Dear Nikunj,
Of course, if viscous effects are important to the response of a given floating platform, simulating without viscous effects would not lead to accurate solutions. In a semisubmersible for example, the viscous drag on the heave plates is likely very important to the overall dynamic response. Until the full implementation of viscous drag elements becomes available in HydroDyn, including an “additional linear (or quadratic) damping matrix” for the platform DOFs would be a suitable option for enabling FAST/HydroDyn to model the proper response. Such an “additional damping matrix” can be derived from known free-decay response data of the given platform.
However, the standard version of HydroDyn is not currently set up to allow a user to specify additional damping matrix coefficients directly through an input file. This feature is not too difficult to add yourself, although it will require a recompile of FAST. Please see the topic on our design codes forum found here for guidance on how the “additional linear damping” was added to HydroDyn for the OC3-Hywind system: Additional linear hydrodynamic damping in OC3 Hywind.
I hope that helps.
Best regards,