In the SubDyn user manual, the guideline for retaining Craig-Bampton modes are: “Until full-system linearization is made available, experience has shown that it is sufficient to enable all C-B modes up to 10 Hz (the natural frequencies of the C-B modes are written to the SubDyn summary file).”
For the monopile-supported offshore wind turbine that I’m working on, even the lowest C-B frequency is above 10Hz. I checked the IEA 15MW reference offshore wind turbine and found that, similarly, even the lowest C-B frequency is above 10Hz:
True Echo - Echo input data to "<rootname>.SD.ech" (flag) "DEFAULT" SDdeltaT - Local Integration Step. If "default", the glue-code integration step will be used. 3 IntMethod - Integration Method [1/2/3/4 = RK4/AB4/ABM4/AM2]. True SttcSolve - Solve dynamics about static equilibrium point -------------------- FEA and CRAIG-BAMPTON PARAMETERS--------------------------- 3 FEMMod - FEM switch: element model in the FEM. [1= Euler-Bernoulli(E-B); 2=Tapered E-B (unavailable); 3= 2-node Timoshenko; 4= 2-node tapered Timoshenko (unavailable)] 3 NDiv - Number of sub-elements per member True CBMod - [T/F] If True perform C-B reduction, else full FEM dofs will be retained. If True, select Nmodes to retain in C-B reduced system. 12 Nmodes - Number of internal modes to retain (ignored if CBMod=False). If Nmodes=0 --> Guyan Reduction. 1 JDampings - Damping Ratios for each retained mode (% of critical) If Nmodes>0, list Nmodes structural damping ratios for each retained mode (% of critical), or a single damping ratio to be applied to all retained modes. (last entered value will be used for all remaining modes).
CB Reduced Eigenvalues [Hz]. Number of retained modes' eigenvalues: 12 1 0.222113E+02 2 0.222113E+02 3 0.398429E+02 4 0.416397E+02 5 0.416397E+02 6 0.425518E+02 7 0.425518E+02 8 0.578223E+02 9 0.578223E+02 10 0.648372E+02 11 0.687125E+02 12 0.687125E+02
Could you comment on:
1.) In this case, is it okay not to include any C-B mode, i.e. use Nmodes=0 that gives a Guyan reduction?
2.) Physically, the C-B modes are the modes of the sub-structure clamped both at the reaction joints and at the platform reference points (in the example of IEA 15MW, 22.2Hz in first bending, 39.8Hz in first torsion, 41.6Hz in second bending). Is my understanding correct?
3.) When will full-system linearization including the SubDyn and HydroDyn models be made available in either the master or the dev branches of OpenFAST?
4.) Before the above becomes available, if I do the following: remove SubDyn and HydroDyn models, and instead model the entire support structure in ElastoDyn essentially like an onshore wind turbine, and let the system undergo linearization analysis, would the result be equivalent/similar to using SubDyn with Guyan reduction?