Investigation of nonlinear difference-frequency wave excitation on a semisubmersible offshore-wind platform with bichromatic-wave CFD simulations

Wang, Lu and Robertson, Amy and Jonkman, Jason and Yu, Yi-Hsiang and Koop, Arjen and Nadal, Adrià Borràs and Li, Haoran and Shi, Wei and Pinguet, Romain and Zhou, Yang and Xiao, Qing and Kumar, Rupesh and Sarlak, Hamid; (2021) Investigation of nonlinear difference-frequency wave excitation on a semisubmersible offshore-wind platform with bichromatic-wave CFD simulations. In: Proceedings of the ASME 2021 3rd International Offshore Wind Technical Conference, IOWTC 2021. ASME, New York. ISBN 9780791884768 (https://doi.org/10.1115/IOWTC2021-3537)

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Abstract

The natural surge and pitch frequencies of semisubmersible offshore wind platforms are typically designed to be below the wave frequencies to avoid direct excitation. However, surge or pitch resonance can be excited by the nonlinear low-frequency loads generated by irregular incident waves. Second-order potential-flow models with added Morison drag have been found to underpredict this low-frequency excitation and response. As part of the OC6 project1 , the authors performed computational fluid dynamics (CFD) simulations to enable a better understanding of the low-frequency loads and the limitations of lower-fidelity models. The focus of this paper is to set up a computationally cost-effective CFD simulation of a fixed semisubmersible platform to investigate nonlinear differencefrequency loads and establish the corresponding uncertainty in the results. Because of the high computing cost, CFD simulations of irregular waves can be challenging. Instead, simulations were performed with bichromatic waves having a shorter repeat period. A preliminary comparison with quadratic transfer functions from second-order potential-flow theory shows that CFD models consistently predict higher nonlinear wave loads at the difference frequency, likely because of flow separation and viscous drag not accounted for in potential-flow theory

ORCID iDs

Wang, Lu, Robertson, Amy, Jonkman, Jason, Yu, Yi-Hsiang, Koop, Arjen, Nadal, Adrià Borràs, Li, Haoran, Shi, Wei, Pinguet, Romain, Zhou, Yang, Xiao, Qing ORCID logoORCID: https://orcid.org/0000-0001-8512-5299, Kumar, Rupesh and Sarlak, Hamid;
CORE (COnnecting REpositories)