Modal analysis of floating NREL 15MW turbine

Hello,

I am currently trying to perform a modal analysis of a tower of the floating turbine with the parameters from IEA Wind TCP Task 37 (https://www.nrel.gov/docs/fy20osti/76773.pdf )
In the article it is precised that the tower was simulated with “free-free” boundary conditions. Does it mean that first natural frequencies were found in an analysis, where both platform and the tower have been considered? The platform is then assumed floating, with no nodes being fixed?
Was the flexibility of a platform taken into acount during the modal analysis?

Thank you in advance for your answer.
Kind regards,
Sofya

Hello Sofya,

Thank you for your question and interest in the 15MW reference turbine. The original floating tower design and analysis was conducted with BModes_jj (referenced frequently in this forum), which can appropriately take into account the mass/inertia of the floating platform, the hydrostatic stiffness, and the mooring stiffness as the boundary condition at the base of the tower. This is what is implied by the “free-free” comment, in that the tower base was not assumed to be clamped.

That being said, we also know that there was an error in the original design loop and the intended natural frequency of about 0.5Hz was not achieved. We are working to address this in the pending v1.1 update currently drafted in a develop-branch on the project repository.

Cheers,
Garrett

Dear Garrett,

Thank you very much for your answer, it really helped me to better understand the results and modeling processus, described in the mentioned article.

Kind regards,
Sofya