FOWT platform steady state pitch angle

Wind & Water Controls
1

Dear all,

An important control objective for FOWT is to stabilize the platform pitch motion in order to avoid negative damping phenomena.
A solution is to force the platform pitching rate to 0, in this case, the platform could stabilize at some non-zero angle.
Another solution is to force the the platform pitch angle and its pitching rate to 0 at the same time, in this case, the platform stays stable vertically.
My question is, which solution is better, with respect to stability and fatigue load?

Thank you
Cheng

2

Dear @Cheng.Zhang,

Just a few comments:

  • You mention control to force the platform pitch angle and rate to zero. Can you clarify what type of control you have in mind?
  • Fatigue is driven more by variations rather than the mean, so, I would think the pitch rate would be more important than the pitch angle as far as fatigue is concerned.
  • To minimize pitch angle and rate, platform and mooring design play a large role in addition to control.

Best regards,

3

Dear @Jason.Jonkman

Thank you for the fast response.

  • My idea is to control the lift force of blades to damp the platform motion.
  • I agree that pitch rate reduction is important to reduce fatigue. However, if the pitch angle could be forced to 0 at the same time, the platform and mooring line will have less fatigue?

Best regards,
Cheng

4

Dear @Cheng.Zhang,

I would not expect the mean pitch angle to have much effect on the fatigue of the support structure.

Best regards,

5

Dear @Jason.Jonkman

I’m not sure about your conclusion.
It is better to make a comparison between the 2 solutions?

Best regards,
Cheng

6

Certainly compare want you want.

I’m also not sure how you would use wind turbine controls to regular the mean pitch angle, which to me is more tied to the floater and mooring system design.

Best regards,

7

Hello @Jason.Jonkman ,

I’m trying to control blade pitch angle to regulate the lift of rotor, then adjust the platform pitch angle. However, it seem not easy to do it, it is not obvious to find a reference for lift force and, the OpenFast can only output the lift of nodes on the blade, rather than the total lift of a blade.

Best regards,
Cheng

8

Dear @Cheng.Zhang,

The control input is the blade-pitch, not the lift force. OpenFAST will naturally compute the impact of the blade pitch on blade lift, which will impact platform pitch. In AeroDyn, you can output the lift force at each aerodynamic analysis node along a blade; you can also output the total aerodynamic load integrated along each blade. But I’m not sure what you want differently than what is already implemented.

Best regards,