Dear Jason,
I am trying to derive tower properties for the OC5 Tower based on the report “Definition of the OC5 DeepCwind Semisubmersible Floating System” by Robertson et al. to use in a OC5 OpenFAST model that I am setting-up. The end goal is to compare OpenFAST and other simulation tools to the OC5 experimental data. To calculate mode shapes and natural frequencies I am modelling the tower in BModes_JJ, which I found in another forum thread (Bmodes download).
I imported linearized stiffness matrix, added mass and hydrodynamic stiffness. The frequencies of the first six modes match the ones in the report for the floating platform so I am assuming the platform is modelled correctly.
The issue is that the frequency of the first tower f-a and s-s modes are extremely low, and are actually pretty close to the heave frequency of the floating platform. I have noticed that adjusting the f-a and s-s stiffness tuners in BModes (flp_stff_mult & edge_stff_mult) of a factor 40 gives me natural frequencies of 0.31 and 0.32 Hz which match the ones measured in the report, but this obviously makes very little sense to me.
Any insight on what I might be doing wrong? I have triple checked my properties file and I believe tower properties are correct, but can provide them if useful. Maybe upscaling from model scale has something to do with this? I have defined the model at full scale as in the report. Would using the OC4 elastodyn input file with appropriately scaled mass be a solution? I read that the OC5 model was designed to match the stiffness and natural frequencies of the OC4 NREL 5MW tower.
Thank you for any help,
Best Regards,
Francesco Papi