SubDyn Craig-Bampton modes and system frequencies

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?

Best regards,

Dear Jing,

I’m not too familiar with the OpenFAST model of the IEA Wind 15-MW turbine atop the monopile, but I’ll try to answer your questions.

  1. I would guess, so, but you could always run both cases (with and without C-B modes) to find out. With the static-improvement method (SIM) enabled (SttcSolve = True), the higher frequency C-B modes that are not enabled are treated quasistatically by the solver (without inertial contributions). This is likely a fine treatment for any high-frequency modes that are not directly excited by the turbine or hydrodynamics.

  2. I have not seen the SubDyn summary file for the IEA Wind 15-MW turbine to confirm, but I would guess you are correct based on the frequency pairs at 22.2 and 41.6 Hz, and the fact that 39.8 Hz does not have an associated pairing.

  3. This functionality is currently being implemented now, followed by testing, and I would expect that this functionality will be released by the end of September.

  4. I’m not sure what you mean by “equivalent”, but the simplified model without SubDyn and HydroDyn will not have Guyan modes nor hydrodynamic loading. However, with this approach, ElastoDyn can likely sufficiently model the first two fore-aft and side-to-side bending modes of the entire support structure (tower + monopile). ElastoDyn cannot model the torsion, axial stretching, or shear (Timoshenko beam) effects that SubDyn can model. But ElastoDyn accounts for geometric nonlinearities in the tower that SubDyn can’t model, so, it is always a tradeoff when selecting which module should be used to model the support structure.

Best regards,

Dear Jason,

Thank you for your time and response!

My thought process of question #4 is, in terms of full-system linearization analysis, if I use an “alternate model” (entire support structure modeled by ElastoDyn) to stay within what’s feasible in OpenFAST today, and if I’m not interested in any system axial/torsional modes that are dominated by the support structure which ElastoDyn is unable to resolve, would the result in terms of Campbell diagram be a close enough approximation to the result I would get in September on the actual model involving SubDyn and HydroDyn.

I think you already answered my question #4 (the alternate model should be an okay workaround for now as far as bending modes are concerned) but please do comment if I missed anything.

Thank you and best regards,

Dear Jing,

Yes, I agree.

Best regards,