Some Doubt about the Fatigue Life of a 5MW Wind Turbine around Cut-in Wind Speed

Wind & Water Computer-Aided Engineering Software Tools
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Dear Jason
I am George, a student who have research on wind turbine. Thank you for providing us this forum to discuss problems concerning academic and technique problems on wind turbine.

I am now trying to use OpenFast to simulate the dynamic response of a 5MW onshore wind turbine (as specified in Test18 Baseline) under different wind speed so as to determine the fatigue damage at tower base under each case. The wind are set to be turbulent with different average velocity for each case as generated from Turbsim.

However, when I compare the “stress-time history series” at the tower base (TwrBsMyt) under 4m/s and 6m/s, the tower seems to vibrate much more severely under 4m/s than under 6m/s, leading to a greater fatigue damage under lower wind speed (as shown in the following two pictures).

All the settings in AeroDyn, ElastoDyn, ServoDyn and primary files are same as default except for the Wind Inflow file are set to read Turbsim file of different wind speed.

My questions is that:
* Is it reasonable for a tower to vibrate severer under lower wind speed? (I guess when the rotor is under a stochastic wind speed around cut-in, the rotor control system will switch back and forth between on and off, thus increasing the tower vibration due to unsteady control command, but it just my personal guess).

  * Or is there anything wrong with my operation of the software, or are there more possible reasons for this scenario that I encountered?

tower_base_moment_6mps.png
tower_base_moment_6mps.png1149×300 25.7 KB
tower_base_moment_4mps.png
tower_base_moment_4mps.png1149×300 29.9 KB

Yours Sincerely
George

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Dear George,

Just a few comments:

  • Unless you’ve changed the controller for the NREL 5-MW baseline turbine, the baseline controller does not include control logic for start up or shut down situations, so, that is not likely playing a role.
  • You haven’t said how you’ve set the turbulence (turbulence intensity or wind speed standard deviation) differently between these two wind speeds, but this could obviously play a role in the tower fatigue.
  • The rotor speed is lower at the lower wind speed, which means that the first tower bending natural frequencies (in both the fore-aft and side-to-side directions) will be closer to the three-per rev (3P) excitation from the rotor aerodynamic excitation, which means the rotor excitation will be closer to resonating the tower at the lower wind speed, which has implications for tower fatigue loading.
  • Fatigue loads are influenced not just by the oscillations in response, but also the mean load, and the higher wind speed clearly shows a higher mean tower load.

Best regards,

3

Dear Jason
Thank you for your instant and conducive reply!

The turbulence model was set to be “IECKAI” with “IEC turbulence category B” for both the 4m/s and 6m/s wind speed. Therefore, I think they would have the same turbulence intensity of 0.14 as specified by IEC standards.

I have checked the rotation speed under both mean wind speed, and found that the corresponding 3P frequency is around 0.36Hz under 4m/s, which is closer to the 1st natural frequency of the tower (0.32Hz). Does it indicate that the resonating effect may play a major role for this phenomenon?

Yours Sincerely
George

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Dear George,

Actually, the turbulence intensity decreases with increasing wind speed when selecting “IEC turbulence category B” turbulence. While you are using an I_ref of 0.14, the turbulence intensity at 4 and 6 m/s is much higher than 14%.

The resonance effect will certainly increase as the 3P excitation frequencies gets closer to the tower natural frequency.

Best regards,

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Dear Jason
The turbulence intensity in IEC Model is decrease with an increase of wind speed, thank you for your correction!

Yours Sincerely
George