Dear Nikunj,
Please see my July 14, 2011 post in the forum topic found here for information on how the “additional linear damping” was added to the OC3-Hywind spar buoy model: Instructions for Compiling FAST.
We can provide you WAMIT input files (e.g., *.gdf) for models in the public domain that we have already generated (such as the for the OC4-DeepCwind semisubmersible), but NREL does not have the resources to develop input files for your own unique design.
Best regards,