Dear @Yosuke.Matsumoto,
Given that it looks like your floater is quite rigid, I would not expect the modeling of hydro-elasticity to have a large impact on the tower-base loads. So, I would expect all three models to give more similar results. I would think you’d be able to set the model properties from each Case to give more similar results.
Regarding Case 1, how you are accounting for viscous effects that are captured in Case 3?
Regarding Case 2, how are you modeling the hydrodynamic loads? Are these matching Case 1 or 3?
Regarding Case 3, are you setting the added mass and dynamic pressure coefficients of the strip-theory model so that the added mass and fluid-inertia loads are similar between strip-theory and potential flow? Moreover, given that you are running Case 3, does this mean that you got your OpenFAST model working from the related forum post (Anomalous response when modeling a tower in SubDyn - #20 by Jason.Jonkman); or did you have to lock platform DOFs or limit the number of Craig-Bampton modes to get this response?
Do you have a sense (based on experimental measurements or high-fidelity (e.g., CFD) modeling) which Case is providing a more realistic solution?
Best regards,