RAO of the pitch motion


I currently work on a modeling of an offshore platform and I get the RAO of its heave motion and pitch motion when it’s only submitted to waves excitation.
I modelized my system with the hybrid theory. In my simulation there is no wind, no current and my platform is composed of the floating platform and the tower (there is no hub, no blades, no generator etc). After solving some problemes, I plotted the RAOs and my RAO for the heave is what I expected, but the pitch’s RAO is still not good. The natural frequency as well as the general shape seem to be coherent with the experiments in the tank tests but the values of the RAO are much smaller than the values of the RAO of the experiments. I wanted to know if it could come from the value of PtfmCzt or the platform pitch inertia in Elastodyn. Indeed I’m pretty sure that the stiffness of my hst file are good. Moreover as my heave RAO seems coherent, I believe that most of the parameters are right. In other words, I want to know what parameters are only related to the pitch motion.
After reading the paper on the definition of the semi-submersible platform OC4 I understood that PtfmCZt, PtfmMass an PtfmIner were parameters of the floating platform only. In my case I only considered the floating platform in steel, that means without the tower and without the ballasts. I defined the ballasts in the filled members sections and I added the coefficients that I needed to add in the AddClin section of Hydrodyn. Is it right ?

Thank you,

Julie Marchebois

Dear Julie,

That all sounds correct. Please be aware that in OpenFAST v2.6 and newer, AddCLin no longer needs to be specified in HydroDyn when adding ballast; this is because hydrostatics in the strip-theory solution are now computed at the displaced position/orientation of the floater.

I’m not sure why your pitch RAO is not what you expect; could it be as simple as a unit conversion problem? Otherwise, items that would effect the pitch RAO are the rotational (pitch inertia), full-system center of mass, hydrostatic restoring in pitch, mooring restoring in pitch, etc. Also, pitch is often coupled to surge. Have you enabled the surge DOF along with pitch and how does the surge ROA compare?

Best regards,