I am simulating the offshore wind turbine of Oc4-jacket, I have some questions for the modal analysis, what is the second frequency of the whole system (including the substructure) of Oc4-jacket? If not, can bmodes calculate the second frequency of the whole system?
Looking forward to your reply and thank you for your kindness!
According to the OC4 Phase I results (nrel.gov/docs/fy12osti/54124.pdf), the second global bending frequency of the tower + jacket of the OC4-jacket is around 1.2 Hz.
BModes can calculate tower natural frequencies, but the jacket must be simplified in BModes as a tower-base boundary condition involving 6x6 mass and stiffness matrices, which can be derived from the SubDyn summary file (hydro_M = MBBt, hydro_K = KBBt, mooring_K = 0).
I hope that helps.