Modeling the floating substructure as flexible structure

[YangLeiTH]

Hi @jjonkman

I performed elastic analysis using SubDyn for the OC3 platform. Below is my SubDyn file. However, I encountered the following error during runtime. I wanted to ask what might be the cause of this issue?

YangLei
NRELOffshrBsline5MW_OC3spar_SubDyn.dat.txt

[jjonkman]

Dear @YangLeiTH,

Just a few comments:

• Did you ensure that the mass, center of mass, and inertias of the OC3-Hywind spar are the same as what is documented in its specifications report (https://www.nrel.gov/docs/fy10osti/47535.pdf) through this SubDyn model? I ask because the thickness of your members in SubDyn is quite small and I don't see any lumped masses that could be used to add ballast. Without the correct mass and center of mass, the floating system may not be hydrostatically stable.
• When modeling floating substructures, SubDyn should be told that the substructure is floating by setting NReact = 0 rather than NReact = 1 with all of the fixity flags set to zero.
• When modeling a floating substructure in SubDyn, I would not suggest using Rayleigh damping of Guyan modes, which are the rigid-body modes when the substructure is floating.

Best regards,

[YangLeiTH]

Hi @jjonkman

I have solved the problem from the forum of NREL. But I have a new question to ask you, when enabling the SubDyn module, should the platform mass and all inertia be set to zero in ElastoDyn module?

Best regards,

[jjonkman]

Dear @YangLeiTH,

You should not double count any mass or inertia. The mass of the substructure accounted for in SubDyn should be absent from the platform in ElastoDyn. If the full substructure is modeled in SubDyn, then the platform mass and inertias in ElastoDyn can be zero.

When you enable both PtfmYDOF and YawDOF in ElastoDyn, please note that it is necessary to set PtfrmYIner nonzero as discussed recently in the following topic on our forum: https://forums.nrel.gov/t/wind-veer-parameter/2103/55.

Best regards,