Problem with yaw DOF on OC3 Spar 5MW wind turbine

[theot-student]

Hello everyone,

I am currently trying to run a simulation of the OC3 5MW wind turbine model with spar buoy (the one with 3 lines mooring system on
the spar platform). Unfortunately I got a problem with the yaw angle DOF of the tower. When I enable this DOF in the elastodyn file, I got the following error during the simulation:

For information, I am trying to run a rigid simulation of the floating wind turbine without waves and without controls.
Here are the hydrodyn and Mooring files I am using:
NRELOffshrBsline5MW_OC3Hywind_FEAMooring.txt
NRELOffshrBsline5MW_OC3Hywind_HydroDyn.txt
Another thing is that the simulation is working correctly when I remove the tower yaw DOF.
Does anyone have an idea where the problem could come from?
Thanks in advance
T. Barbier

[jjonkman]

Dear @theot-student,

Can you clarify other settings about your simulation? Is the rotor operating or parked/idling? What wind and wave conditions are you inputting? By "tower yaw DOF", do you mean platform yaw (PtfmYDOF = TRUE) or nacelle yaw (YawDOF = TRUE)?

Best regards,

[theot-student]

Dear @jjonkman,

Thank you for your reply. In my simulation, the rotor should operate normally. I have remove the initial speed but the problem is the same with an initial velocity. Here are the DOF I use:

By tower yaw DOF, I mean the platform yaw (PtfmYDOF = TRUE).
For the environnement condition, I use a steady 12 mps wind with 0° direction. I have also deactivated the waves (WaveMod = 0) and the current, because I just wanted to make a simulation in still water. Here is my SeaState file:
SeaState.txt

Best regards,
T. Barbier

[jjonkman]

Dear @theot-student,

Thanks for clarifying.

You mentioned in your first post that you don't have control enabled (presumably you mean that you disabled ServoDyn). But I see that you have the GenDOF enabled in ElastoDyn. Without ServoDyn enabled, there will be no generator reaction torque, so, the aerodynamic loads will quickly accelerate the rotor, making the model unstable. For an operational rotor, you should enable a generator torque or controller model in ServoDyn if you have GenDOF = TRUE; alternatively, if you really don't want control, you should set GenDOF = FALSE to keep the generator spinning at a constant speed regardless of the torques applied.

Best regards,

[theot-student]

Dear @jjonkman,

Thanks again for the reply. As you suggest, I have added a simple VS control to make sure the acceleration of the rotor is not too high. However, I still got the warning message about small angle assumption violated in yaw position and then the simulation is stuck on this warning:

I wonder if it's normal to have a warning message make the simulation stop like this?

Regards,
T. Barbier

[jjonkman]

Dear @theot-student,

This is not "normal behavior" and suggests a problem with your simulation set-up. I see the initial generator speed and pitch are zero even though the wind speed is rated. With poor initial conditions, the controller is likely not responding properly. We generally recommend initializing the rotor speed and blade-pitch angles to their expected (mean) values conditioned on the mean hub-height wind speed, which for the NREL 5-MW baseline wind turbine at 12 m/s should be RtSpeed = 12.1 rpm and BlPitch = 3.823deg.

Best regards,