Question about the free vibration of offshore wind turbine tower

Wind & Water Computer-Aided Engineering Software Tools
1

Dear All

I have a question about the vibration of the wind turbine tower. I use Fast 8 and only enable Tower fore-aft displacement DOF and platform pitch DOF and enable the use of TTMD in X direction.

I set the wind to be 0.001m/s and wave condition to be still water. The initial platform pitch angle is set to 0.1 degrees. In my opinion, the TTMD will damp the tower and platform motion so the final position of the tower and barge-platform will be 0. However, I found the final position of the platform is -0.055 degree and the final position of the tower is -0.02m when I set rotor speed to 0 rpm. The final position of the platform is 0.048 degree and the final position of the tower is 0.0036m when I set rotor speed to 12.1 rpm. I am wondering if this is normal? If it is normal, Is there anything wrong about my settings of just because of the FAST model itself?

Best

XING WEI

2

Dear Xing Wei,

I’m not sure which wind turbine you are simulating (are you using the NREL 5-MW turbine?), but I would not expect the platform and tower deflections to be zero in the absence of aerodynamic or hydrodynamic forcing if there is an offset of the center of mass of the rotor-nacelle assembly from the tower centerline i.e. the overhanging weight will cause a tilt of the tower/platform in that direction due to gravity. A spinning rotor will also induce a thrust (depending on the blade-pitch angle) even if the wind inflow is still.

Best regards,