Offshore wind turbines have been investigated and developed as one of the renewable energies. In Japan, the research and development of floating type offshore wind turbines have been carried out because the water around the country is too deep to settle the bottom mounted type.

In this paper, we investigate the effects of the diameter of the column floater of the advanced spar in regular wave by using open source computational fluid dynamics software OpenFOAM. We use olaFOAM which equipped with the functions to set the boundary conditions of wave generation at the inlet and wave absorption at the outlet. The forces acting on a spar obtained by the numerical simulation and Morison’s equation are compared to examine the validity of the numerical model. A good agreement between them confirms the validity of the numerical method. Then we simulate numerically the effects of the column diameter on the flow around the advanced spars and the wave load. The result clarifies that Morison’s equation overestimates the wave force, and the difference increases with the column diameter. For more detailed analysis we divide the advanced spar into three parts, the upper spar above the column, the column floater and the lower spar below the column. As a result, we find that the difference between the wave load acting on the column by Morison’s equation and that by numerical simulation is dominant due to the flow separation around the column. Finally we modify the load coefficients of Morison’s equation for the column so that the modified equation can accurately evaluate the wave load acting on the advanced spar.

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