Water ballasting & buoyancy loads

Hello everyone,

I am currently trying to model a semi-submersible platform, and I am having trouble with the water ballast. Some of the members partially filled with water are horizontal pontoons, and when I check the summary file produced by HydroDyn I can see that the mass of water inside the pontoons corresponds to their whole internal volume (despite the fact that the chosen FillFSLoc should fill only about a seventh of their volume). Should I conclude that in its current version, the code only models water ballast properly when it fills a vertical member ? Or am I missing something ?

One more question about water ballast : the way I see it, if I fill my structure with a volume Vb of water, I will have to add rhogVb to the weight of the platform. The way it is done in FAST V8 (that is, if I’m not mistaken), part of the buoyancy load is substracted, which is completely equivalent provided that the substracted part is also equal to rhogVb. But the summary file from HydroDyn tells me that, for my ballasted volume of 9.5E+03 m^3, the internal buoyancy substracted is 3.3E+07 N… And I don’t understand that figure, even though it seems to be pretty basic physics.

Thanks for your help and for all the good work on FAST,



Dear Romain,

I wouldn’t say that the filled members calculation of HydroDyn only applies to vertical members, but the filled member calculation is less sophisticated than you are assuming. The calculation using FillFSLoc only considers the centerline of the member, so in effect, the entire member will be filled up to the point where the member centerline crosses FillFSLoc. Thus, HydroDyn won’t permit partial filling of horizontal members.

Regarding the difference you are seeing between the ballasted volume and the internal buoyancy in the HydroDyn summary file, I think I’d have to see the model to comment. I looked at the summary file for the OC4-DeepCwind semisubmersible model, and in that model, the relationship between the ballasted volume and the internal buoyancy is as you expect it would be.

Best regards,

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,



Thank you for reporting this issue. I have verified the problem in the current version of HydroDyn (v2.05). This bug was created when we added user supplied wave kinematics and with our wave stretching implementation. Some additional code is needed in the WriteSummaryFile() subroutine of Morison.f90. I’ve generated a fix, but it hasn’t been verified. As you state, this bug does not affect the time-marching load calculations. We will incorporate this bug fix into the next released version of HydroDyn.

Thank you,

Greg Hayman

Dear Jason,

When we imply the apparent fixity method there would be some new joints (i.e. for jacket 4 new joints) added into the joint lists of the SubDyn and Hydrodyn and either four new members are taken into account. Do the new members below the seabed level have to be considered as filled members despite they are assumed like piles ?

Best regards,

Dear Arsalan,

HydroDyn will not apply hydrodynamic loads in the strip-theory solution to nodes/elements below the seabed, so, there is no reason to change the HydroDyn model when adding or changing an apparent fixity foundation in SubDyn. See section 6.3 of the draft HydroDyn User’s Guide and Theory Manual for more information: wind.nrel.gov/nwtc/docs/HydroDyn_Manual.pdf.

Best regards,

Dear Jason,

Thank you so much for your helpful advices.