Dear Jason,
Thank you very much for your quick reply. Regarding the influence of the platform center of mass and the turbine mass(I guess also nacelle, motor, and hub), on the roll and pitch inertia of the platform, if you mean I should use the 6x6 mass matrix of the full system given in the marin_semi.frc file (mentioned in this discussion OC4-DeepCwind semisubmersible - WAMIT Files).
Yes, this seems to work well to reduce the surge motion frequency a little be to 0.0093 Hz, which perfectly agreed with the existing result, and also a reduction in the heave motion frequency. But, the natural frequencies of pitch and roll are also reduced and still incorrect.
Could you please give me more comments and suggestions.
Thanks again.
Best regards,
Feng