Pitch Free Decay Test

Wind & Water Computer-Aided Engineering Software Tools
1

Good Morning,
I am attempting to perform free decay tests using the OC4 Semi-submersible (5MW_OC4Semi_WSt_WavesWN) model. These results are being compared to a paper titled, “Systematic Comparisons among OpenFAST, Charm3D-FAST Simulations and DeepC Wind ModelTest for 5MW OC4 Semisubmersible Offshore Wind Turbine”.

I am using OpenFAST 3.5.0 on Windows compiled using Visual Studios 2022. I successfully ran the surge and heave free decay tests, noticing some minor differences. However, when I attempted to run a pitch free decay test with an offset of 7.5 degrees I encountered the following error:

FAST_Solution:CalcOutputs_And_SolveForInputs:SolveOption1:ED_HD_InputOutputSolve:HydroDynCalcOutput: Angles in GetSmllRotAngs() are larger than 0.4 radians.
HydroDyn_CalcOutput:HDOut_MapOutputs: Angles in GetSmllRotAngs() are larger than 0.4 radians.
ED_HD_InputOUtputSolve:HydroDyn_CalcOUtput: Angles in GetSmllRotAngs() are larger than 0.4 radians.
HydroDyn_CalcOutput:HDOut_MapOutputs: Angles in GetSmll RotAngs() are larger than 0.4 radians.

I can successfully run the free decay test with a maximum displacement of 1 degree. Anything higher causes the simulation to fail with the same error message at the 0.4s time step. Plotting the results indicates that the pitch (along with a couple other variables) quickly approaches infinity.

Configuration:

  • CompInflow, CompAero, CompSub, and CompIce are disabled.
  • CompMooring = 2
  • RotSpeed = 0
  • WaveMod = 0
  • PtfmPitch = 7.5 (all other initial conditions in ElastoDyn are 0)

Troubleshooting:

  1. Disable all DOF except PtfmPDOF
  2. Set PotMod to 0 (and set PropPot to FALSE for all members)
  3. Play with various initial conditions

All my run files and plots of the results I have gotten so far are in this google drive folder: NREL Troubleshooting Please let me know if you have any issues accessing or viewing it.

Do you have any suggestions of what I can do to successfully run the Pitch Free Decay Test?
Thank you,
Jolie

2

Dear Jolie,

I noticed that (1) you are using FEAMooring instead of MoorDyn, which is the default mooring module used by the OC4-DeepCwind semi r-test, and that (2) the properties you’ve set in FEAMooring are different than the default model. I would suggest switching back to MoorDyn or verifying that all of the inputs to FEAMooring are set correctly.

Best regards,

3

Hi,

Thank you for your suggestion. Changing the Mooring system from the FEAMooring file to the MoorDyn file allowed the simulation to run successfully! The results don’t match quite the paper I am comparing it to yet, but I will be troubleshooting that. I attached the pitch displacement plot below along with the calculated natural frequency.
Thank you so much for your help,
Jolie

FreeDecay_7.5PitchResults
FreeDecay_7.5PitchResults800×946 35.3 KB

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