The natural frequencies of the blade asymmetric flapwise modes of the 5MW wind turbine

Wind & Water Structural Analysis
1

Hello all.
I have a question about the blade flapwise natural frequencies of the OC4-DeepCwind semisubmersible.

In the OC4 results of the link provided by Jonkman(Phase II Floating Semisubmersible - Google Drive), the LC1.1 natural frequencies of the 1St Blade Asymmetric Flapwise Pitch and Yaw modes are 0.6695Hz and 0.6667Hz, which are very close. However, I found different result in the discussion in the NREL Forum( OC4 DeepCWind Eigenanalysis/Static Equilibrium/Free Decay tests), as shown in the following figure. The results in the figure are 0.672Hz and 0.635Hz, which is different from the former, and the results of the latter are more consistent with my simulation results.

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Can you help me distinguish which one is right? If the results in the figure are correct, can you tell me in which document these results in the figure exsit?

Thank you very much,

Best regards.

2

Dear @Dehuang.Gong,

I’m not sure; I have not computed these natural frequencies recently. NREL generated the original OC4 Phase II results, but these are from a much older version of FAST. The results you share were from a user, not NREL. You mention that the latter results are are more consistent with yours; can you clarify how you are generating results and what you are comparing?

FYI: Linearization of an OpenFAST model of the NREL 5-MW baseline wind turbine atop the OC4-DeepCwind semisubmersible is a standard r-test: r-test/glue-codes/openfast/5MW_OC4Semi_Linear at main · OpenFAST/r-test · GitHub.

Best regards,

3

Dear Jason,

Thank you for your reply.

I use the finite beam element method to model the OC4 full system (as shown in the following figure), and the comprison between my results and the exsiting results is shown in the following table, in which the Fast and Bladed results are all taken from the link you provided(Phase II Floating Semisubmersible - Google Drive ).

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In the table, I found that there is obvious deviation between the natural freuencies of the 1st Blade Asymmetric Flapwise Pitch and the 1st Blade Asymmetric Flapwise Yaw, which is similar to that in Bladed, however, the results of Fast show that these two frequencies are very close. Besides, by observing the modal shapes, I found that the 1st Blade Asymmetric Flapwise Pitch is mainly coupled with the fore-aft tower bending, while the 1st Blade Asymmetric Flapwise Yaw is mainly coupled with the torsion of the tower.

Can you explain why these two natural frequencies obtained by Fast are so close?

Thank you,

Best regards.

4

Dear @Dehuang.Gong,

Those OC4 Phase II results where generated a long time ago in a much older version of FAST, so, I can’t explain the results, and I would encourage you to rerun the case using OpenFAST to verify if the results have changed and are closer to what you expect.

Best regards,