# Modeling multi-member floating wind turbine

Dear Dr. Jason,
I met some problems when modeling Telwind using OpenFastv3.0. The Telwind floating platform consists of two floaters and six tension cables connecting them.
1.If two floaters are assumed rigid, do I only need to model the six cables in SubDyn? And the twelve joints of cables are all set as interface joints?
2.In the properties of cables, does the positive T0 value mean stretch?
3.In ElastoDyn, do I need to definite the mass and inertia of the two floaters separately?
I would appreciate it if you could answer these questions.

Best regards

Dear Chen.Gu,

Are you trying to model this floater as a single six degree of freedom rigid body? If so, you can model the entire floater as the platform in ElastoDyn.

Or are you trying to model the floater as a two six degree of freedom rigid bodies connected by elastic cables? If so, you’ll need to model the floater in the SubDyn module. SubDyn models the substructure as an interconnection of beam, taut cable, and/or rigid members. Presumably you’d want to model the six cables with the taut cable elements, but I’m not fully sure how you’d want to model the two hulls.

For cable elements in SubDyn, the pretension force must be positive, indicating stretching of the line (to keep the cable taut).

Best regards,

Dear Jason

Yes, I want to model the floater as two 6-Dof rigid bodies connected by elastic cables in SubDyn, in that case, how to define the PtfmRefzt and PtfmMass properities in ElastoDyn, should I follow the rules applied in fixed offshore wind turbine and set those parameters as the TP(Transiton piece) 's position and mass, which is PtfmCMzt=PtfmRefzt=TP center height, PtfmMass=TP mass?

Best regards
Chen.Gu

Dear Chen.Gu,

It probably makes sense to model the entire floating substructure (upper and lower rigid bodies and cables) in the same module (SubDyn). In that case, the 6 platform DOFs should be enabled in ElastoDyn, but the mass, center of mass, and inertia of the upper rigid body needs to be modeled in SubDyn. So, you can set PtfmRefzt = PtfmCMzt = PtfmMass = 0 (etc. for all platform mass, center of mass, and inertia properties in ElastoDyn, except PtfmYIner, which you should set equal to the rotational inertia of the undeflected tower about its centerline, at least when YawDOF = True, as discussed in other forum topics).

To model the upper rigid body in SubDyn, ideally you’d place rigid links between the interface node and the nodes connecting the cables, but there is a bug in SubDyn that was recently found regarding this functionality, as reported recently on the OpenFAST issues page: github.com/OpenFAST/openfast/issues/854. As such, I’d recommend using quite stiff beam elements. You can add lumped (concentrated) masses/inertias at the node endpoints of the beams to ensure that the overall mass, center of mass, and inertia of the upper rigid body are as you want.

To model the lower rigid body in SubDyn, you can use rigid links between nodes of the cables, again, adding lumped masses/inertias as needed to get the overall mass, center of mass, and inertia of the lower rigid body are as you want.

Best regards,

Sir，If I want to built up new offshore floating foundation，how can I need to do in FAST to built up my floating foundation，how I need to have data about foundation，Thanks！

Dear Hairui,

I’m not sure I really understand your question. You’ll obviously need the properties of the floating substructure, including structural and hydrodynamic properties of the floater, the properties of the mooring system and tower, and any modifications necessary in the wind turbine controller.

Best regards,

Thank for your help! So where should I modify the floating foundation ？Is HydroDyn or SubDny？Another question is how can I obtain data about floating substructure ，is other software？because I want to built up new floating foundation by FAST to analyse.
Be regards！

Dear Hairui,

The hydrodynamic properties are defined in HydroDyn, the floating substructure properties are defined in SubDyn (if the substructure is modeled as flexible) or ElastoDyn (if the substructure is modeled as a lumped mass/inertia), the tower properties are defined in ElastoDyn, and the mooring properties are defined in one of the mooring modules (e.g., MAP++, MoorDyn, or FEAMooring).

The data necessary to build a floating substructure model depends on the floater. If the floater is of large volume, presumably you’d want to model the hydrodynamics via potential flow theory, which requires the use of WAMIT or similar software to generate the hydrodynamic coefficients as a preprocessing step.

Best regards,