Mode shapes for Floating offshore wind turbine

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Mode shapes for Floating offshore wind turbine

Postby Danupon.Subanapong » Thu Dec 05, 2019 12:22 am

I am working with floating offshore wind turbine modeling in FAST. However, I have a little confuse about the tower mode shapes. I use BModes to derive mode shapes. I would like to ask if we need to use a tower only mode shape or tower with a floater mode shape. Also, BModes does consider floater as a rigid body. I know that NREL is now using ADAMS to derive mode shapes. Do you consider floater in modal analysis for tower mode shapes or not? If you consider a floater in tower mode shapes analysis, do you consider floater as a flexible body?
Thank you so much in advance.

Danupon Subanapong

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Location: Boulder, CO

Re: Mode shapes for Floating offshore wind turbine

Postby Jason.Jonkman » Thu Dec 05, 2019 7:00 am

Dear Danupon,

Correction: NREL used ADAMS to derive tower mode shapes for floating offshore wind turbines before BModes was capable of doing that. NREL has now used BModes to derive tower mode shapes for floating offshore wind turbines.

When deriving tower mode shapes for ElastoDyn--whether fixed or floating, it is important to properly consider the tower-top and tower-base boundary conditions. FAST currently treats the floater of a floating offshore wind turbine as a six degree of freedom rigid body (but we are working to allow for flexible floaters in a future release of OpenFAST). As such, when deriving the tower mode shapes, it is important to use the mass/inertia, added mass, and hydrostatic stiffness of the floater as a tower-base boundary condition. The tower-top boundary condition should be free to move (not constrained) and include the mass/inertia of the rotor-nacelle assembly as a tower-top boundary condition. These boundary conditions are able to be set in what is called BModes_JJ on this forum.

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
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