A time domain simulation tool is used to setup a numerical simulation model in which the wave loads of the potential flow are combined with the viscous drag force computed by the Morison equation. The drag coefficients in the Morison equation are determined to provide the best numerical results compared to model test data.
The numerical simulations and analytic solutions have been used to examine the characteristics of mean viscous drift force on the semi-submersible for fixed condition. The results show that the mean viscous drift force is mainly produced by the column. The column is divided into splash zone and submerged zone because they give much different mean viscous drift force. In high waves, the splash zone leads to mean drift force much larger than the submerged zone for both wave-only and wave-current coexisting flow fields.
The present results clearly show that the drag coefficients can be expressed as a function of the Keulegan-Carpenter number, especially for high waves. There is strong correlation between regular and irregular waves for both wave-only and wave-current coexisting flow fields. If current is present, the drag coefficient decreases significantly.
