Dear Neeraj,
FAST augments the potential-flow loads with viscous effects by taking only the viscous drag term from Morison’s equation and superimposing this load with the added mass, radiation damping, hydrostatic, and wave-excitation loads from potential flow. This should impact the damping, but not the natural frequencies of the response.
The masses, inertias, and CG you report are correct for the OC3-Hywind spar alone, without the tower, nacelle, rotor, or mooring system. However, while the system response certainly depends on the mass/inertia of the system, the added mass and damping coefficients do not. My guess is that you are not referring the rotational modes to the same reference point. When NREL derived the added mass, damping, and wave-excitation loads for the OC3-Hywind spar, we used WAMIT, which references the rotational modes relative to the centerline of the body at mean sea level. What is the reference point for the ANSYS/AQWA results?
Best regards,