Dear Riccardo,
I’m not sure I understand what you are plotting to really comment.
I would suggest simplifying the model to debug, e.g., do you get the results you expect for a still-water simulation (WaveMod = 0)?
One thing I see missing in your block is the gravitational restoring from body weight, unless you’ve included that in your hydrostatic stiffness matrix (which is not where that term is accounted for in FAST). But this will effect the moments, not the forces.
Best regards,