Low‐frequency dynamics of a floating wind turbine in wave tank–scaled experiments with SiL hybrid method

Abstract

The design of floating wind turbines needs the validation of numerical models against measurements obtained from experiments that accurately represent the system dynamics. This requires solving the conflict in the scaling of the hydrodynamic and aerodynamic forces that arises in tests with wind and waves. To sort out this conflict, we propose a hybrid testing method that uses a ducted fan to replace the rotor and introduce a force representing the aerodynamic thrust. The force is obtained from a simulation of the rotor coupled in real time with the measured platform displacements at the basin. This method is applied on a test campaign of a semisubmersible wind turbine with a scale factor of 1/45. The experimental data are compared with numerical computations using linear and non-linear hydrodynamic models. Pitch decays in constant wind show a good agreement with computations, demonstrating that the hybrid testing method correctly introduces the aerodynamic damping. Test cases with constant wind and irregular waves show better agreement with the simulations in the power spectral density's (PSD's) low-frequency region when non-linear hydrodynamics are computed. In cases with turbulent wind at rated wind speed, the low-frequency platform motions are dominated by the wind, hiding the differences from hydrodynamic non-linearities. In these conditions, the agreement between experiments using the proposed hybrid method and computations is good in all the frequency range both for the linear and the non-linear hydrodynamic models. Conversely, for turbulent winds producing lower rotor thrust, non-linear hydrodynamics are relevant for the simulation of the low-frequency system dynamics.

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

“…Nevertheless, it was discovered that varying degrees of underprediction at low frequencies are ubiquitous across engineering-level tools and cannot be addressed simply by further tuning the model coefficients; rather, improvements to the formulation of the engineering models are required. Similar underpredictions have also been encountered by other researchers (see, e.g., [7,8]).…”

“…Nevertheless, it was discovered that varying degrees of underprediction at low frequencies are ubiquitous across engineering-level tools and cannot be addressed simply by further tuning the model coefficients; rather, improvements to the formulation of the engineering models are required. Similar underpredictions have also been encountered by other researchers (see, e.g., [7,8]). The underprediction of the low-frequency wave-exciting force in surge (in the direction of wave propagation) and moment in pitch (about a transverse axis perpendicular to the wave propagation) on a fixed OC5-DeepCwind semisubmersible is shown in Figure 1.…”

Wave- and drag-driven subharmonic responses of a floating wind turbine

“…A number of numerical modelling studies have reported difficulties in accurately predicting subharmonic resonant load and/or motion responses of floating wind turbines. The large code comparison initiatives OC5 and OC6 (see Robertson et al 2017Robertson et al , 2020, as well as works by Azcona, Bouchotrouch & Vittori (2019) and Li & Bachynski (2021) for example, reveal model underestimation of surge and pitch resonant responses, when compared to wave-basin experiments. It is hoped that the detailed examination of small-scale floater measurements presented in this paper, which allow identification of different excitation mechanisms affecting the resonant floater dynamics, could be useful in model validation.…”

Seakeeping Tests of a FOWT in Wind and Waves: An Analysis of Dynamic Coupling Effects and Their Impact on the Predictions of Pitch Motion Response

“…On the other hand, experimental methodologies for studying FOWT behaviour under wave and wind have been adopted. Different approaches for emulating the rotor wind forces can be found in the literature, such as using Reynolds scaled wind [4], cables [5,6] and active fans [7][8][9][10]. The use of cables or active fans demands a code integration with real-time rotor load prediction in a software-in-the-loop scheme (SIL).…”