Combined wave, wind, and current simulation in laboratory basins with floating offshore wind turbines
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Full Text E-thesis
Date
2022-09-16
Authors
Otter, Aldert
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Publisher
University College Cork
Published Version
Abstract
Testing scale models of Floating Offshore Wind Turbines (FOWT) under realistic offshore conditions at scale in wave basins is challenging. There exists a strong coupling between the turbine aerodynamics and platform hydrodynamics, and working in the two different fluid domains of air and water causes a scaling mismatch between Reynolds- and Froude scaling.
Furthermore, not every test facility with wave basins has the equipment to generate wind and current to simulate combined environmental loadings.
To overcome these challenges a hybrid test method to simulate wind and current was developed for this thesis. Hybrid testing is a combination of real-time numerical modelling and scale model testing. The aerodynamic loads of wind and hydrodynamic loads of currents are calculated in real-time and applied to the FOWT scale model via mechanical actuators.
To emulate aerodynamic loads a Multi-Propeller Actuator (MPA) was developed using off-the shelve parts from recreational aerial drones on a custom-made frame. By using several propellers with different thrust directions, multiple aerodynamic loads can be emulated simultaneously, and emulating forces rather than viscous loads solves the scaling mismatch.
Aerodynamic loads have been emulated by other researchers with propeller actuators, however, only very few examples of using multiple propellers were found in the literature. The study with the MPA adds to this knowledge gap.
A winch actuator was developed to simulate sea currents. By emulating the drag force of a current on the platform of the FOWT, and approximating wave-current interactions by adjusting wave spectra, currents can reliably be simulated.
No other examples of this method to simulate current were found, representing a clear knowledge gap. The study with the winch actuator fills this gap in the literature.
Both actuators are controlled with a Software-in-the-Loop (SIL) application. This control method uses real-time feedback from a load cell and motion tracking system to update the loads calculation with the real-time numerical simulation for each time-step, improving the accuracy of the simulation. Simulating current with the winch actuator is referred to as SIL current.
Experimental results throughout this body of work have been validated with offline numerical simulations using FAST and AQWA. Two validation metrics, developed for this study, have been applied to the results. Experiments with SIL current have also been validated by repeating the experiments with physical current, referred to as the full physical method, and comparing the results of both methods.
Both actuators were applied to a 1/50 scale model of the INNWIND semisubmersible platform with the NREL 5 MW as the simulated wind turbine.
The results have shown the winch actuator can reliably and accurately emulate the drag force of a current on the FOWT platform and the method to approximate wave-current interactions was found adequate. The results also showed that the MPA can reliably and accurately emulate thrust- and torque loads of the NREL 5 MW turbine.
The SIL current and aerodynamics emulation with the MPA, in combination with physical waves, were found suitable to replicate realistic offshore conditions at scale in the wave basin.
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Keywords
Hybrid testing , Floating offshore wind , Software in the loop
Citation
Otter, A. 2022. Combined wave, wind, and current simulation in laboratory basins with floating offshore wind turbines. PhD Thesis, University College Cork.