Influence of Viscosity and Non-Linearities in Predicting Motions of a Wind Energy Offshore Platform in Regular Waves

Abstract

Motion predictions of floating bodies in extreme waves represent a challenging problem in naval hydrodynamics. The solution of the seakeeping problem involves the study of complex non-linear wave-body interactions that require large computational costs. For this reason, over the years, many seakeeping models have been formulated in order to predict ship motions using simplified flow theories, usually based on potential flow theories. Neglecting viscous effects in the wave-induced forces might largely underestimate the energy dissipated by the system. This problem is particularly relevant for unconventional floating bodies at resonance. In these operating conditions, the linear assumption is no longer valid, and conventional boundary element methods, based on potential flow, might predict unrealistic large responses if not corrected with empirical viscous damping coefficients. The application considered in this study is an offshore platform to be operated in a wind farm requiring operability even in extreme meteorological conditions. In this paper, we compare heave and pitch response amplitude operators predicted for an offshore platform using three different seakeeping models of increasing complexity, namely, a frequency-domain boundary element method (BEM), a partly nonlinear time domain BEM, and a non-linear viscous model based on the solution of the unsteady Reynolds-averaged Navier–Stokes (URANS) equations. Results are critically compared in terms of accuracy, applicability, and computational costs.

Highly Accurate Experimental Heave Decay Tests with a Floating Sphere: A Public Benchmark Dataset for Model Validation of Fluid–Structure Interaction

“…At the low-fidelity end of CFD models to simulate WECs are the LPF models, which despite rather gross assumptions of linearity in both the governing equation (Laplace) and the boundary conditions, produce useful simulations for engineering purposes and indeed are very time-efficient; see, e.g., [23]. The dynamic response of marine structures is commonly analyzed in the frequency domain using LPF theory [23][24][25][26]. Time-domain models are based on hydrodynamic coefficients solved in the frequency domain and inserted into the Cummins equation [27,28]; see Appendix C for further information.…”

Uncertainty Assessment of CFD Investigation of the Nonlinear Difference-Frequency Wave Loads on a Semisubmersible FOWT Platform

“…Wall On the upstream and bottom boundaries, the flow velocity was prescribed according to the linear wave theory [16]; however, because of the large water depth relative to the wavelengths, the bottom boundary was essentially a no-slip boundary. In fact, effectively identical results for the present problem down to the small nonlinear wave loads have also been obtained with no-slip condition on the bottom.…”

“…A systematic numerical uncertainty analysis was performed and is described in Section 5 for the baseline setup by repeating the simulation with different numbers of outer iterations, time steps, and grid sizes. On the upstream and bottom boundaries, the flow velocity was prescribed according to the linear wave theory [16]; however, because of the large water depth relative to the wavelengths, the bottom boundary was essentially a no-slip boundary. In fact, effectively identical results for the present problem down to the small nonlinear wave loads have also been obtained with no-slip condition on the bottom.…”

“…Wang et al computed the surge, heave, and pitch response amplitude operators for the same floater geometry by simulating the motion of the floater with various incident wave frequencies [15]. Del Águila Ferrandis et al computed the response amplitude operators of an unconventional FOWT platform in regular waves using CFD and compared the results to both frequency-and time-domain boundary element methods [16]. More recent works incorporated fully coupled aero-and hydrodynamics to investigate the effect of wave-induced motion on the performance and operation of the mounted wind turbines [17][18][19][20].…”