Dear Michael,
We have experienced platform-yaw instability problems for models with too little yaw stiffness. For example, the OC3-Hywind spar system is commonly modeled with a simplified mooring model without the delta connection to the platform. The delta connection in real life greatly increases the yaw stiffness of the mooring system; thus, a simple yaw spring is commonly modeled to mimic the effect of the delta. But without this yaw spring, the model is likely to go unstable. Alternatively, the details of the delta connection could be modeled.
I’m not aware of any “extra” damping brought about by the FAST-OrcaFlex interface. Is aerodynamics disabled in the coupled FAST-OrcaFlex model? Aerodynamic damping could be significant. For a coupled FAST-OrcaFlex model without aerodynamics, I would expect consistent damping to an equivalent OrcaFlex-only model.
Best regards,