Comparison of Nonlinear Wave-Loading Models on Rigid Cylinders in Regular Waves

Abstract

Monopiles able to support very large offshore wind turbines are slender structures susceptible to nonlinear resonant phenomena. With the aim to better understand and model the wave-loading on these structures in very steep waves where ringing occurs and the numerical wave-loading models tend to lose validity, this study investigates the distinct influences of nonlinearities in the wave kinematics and in the hydrodynamic loading models. Six wave kinematics from linear to fully nonlinear are modelled in combination with four hydrodynamic loading models from three theories, assessing the effects of both types of nonlinearities and the wave conditions where each type has stronger influence. The main findings include that the nonlinearities in the wave kinematics have stronger influence in the intermediate water depth, while the choice of the hydrodynamic loading model has larger influence in deep water. Moreover, finite-depth FNV theory captures the loading in the widest range of wave and cylinder conditions. The areas of worst prediction by the numerical models were found to be the largest steepness and wave numbers for second harmonic, as well as the vicinity of the wave-breaking limit, especially for the third harmonic. The main cause is the non-monotonic growth of the experimental loading with increasing steepness due to flow separation, which leads to increasing numerical overpredictions since the numerical wave-loading models increase monotonically.

Nonlinear time‐domain wave‐structure interaction: A parallel fast integral equation approach

“…Additional simulations showed that instabilities only occurred for thin cylinders, that is, when the surface Keulegan-Carpenter number is moderate. When running the model without filtering and at sufficiently high-resolution, wave patterns similar to those identified by Swan and Sheikh 104 as type-2 waves appeared near the cylinder, which are nonlinear waves that would eventually break, and discussion of the physics has been considered by Mockute et al 105 for this type of problem. In order to investigate the stability question further, tests for kA = 0.05 and kR = 0.378 were conducted, without using any filtering of the free-surface.…”

Optimal Dimensions of a Semisubmersible Floating Platform for a 10 MW Wind Turbine

“…FD models neglect or approximate non-linear effects and are restricted to steady load conditions. Several important nonlinear and transient effects on fixed-bottom WTs have been observed in [28][29][30][31][32][33][34].…”

Influence of Clumps-Weighted Moorings on a Spar Buoy Offshore Wind Turbine

“…However, the OC3 Hywind test case provided by NREL includes all the hydrodynamic data obtained with the external software WAMIT [33]. More accurate models capable to account for important nonlinear effects, see for example [34][35][36][37][38][39] for fixed bottom platforms and [40][41][42][43] for floating platforms, should be used for higher fidelity simulations. However, when high efficiency is required as in the present case, the potential-flow theory together with the assumption of small waves and small body motion represent an acceptable trade-off between accuracy and efficiency.…”