations to study the wave loads on a fixed semisubmersible FOWT platform in in the future when the present CFD solution will be validated against the measurements bichromatic incident. At this KC number, inertial effects dominate (see, e.g., [26]), and we do not expect the formation of viscosity-induced asymmetric wakes from the columns
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