Blade Pitch Control Before Rated Wind Speed-IEA 15 MW on UMaine Semi-Sub Floater

Dear all,

I am investigating the effect of static floater pitch angle on the IEA 15 MW wind turbine, placed on the UMaine semi-sub floater. More specifically, I am trying to undestand how the static pitch angle affects the power produced by the turbine. For this reason, I am comparing the power curve of the IEA 15 MW fixed bottom wind turbine with the one produced for the fixed bottom configuration under a constant platform pitch, which is the average value of the time series observed for the UMaine floater. The configuration I am using is the following:

1.) HydroDyn and MoorDyn disabled.
2.) Platform 6 DoFs disabled. ElastoDyn initial conditions are the ones for the fixed bottom model, except for the platform pitch. It is set to be constant during the simulation at a value equal to the mean ploater pitch angle value observed for the FOWT simulations (presented in the figure that follows)
3.) The DISCON.IN file used is the one for the UMaine floater.

My issue is that the blade pitch control starts earlier than expected, at 10 m/s and not at 10.59 m/s, as with the controller of the IEA 15 fixed bottom wind turbine, while the blade pitch angle values for a wind speed of 11 m/s are higher than the expected for the IEA 15 MW fixed bottom wind turbine. Rater power is achieved at 12 m/s and not at 10.59 m/s, as with the fixed bottom wind turbine.

In addition, I observe an increase in the inflow angle near the hub (up to r/R<0.3), which suggests that the hub height may experience higher wind speeds, thus triggering the blade pitch control. It should be noted that, in the configuration I am working on, both the average rotational speeds and average induction factors are lower when compared to the fixed bottom configuration, which is explained by the blade pitching.

Therefore, I am wondering whether the inflow angle increase indeed occurs due to an increase of the wind speed at hub height or there is another unforeseen factor, which results in early blade pitching.

Best regards,
Ioannis Voultsos.

Dear Ioannis,

Can you clarify what type of wind inflow you are using in these comparisons? And when you refer to inflow angle, can you clarify which OpenFAST output(s) you are using?

Best regards,

Dear Jason,

I am using steady wind conditions with a power law exponent of 0.

As for the inflow angle, since in my project I am mostly interested in the angle of attack variation, I have as output the angle of attack for nine nodes (B1N#Alpha). Since in the windspeeds ~6-10 m/s the blade pitch angle is set to zero and the section twist angle is constant, I assume that the angle of attack variations are mostly affected by the inflow angle variation.

Best regards,
Ioannis Voultsos.

Dear Ioannis,

Given that you are using steady uniform inflow without shear, the hub height wind speed should not be changing. However, AeroDyn will use the flow normal to the disk in its induction calculation, and effectively the fixed platform-pitch angle you are specifying would add to the existing shaft tilt, and the combined effect would be a reduction of the wind speed normal to the disk. So, I’m also puzzled by the results you are describing. That said, I’m not familiar enough with how the ROSCO controller is handling this case. Hopefully someone more familiar with the ROSCO controller can comment.

Best regards,

Dear Jason,

Indeed it is a controller issue. The controller is tuned to sense the nacelle fore-aft velocity and pitch the blades accordingly.

However, I am trying to understand what is the wind speed that the controller senses in order to pitch the blade. Although the blade pitch controller does not directly use the inflow wind speed in regulating blade pitch, is there a way of nevertheless observing which wind speed is actually involved into the controller’s actions?

I tried to open the .dbg file generated by the DISCON.IN file, but it does not include the wind speed. Only the following variables are included:
NacIMU_FA_AccF (m/s)
WE_Vw (rad)
NacIMU_FA_Acc (rad/s^2)
FA_Acc (m/s^2)
Fl_Pitcom (rad)
test (rad/s)

Best regards,
Ioannis Voultsos.

Hi Ioannis,

ROSCO uses a wind speed estimate to change the lower pitch limit of the controller for the IEA 15-MW Semi. That is why the pitch starts at a lower wind speed and rated power is reached at a higher wind speed than you expect.

For your study, you may want to disable these features. You can do this by setting WE_Mode = 0 (no wind speed estimate, a low pass filtered wind speed signal is used) and PS_Mode = 0. More details can be found here: 4. ROSCO Controller Structure - ROSCO 2.4.0 documentation

Best, Dan