Abstract
A shallow draft cylindrical barge type floater with footing close to the water surface was experimentally evaluated in waves to investigate non-linear motion characteristics. The floater was designed to be used as an option for FOWT—floating offshore wind turbines. The non-linear mechanism can be promoted due to the viscous force that acts on the footing edges and the footing interaction with the free surface. In general, the observed non-linear viscous damping is modeled as a force proportional to the square of the relative velocity between the floater and the water. Therefore, the viscous damping levels is expected to increase, and the response in waves, to decrease. However, an increase in motion responses was observed for a broad range of wave periods. An attempt was made to clarify the hydrodynamic mechanism by comparing wave tank experiments, numerical calculations by CFD—computational fluid dynamics codes, and linear potential theory codes. Regular wave tests for three different wave height conditions were carried out, including free decay tests in still waters. For CFD simulations, the OpenFOAM code was selected. For potential theory simulations, the WAMIT code was chosen. As a result of the research, three points could be highlighted and discussed: first, the hydrodynamic phenomenon that contributed to the non-linear motion of the floater was identified; second, the increase and coupling of the motions response of heave and pitch motions; and finally, the phenomenon that the footing held water mass and lifted it to the water surface. The CFD calculations were able to get good qualitative results compared with the experiments and confirmed the use of CFD as a useful tool to capture the non-linear hydrodynamic phenomenon. The linear potential theory was not able to capture the phenomenon discussed herein.
Highlights
In relation to the use of renewable energies in Japan, the Basic Energy Plan [1] describes that energy should be economically feasible, and non-carbon emission sources must be used to generate electric power
CFD simulations were performed corresponding to the wave test conditions and compared with the experimental result to verify the accuracy of CFD calculation and to clarify the hydrodynamic phenomenon
Regarding the non-linear motion characteristics observed in regular waves for a shallow draft cylindrical barge type floater with footing, this hydrodynamic phenomenon was investigated to clarify the mechanism of the non-linear motion characteristics by comparing the experimental regular wave tests using the 1/73.5 scale model and numerical simulations by CFD and potential theory codes
Summary
In relation to the use of renewable energies in Japan, the Basic Energy Plan [1] describes that energy should be economically feasible, and non-carbon emission sources must be used to generate electric power. The top footing (close to the free surface) is responsible for large non-linear motion behaviors, mainly in the heave and pitch directions. The RAOs obtained from the experiments showed a remarkable increase in the wave period range around the cancellation points This phenomenon can be seen in detail for a two-dimensional (2D) case in [20]. This paper attempts to clarify the mechanism of a non-linear hydrodynamic phenomenon that significantly increases RAO of the heave and pitch of a cylindrical barge type FOWT for shallow waters in specific wave periods. The non-linear hydrodynamic phenomenon and its effect on the RAOs were clarified by comparisons between regular wave experiments, CFD simulations (by OpenFOAM code), and linear potential theory calculations (by WAMIT code).
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