This paper presents a study of the characteristics of motion responses and tether tensions of a floating structure with four rectangular moonpools and one or two vertical axis wind turbine (VAWT) models. To complement our previous research work reported by Ikoma et al. (IOWTC2019-7598), new physical experiments and numerical calculations were carried out. The effects of the sloshing behavior in moonpools and the gyroscopic moments of the turbines on the body motions and tether tensions were studied.&nbsp;First, a series of physical model tests were performed in the wave tank of Nihon university, through which the gyroscopic effects due to the rotor rotations on the motion responses of the floating system were examined. Specifically, VAWTs with varied rotating speeds (0, 40 and 80 rpm), masses and moments of inertia were considered in the experiments. Regular incident waves with a wide range of periods (0.6 to 2.0 seconds) and varied amplitudes (0.01 and 0.02 m) were generated in the model tests. The motion responses including sway, heave, roll and yaw motions of the floating structures in water waves and the tether tensions were measured. The first- and second- order components of the experimental results were shown and discussed.&nbsp;Second, numerical calculations were carried out by our in-house program code and the hydrodynamic software WAMIT. The in-house program code was developed based on potential flow theory at first and second orders, using the element boundary method, which can be used to solve diffraction and radiation problems of floating bodies. Numerical predictions were made by the in-house program code and compared with the experimentally measured data and the calculation results of WAMIT. Satisfactory agreements were observed among the results for various wave and geometric conditions. The sloshing behavior in the moonpools and its effects on the body motions and tether tensions were investigated through numerical computations.&nbsp;Finally, the effects of the four moonpools on the motion performance were discussed for a wide range of wave period. It is found that through the optimization of the geometries of moonpools, the motion responses of the floating system could be significantly reduced within a wide wave frequency band. The complex sloshing behaviors in the moonpools is responsible for the motion reductions, particularly for sway, heave and roll motions. The numerical results in this paper suggest that the floating systems with four moonpools could have advantages in the hydrodynamic performance compared with those installed with mono moonpool.&nbsp;Key words: VAWT, Moonpool, Experiment, Calculation, Motion reduction

Study of Motion Performance of a Floating System With Four Moonpools a VAWT

Paper Proceedings: https://asmedigitalcollection.asme.org/OMAE/IOWTC2021/volume/84768

ASME 2021 International Offshore Wind Technical Conference (IOWTC2021)

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