Modeling Platforms with Tripod Towers

I was looking for information on how to model structures with a tripod-like tower when I came across this forum post: [Modelling a tripod turbine tower](https://NREL Forum - Modelling a tripod turbine tower)

Is this still the case or this functionality has been added to OpenFAST in a past update?

Kind regards.

Dear @Pere.Palacin,

The ability to model floating substructures with multiple flexible members mentioned in the forum post you linked to was added in OpenFAST v2.6. So, it is now possible in model tripod-like towers in floating substructures and be able to extract loads in each member of tower.

That said, while we have used this capability at NREL, we don’t currently have a floating offshore wind turbine example model with this functionality available in the public domain. See my post dated Aug 6, 2021 in the following discussion on the OpenFAST github page for more information and guidance: FeaCoupling of an External floater (platform+mooring) with Openfast #791 · Discussion #801 · OpenFAST/openfast · GitHub .

Best regards,

Hi @Pere.Palacin,

I confirm that you are able to model this kind of tripod tower in OpenFAST using the SubDyn module. You can find information about the theory used here: SubDyn Theory — OpenFAST v3.4.1 documentation

You can also find an example of flexible floater with arbitrary geometry in OpenFAST here: Energies | Free Full-Text | Modeling the TetraSpar Floating Offshore Wind Turbine Foundation as a Flexible Structure in OrcaFlex and OpenFAST

I hope that helps!


Thanks a lot for your reply @Roger.Bergua and @Jason.Jonkman,

From what I understood, in order to simulate a platform featuring a tripod tower, I would need to tweak the SubDyn (or even the HydroDyn?) input file, to include both the geometry of the tripod tower and the semisubmersible, and then remove the conventional tower from the ElastoDyn_Tower file, right?

At the same time the issue that prevents Paraview from rendering the SubDyn has not been fixed?

Best regards,


Dear @Pere.Palacin,

That is correct. To model a floating substructure with tripod tower would require that you eliminate the tower in ElastoDyn, define the structural properties in SubDyn, and define the hydrodynamic properties in HydroDyn.

The issue preventing visualization of the substructure in ParaView (Visualizing Offshore Support Structures · Issue #776 · OpenFAST/openfast · GitHub) has not been fixed. That said, you can visualize the Guyan and Craig-Bampton modes of the SubDyn model through the following website created by Emmanuel Branlard of NREL: Mode shape visualization; to use this feature, simply load the .json summary file generated by SubDyn.

Best regards,

Thanks a lot for your reply Jason,

Please forgive my ignorance, I’m still a beginner when it comes to using OpenFast. I’ve seen that the sample code corresponding to the case: “5MW_OC4Semi_Linear” does not have a SubDyn file and instead the geometry of the semisubmersible is introduced through the HydronDyn file. I was trying to edit this file to include my substructure. Do I have to create a SubDyn file for it then? Or I can use the HydroDyn file to do so, just like the example code?

Dear @Pere.Palacin,

Again, NREL has not provided a sample OpenFAST model (with input files for SubDyn, HydroDyn, etc.) for a floating offshore wind turbine (FOWT) with substructure flexibility, so at this time, we can’t share an example with you. (All of the FOWT OpenFAST models we have made with substructure flexibility to date include proprietary information that cannot be shared publicly, per the following OpenFAST issue: Issues · OpenFAST/r-test · GitHub)

OpenFAST is modular. So, for an FOWT with a tripod tower, the support structural dynamic properties must be specified within SubDyn (including floater and tripod tower), the floater hydrodynamic properties must be specified in HydroDyn, and the tower must be removed from ElastoDyn.

If you want to start with the 5MW_OC4Semi_Linear example, you must also enable SubDyn and include a SubDyn file.

Best regards,

Dear @Jason.Jonkman,

Thanks a lot for your reply. Understood, I only have to include a SubDyn input file if I want to analyse the structural dynamics of the platform and the HydroDyn for the HydroDynamic properties.

Many thanks!

Greetings @Jason.Jonkman,

Coming back to this topic, how can I remove the tower from the ElastoDyn File? I have tried removing the ElastoDyn input file completely in the .FST file but that didn’t work.

I have also tried setting the variables TowerHt, TowerBsHt, PtfmCMxt, PtfmCMy, PtfmCMzt and PtfmRefz on the ElastoDyn file accordingly but I couldn’t manage to do so.

I’m talking about the r-test case: NRELOffshrBsline5MW_OC3Tripod_ElastoDyn input file. I have created a SubDyn File with a floater that already includes a tripod tower structure and now I just want the RNA to sit on top of the highest joint in the subdyn file.

I would really appreciate some guidance on this topic.

Thanks a lot!

Dear @Pere.Palacin,

There is some guidance on having the SubDyn model extend up to the yaw bearing here: Modeling Considerations — OpenFAST v3.4.1 documentation (second paragraph). But basically, you’ll want to keep ElastoDyn enabled and set PtfmRefzt, PtfmCMzt, and TowerBsHt all to be close to TowerHt (ElastoDyn will not allow them to exactly equal TowerHt) such that the tower length is negligible and disable the tower DOFs.

Best regards,

Thanks a lot, that worked!

Have a nice day!!

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