Non-linear hydrostatic and wave forces on floating structures are very important during large amplitude waves. The computer program simdyn is a blended time domain program developed by Marine Dynamics Laboratory at TAMU and is capable of capturing the role of non-linear fluid forces. simdyn has previously been used to demonstrate that nonlinear hydrostatics have become very important in the problem of parametric excitation. In the current work simdyn is coupled with the computer program fast developed by U.S. National Renewable Energy Laboratory (NREL) for numerical simulation of floating offshore wind turbines (FOWTs). fast-simdyn is now a tool that is capable of studying large amplitude motions of FOWTs in extreme seas. fast-simdyn was then used to study the classic instability of negative damping that occurs in FOWTs that use conventional land-based control. The development of platform pitch and platform surge instability are studied in relation to different wave and wind scenarios. The intent was to do an analysis to see if the non-linear forces do play a significant role in large amplitude motions induced by negative damping. This study gives an indication of whether the development and application of higher fidelity hydrodynamic modules are justified.