Dear Jason
Thank you very much for the information provided - it was very insightful. I have eliminated RNA inertial loads by disabling all DOFs (and setting ShftTilt to 0, since Blade DOFs are always off) and obtained rotor thrust which I applied to my model (I have not yet applied other loads and external moments as I presume they have little effect on response in X direction, please correct me if I am wrong). When comparing to FAST results of the tower top displacement (all DOFs were on except Blades, Teeter, Drivetrain and Gen Dofs, no wave action) I got a good agreement for amplitudes but the frequencies were slightly different.
For my model I get the first natural frequency as 0.2912 Hz. Based on the posts from this thread - Using Aggregate Mass in ADAMS to Check NREL CS_Monopile.bmi still be applicable to it?
Another issue is with fore-aft (My) mudline moments - for some reasons my amplitudes during transient response are much larger even though ttds are more or less the same - could that come from omission of RNA’s inertias again?
Lastly, I do understand that platform DOFs are related to the base in case of a monopile OWT, but is it important to always have all 6 platform DOFs on (how exactly do heave, sway and surge relate to it)?
Thank you!