OC$-Phase II Centre of Gravity and Mass

Dear Jason,

I have one question regarding the OC$ platform. The CM of the floater is -8.6588 in the elastodyn file, but -13.46 in the report ‘Definition of the Semisubmersible Floating System for Phase II of OC4’. I assume this is because of the additional ballast mass. Suppose, I change the draft of the platform, will Hydrodyn calculate the ballast mass automatically from the underwater volume and use it for the simulation ?

Regards,

Rahul.

Dear @Rahul.Ramachandran,

Correct; for the OC4-DeepCwind OpenFAST model available in the r-test, the ElastoDyn model considers only the structural mass and the water ballast is modeled within HydroDyn.

The water ballast is defined in HydroDyn via the filled member inputs, which are tied to specific HydroDyn members. If you change the properties or joint locations of these members, the water ballast will change accordingly. See the filled member section of the HydroDyn input file, and the associated HydroDyn documentation (4.2.8.2. Input Files — OpenFAST v3.2.0 documentation) for more information.

Best regards,

Thank you Jason,

Regarding the mass moments of inertias of the components, I can find the hub, generator and nacelle inertias in the elastodyn file. Are they the same about all x,y,z axes ? What about the tower and the blades ? I assume that the platform inertia values just depends on the mass distribution of the platform and doesn’t include any other components?

Dear @Rahul.Ramachandran,

The inertias of the hub and generator specified in ElastoDyn are the inertias about their centerline; other inertias are assumed to be zero. The inertia of the nacelle specified in ElastoDyn is the inertia about the yaw axis; the pitch and roll inertias are assumed to be zero about the nacelle CM. These other inertias could be added to ElastoDyn in the future, but likely have little effect on the dynamic response for most wind turbines.

The tower and blade inertias about their transverse axes are calculated within ElastoDyn based on the the specified mass distributions. The inertia of the tower about its centerline and the blade inertia about the pitch axis are assumed to be zero within ElastoDyn. These inertias could also be added to ElastoDyn in the future.

You are correct that the platform inertias should be specified based on the actual mass distribution of the platform/substructure.

Best regards,

Thank you so much @Jason.Jonkman