Dear Sir/Miss:
Refer to the following article:
July 2020
IEA Wind TCP Task 37
Definition of the UMaine
VolturnUS-S Reference Platform
Developed for the IEA Wind 15
Megawatt Offshore Reference
Wind Turbine
Technical Report
Please kindly be advised of whether the tower bending mode frequency calculations in FOWT free floating conditions should include the Nacelle and blades and floating foundation masses and water added masses and water ballast masses, etc.
Your help will be highly appreciated.
Warmest regards.
kuangjinlu
Dear @Jinlu.Kuang,
I was not the one who authored this report, but I am quite certain these are the tower natural frequencies taking account of the floater mass/inertia (including added mass and ballast) and rotor-nacelle assembly (RNA) mass/inertia.
Best regards,
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Dear Prof. Jonkman:
Many thanks for your information to the wonderful NREL report: IEA Wind TCP Task 37 (VolturnUS-S Reference Platform). Would you please possibly clarify whether the flexibility of FOWT foundation (floating vessel/platform) is included or not?
When the “flexibility” of the complicated platform of foundation structure is included, the natural frequencies of the coupled tower (beam + platform) in free floating conditions are very difficult to reach the dynamic target values based upon the structural vibration eigenvalue assessment software. That is why we ask for your advice. Looks the BModes program does not include the flexibility of the platform. Am I right?
Your help would be highly appreciated.
Warmest regards.
kuangjinlu
Dear @Jinlu.Kuang,
The OpenFAST model of the VolturnUS semisubmersible considers the floater as rigid, but I do know there are projects where substructure flexibility has been considered.
I agree that BModes would treat the floater rigidly for the purposes of computing tower mode shapes. This assumption may be OK for the VolturnUS semisubmersible, that is quite massive and stiff, but may not be a good assumption for lighter and more flexible floaters. In this case, I would recommend deriving tower mode shapes for ElastoDyn from a different software, e.g., an FEA model of the entire support structure (flexible tower + flexible floater).
Best regards,
Dear Prof. Jonkman:
Sincerely, many thanks for your information and recommendation.
Eager to know more about the ELASTODYN program. Browsed the introduction for ELASTODYN and not got to know what kind of FLEXIBLE mechanism/connection between the tower and platform is being used. Would you please possibly say a few words on how to consider the floating platform flexibility in the ELASTODYN subroutine?
Your help will be highly appreciated.
Warmest regards
Kuangjinlu
Dear @Jinlu.Kuang,
When modeling a floating offshore wind turbine with flexible substructure in OpenFAST, the floater flexibility is not considered in ElastoDyn, but in SubDyn. That is, ElastoDyn will provide the 6-degree of freedom (DOF) rigid-body large-displacement motion of the floater through its platform DOFs, whose motion is transferred to the SubDyn module. SubDyn, in turn, will solve for the elasticity of the floater relative to its rigid-body motion, which can be represented as a set of beam, cable, and rigid-link elements interconnected by various joints (rigid, pin, universal, ball), combined with a Craig-Bampton reduction and static-improvement method. SubDyn returns the reaction loads from the floating substructure back to ElastoDyn to ensure proper sturcture-structure coupling. In this case, SubDyn is also coupled to HydroDyn for hydro-elastic simulation, and to a mooring module (MoorDyn, MAP++, FEAMooring).
Best regards,
Dear Prof. Jonkman:
Many thanks for your introduction on OPENFAST SUBDYN, truly comprehensive and quite phenomenal.
No wonder why the OPENFAST program becomes so popular.
Would you mind if we ask you to recommend one or two typical pure technical reports/articles/thesis… on the FOWT foundation fatigue damage (in frequency domain or time-domain) assessment including fatigue loading cases?
Your help will be highly appreciated.
Warmest regards,
kuangjinlu
Dear @Jinlu.Kuang,
Sorry, but I don’t have something specific to recommend.
Best regards,
