OC4 Task 30 Phase II: Unstable or Changing the stipulated timesteps.

Dear Dr. Jonkman,

I am trying to reproduce the load case 3.1 (steady wind and regular waves) from the OC4 phase 2 analysis of the DeepCWind floating wind turbine.

The issue is I get a Fatal Error due to an unstable model as I saw on other posts. I can just solve it by reducing the timestep of the main input file to make it work. But I followed the instructions given by the document “Load Cases for OC4 phase II” of 2013, found in the following link you provided in another post: https://drive.google.com/drive/folders/1DCtqqoRvu_kvof8mcagQ3imrK7xmOLv3

It stipulates that:

  • DT output = 0.05 s, so the Nyquist frequency is 10 Hz,
  • DT wind = 0.05 s in the wind paragraph (although it is written 0.1 s in Table 1: General Settings),
  • and DT wave = 0.1 s.

I applied it, with DT wave = 0.1 s, DT wind = 0.05 s, and DT simulation of the main input file (.fst) = 0.05 s. In that case, the model is unstable and I get instantly a Fatal Error.

But if I reduce the DT simulation (.fst), I get a warning that DT wind cannot be less than DT simulation.

In the end, I cannot reproduce the load case 3.1 without having an unstable model, or I can do it but reducing DT wind to match DT simulation that I reduced to get a stable model.

I hope it is clear enough, and I thank ou in advance for your help so I can have a stable model reproducing exactly the load case 3.1.

Have a nice day,


Dear Bertrand,

The FAST model of the OC4-DeepCwind semisubmersible requires a time step of 0.0125. Of course, you can output the time-series data at 0.05 s.

But I’m not familiar with the error, “DT wind cannot be less than DT simulation.” Can you send the exact error message? We normally set the time step in TurbSim equal to 0.05 regardless of the FAST time step, so, I would not expect any error related to this.

Best regards,

Thank you for your answer, and here is attached the screenshot of the error message.

Dear Bertrand,

This error is referring to the AeroDyn v14 time step, not the InflowWInd time step. Simply set DTAero in the AeroDyn v14 input file to 0.0125 s and the simulation should run without error.

Best regards,

Dear Jason,

Oh ok, I was confused between DT wind and DT Aero.

Thank you for your answer.

Best regards