Jacket modeling - Large displacements error

Wind & Water Structural Analysis
1

Hi everyone,
i’m trying to modify the OC4 NREL5MW wind turbine by introducing a different jacket+tower. These are the steps i’ve followed up to now:

  1. i’ve generated an ElastoDyn tower input starting from the tower distributed properties and considering the tower fixed to the ground (tower modes were computed with Modes)
  2. i’ve run simulations with the “onshore” configuration to check the model and everything was fine
  3. i’ve modeled the jacket in SubDyn. The upper-most jacket member defined in Subdyn corresponds to the bottom end of the turbine tower. Thus, in SubDyn, i’ve indicated the upper-most joint as the unique TP joint and i’ve constrained all 6DOFs
  4. in ElastoDyn i’ve enabled all 6 platform DOFs and i’ve set the following properties:
    • TowerBsHt = “TP-joint” z-coordinate
    • PtfmCMzt = 0
    • PtfmRefzt = “TP-joint” z-coordinate
    • PtfmMass = 0
    • PtfmRIner = 0
    • PtfmPIner = 0
    • PtfmYIner = 0
  5. i didn’t modify the tower input file (i’m using the one generated for the onshore wind turbine)

When i try to run a FAST simulation (ElastoDyn + SubDyn only) i keep getting into this error:

FAST_Solution: CaclOutputs_And_SoveForInputs:SolveOption1:ED_SD_HD_BD_Orca_InputOutputSolve:SD_CalcOuput:Small angle assumption violated in SUBROUTINE SmllRotTrans<> due to a large UL input angles. The solution may be inaccurate. Simulation continuing, but future warning from SmllRotTrans<> will be suppressed.

I’ve tried to modify the ElastoDyn parameters reported above as well as the SubDyn simulation options (number of elements per member, CB modes, etc…). Any tentative resulted in the error above
followed by an abort (FATAL ERROR).
If i run the simulation with disactivtated platform DOFs i get the same error, the simulation ends but SubDyn channels become NaN after few simulation instants.

Has someone already faced the problem above? I was thinking to compute the tower modes considering the jacket effect (as described in the SubDyn user guide) but before doing any calculation (it is not trivial to get some parameters required by BModes) i was wondering if this would be enough to solve the problem.
If i decide to model also tower with SubDyn, who do i have to modify the ElastoDyn input file? Do i need to specify a tower input file?

Thank you in advance,

Alessandro

2

Dear Alessandro,

You’re overall approach sounds fine, except for the setting of PtfmYIner in ElastoDyn. Section 5.4 of the draft SubDyn User’s Guide and Theory Manual (wind.nrel.gov/nwtc/docs/SubDyn_Manual.pdf) recommends that PtfmYIner be set equal to the total rotational inertia of the undeflected tower about its centerline. Setting PtfmYIner to zero as you’ve done could result in a numerical instability. I would expect disabling the platform-yaw DOF in ElastoDyn would also eliminate the instability, but with less a less accurate solution. Does setting PtfmYIner in ElastoDyn yield an appropriate solution?

Changing the mode shapes in ElastoDyn to be appropriate to the substructure boundary condition should improve the accuracy of the coupled ElastoDyn-SubDyn solution, but I would not expect it to solve the numerical problem your model is suffering.

I’m not sure I understand your question last question regarding modeling and tower in SubDyn and modifying the tower in ElastoDyn.

Best regards,

3

Dear Jason,

thank you for your kind reply! I will try for sure to set a more appropriate value for PtfmYIner.
For what concern the last question in my previous post i was asking if it is possible to solve the problem i’m facing by modeling the entire jacket+tower in SubDyn instead of running a ElastoDyn/SubDyn model. In these days i tried to model also the tower in SubDynbut, always setting PtfmYIner = 0, i run in the same numerical instability.
I will inform you (and this thread) of any update

4

Dear Alessandro,

Yes, you can certainly model the entire support structure in SubDyn and avoid the tower model in FAST altogether. This is also discussed a bit in Section 5.4 of the SubDyn User’s Guide and Theory Manual. Let us know if setting PtfmYIner to a nonzero value resolves the problem.

Best regards,

5

Dear all
this is to inform the forum that i’ve solved the numerical instability issue by taking three actions:

  • the jacket model was simplified removing joints inside “physical members” (a single lattice element is now enclosed by two joints)
  • PtfmYIner was set to a value different from zero (equal to the tower inertia around the tower centerline as suggested by Jason)
  • the recommended module time step (.fst) was reduced from 0.05 to 0.001

I will post any further news about the model

6

I’m glad to hear that you solved your problem!

Best regards,