Dear Anuj,
I wouldn’t suggest simulating a floating offshore wind turbine without moorings – without moorings, the rotor thrust would cause continual drift of the platform! And for some floating platforms (e.g., tension-leg platforms (TLPs)), the mooring system is very important to the overall hydrostatic stability of the platform.
Please see my Apr 20, 2011 post in the forum topic found here for information on how to deal with the error regarding “unstretched mooring line length too large”: RAOs in FAST.
Best regards,