Investigation of Nonlinear Difference-Frequency Wave Excitation on a Semisubmersible Offshore-Wind Platform With Bichromatic-Wave CFD Simulations

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 difference-frequency 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.

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

“…Previously in OC6 Phase I, we focused on using CFD to obtain estimates of the wave diffraction loads on the OC6-DeepCwind semisubmersible in a fixed condition, especially the nonlinear, low-frequency loads that potentially excite the surge and pitch resonance motion of a semisubmersible FOWT [2][3][4]. Tuning the engineering models also requires knowledge of the motion-damping characteristics of the floater, which are strongly influenced by viscous effects (see [5]); therefore, we now focus on the validation of CFD simulations of the free-decay motion of the OC6-DeepCwind semisubmersible.…”

“…Approximately corresponds to the first mesh refinement zone of Table5 3. Approximately corresponds to the first mesh refinement zone of Table6 4. Approximately corresponds to the rest of the mesh refinement zones of Table6 5.…”

OC6 Phase Ib: Floating Wind Component Experiment for Difference-Frequency Hydrodynamic Load Validation

“…At the difference frequency, small, spurious free waves might also have been present because of imperfect wave generation caused by the mismatch between the wavemaker paddle motion and the exact wave-field velocity of nonlinear bichromatic waves [33]. The free waves, while small, can lead to nonnegligible contamination of the equally small difference-frequency nonlinear wave loads, an issue first identified in the CFD analysis of OC6 Phase Ib on bichromatic waves [38,39]. The errors from these spurious wave components that might have been present in the basin were estimated from a wave-splitting analysis based on the wave measurements taken at multiple locations along the length of the basin during wave calibration, and corresponding uncertainties in the final normalized wave loads were developed.…”

An investigation of the applicability of SPIV for the analysis of the dynamics of floating offshore wind platforms

“…Among the different phenomena caused by viscousity, the viscous drift forces in extreme sea states (Tan et al, 2021), as well as viscous damping 1 effect (Clement et al, 2021) on floater and mooring line could be mentioned. The point is that the FOWTs models with second-order potential-flow theory useslly shows underestimate forces and results, comparing to CFD and lab testing approach (Wang et al, 2021a). This is mainly because of ignoring the separation and viscous drag in potential-flow theory.…”