Designing for yaw errors using FAST

I am trying to design monopiles for the NREL baseline 5MW turbine using site specific hurricane hazard according to IEC 61400-3 load case 6.2a, which requires the consideration of all yaw errors from -180 to +180 degrees. I have read some literature mentioning that there is an “instability” that occurs for the NREL baseline turbine at around +35 degrees. This is described as an “aero-elastic interaction causing negative damping in a mode that couples rotor azimuth with platform yaw”, notably from pages 118-119 of “Dynamics Modeling and Loads Analysis of an Offshore Floating Wind Turbine” by Jason Jonkman. Even if there are fail safe measures in the yaw control system such as breaks or slips, DLC 6.2a assumes that there is an error in these systems, so one can argue that these specific yaw angles must still be considered in design.

My question is, if we assume that this instability is a possible physical instability of the turbine/blade/tower configuration, then is it reasonable to consider the structural response generated at this yaw error as a reasonable response to design for? If it is unreasonable to design for these yaw angles, then what is the range of angles that should be considered for DLC 6.2a?

Dear Spencer,

Yes, I’m familiar with the problem, as explained in the report you mentioned. I have talked to other modelers in Europe and heard that this problem has been seen in many different turbines and with different aero-elastic software. I’ve also met a PhD student at DTU in Denmark that researched the causes/solutions of this problem. My understanding is that the current belief is that the instability would likely not occur in the physical world and that the aero-elastic software only predict a problem due to simplifications in how the software treat the dynamics of deep stall. My understanding is that the industry’s current approach to dealing with this problem is to either (1) bypass it by choosing yaw errors that don’t result in the instability or (2) increase the structural damping in the blade edge / tower side-to-side mode until the instability goes away. For (1), instead of limiting the range of yaw errors, one may simply ignore the case that causes the instability (in your case this could mean dropping the 35-degree case).

Best regards,

Hi Jason,

Thank you for your response. Are you able to pass along any further information about the work done at DTU on this topic? It would be great to be able to reference a conference paper or journal article citing that the two solutions proposed above are reasonable and generally accepted by industry.

Thank you for your help.
Spencer Hallowell

Dear Spencer,

I suggest reviewing the work of Witold Skrzypinski et al; Witold completed his PhD from DTU Wind Energy in 2012 regarding the analysis and modeling of unsteady aerodynamics with application to wind turbine blade vibration at standstill conditions.

Best regards,

Once again thank you for your response and for the reference. I will look through his work to better understand this phenomenon.


Dear jason:
I am trying to model a “standstill” OC4 Jacket supported OWT(test21) in OpenFAST with a Vhub=50.83m/s and a yaw misalignment of 30º.WakeMod =0.AFAeroMod=1.turbulence intensity=11%.I have some questions about the result I get, as shown in following pic

I did not modify any of the structural parameters.The displacement response at the top of the tower peaked near 1.07 Hertz.The same situation also occurs in the out-of-plane and in-plane tip deflection. Although it’s close to 1st aysmetric edgewise yaw, but I’ve set YawDOF to False,YCMode to 0.Could it be the 1st edgewise collective or the 2nd global fore-aft and side-to-side mode according to the Phase I – Results of Coupled Simulations of an Offshore Wind Turbine with Jacket Support Structure ?so I have the following questions:
1.Are my results credible?or I make a mistake somewhere.
2.Which mode of the OWT corresponds to 1.07 hz?
I just started learning the dynamic response of the OWT, and I’m sorry if my question is silly.Looking forward to your reply.
Best regards

Dear Chenxu.Zhao,

Indeed your simulation is suffering the same edgewise / side-to-side instability discussed earlier in this forum topic. For the NREL 5-MW wind turbine, the instability will be present in parked/idling simulations in high winds with yaw errors between 20-40 degrees. The frequency you are reporting is the frequency of the blade-edgewise modes. Disabling the nacelle-yaw DOF (YawDOF) will not solve the problem, but disabling the edgewise DOF (EdgeDOF=False) or increasing the edgewise damping (BldEdDmp(1)) would. See the discussion above for further guidance.

Best regards,