Pitch Free Decay Test

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.


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


  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,

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,


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,

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