HydroDyn buoyancy term

[Yingxin.Lv]

Dear @Jason.Jonkman,
I am conducting analysis of the 5MW OC3 offshore wind turbine using OpenFAST , The output parameters M1N1FBxi, M1N1FByi, M1N1FBzi, M1N1MBxi, M1N1MByi, and M1N1MBzi represent distributed buoyancy forces/moments and I found their calculation formulas in “(https://webserver.nrel.gov/wind/nwtc/assets/downloads/HydroDyn/HydroDyn_Manual.pdf)”.

However:In the cylindrical section (constant radius R(z)), ∂z/∂Rshould theoretically be zero.The solver outputs M1N1MByi non-zero .I would greatly appreciate it if you could clarify why this occurs

-When modeling the platform as a rigid body, I would greatly appreciate clarification on how to calculate the forces and moments generated by dynamic pressure.
I would be appreciated it if you could reply. Thank you
Best regards,

[Jason.Jonkman]

Dear @Yingxin.Lv,

The equation you grabbed was from an older version of HydroDyn and is not used in newest version of OpenFAST.

More up-to-date theoretical details are provided here: 4.10. HydroDyn User Guide and Theory Manual — OpenFAST v4.0.2 documentation, for example, the buoyancy and dynamic pressure terms used by the strip-theory solution are documented here: https://openfast.readthedocs.io/en/main/_downloads/39aafbb61663ae16bd1a7e80f305c20b/HydroDyn_Plan_TCF_Morison.docx.

That said, I know there are updates to these documents, as well as additional implementation plans, that have not been uploaded yet to readthedocs.

Best regards,