Dear SungWon,
The FAST v7 executable provided with the OC3-Hywind model (here) includes the “additional linear damping” documented in: [url]http://www.nrel.gov/docs/fy10osti/47535.pdf[/url]. More information on this topic is provided in the following forum topic: Additional linear hydrodynamic damping in OC3 Hywind.
In FAST v8, it is easy to add (or remove) additional linear damping, additional quadratic damping, and additional linear stiffness directly through the HydroDyn input file, with no need to customize the source code.
Best regards,