The Floating Offshore Wind Turbine (FOWT) is a fairly new concept. There are limited number of full-scale prototypes to provide real data. Therefore, most of the research today relies on numerical models. This is required, so that an adequate amount of confidence can be gained before venturing into large scale production. The major challenge ahead is proving their reliability and robustness. There needs to be supporting studies that consider most factors that can go wrong. The computer program FAST was a groundbreaking contribution from NREL in this regard. FAST is capable of doing combined loading analysis of FOWTs. The numerical model used for the hydrodynamics can, however, be improved further.

Non-linear hydrostatic and wave forces on floating structures become 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 become very important in the problem of parametric excitation. In the current work, SIMDYN is coupled with FAST. The 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 of an even more sophisticated hydrodynamic modules is justified.

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