Dear Jason,
Thank you for your reply, I have decided to count that water ballast as structure mass, which doesn’t make much difference if I adapt the inertia and the platform CM position. Regarding the difference in the internal buoyancy result, I think I understand what happened. For a horizontal member, the ballast volume computed by FAST will be the whole internal volume even if it is only partially filled (like you explained). But for the value of the internal buoyancy, there are 2 cases : if FillFSLoc is above the member’s centerline, then BuoyFzi will also take into account the whole member volume as though it was full of water ; but if FillFSLoc is below the centerline, the internal buoyancy for that member will be zero.
While observing those results, I noticed something strange, still in the HydroDyn summary file : in version 8.15 (it wasn’t the case in 8.12), the external buoyancy load BuoyFzi corresponds to the full structure volume, instead of only taking into account the submerged part. However, it doesn’t make any difference in the simulations : that (false ?) value doesn’t seem to be used for anything other than printing the HD.sum file.
Thanks again for your help,
Best regards,
Romain