Tower top loads calculation

Hi everyone,

I am using MLife for computing the lifetime of the tower top.
For this I try to determine the tower top bearing loads.
I have the acceleration measured on the three directions of the tower, so with the second law of Newton I calculate the shear force by multiplying the acceleration by the mass of the rotor and the nacelle.

But my questions are about the moments, as I am new in the wind turbine field, I don’t really see what represents the bearing yaw moment, bearing roll moment and bearing pitch moment and how to compute them.

If anyone can enlight me about these physical parameters, I would be really grateful.

Thanks in advance for your help,

Dear Alexandre,

If you are running FAST, you can simply output the tower-top loads (both forces and moments).

Your calculation for the tower-top forces based on accelerations will miss the contribution of the applied aerodynamic forces on, and component weights of, the rotor and nacelle, plus some smaller effects due to the flexibility of the rotor.

For the tower-top moments, you could do a similar calculation using the moment arm for the forces (applied and weight) and masses, but you’d also want to include the rotational inertial effects.

Best regards,

Dear Jason,

thanks for your reply.

I know that I could output these values by using Fast, but the objective is to estimate the effect of the rotor imbalance measured by accelerometer in the lifetime of a machine.

I don’ see why the aerodynamic forces are not contributed, is the sum of all the forces applied in a point that are taken into account in the operation Mass*acceleration.

Yes i forgot to consider the mass of the nacelle components.

For the moment, it means that I have to allow for all of the forces their apllication points. (Does that mean for the aerodynamic forces I need to compute the wind speed in the downwinds and upwind blades ?).

Thanks again for your reply,

Dear Alexandre,

Consider the simple one degree-of-freedom mass-spring damper system with applied force:

mqdd + cqd + k*q = F

The reaction load transferred to the fixed frame is:

F_reaction = F - mqdd = cqd + k*q

Relating this your case, “F” is the applied aerodynamic force and weight of the rotor-nacelle assembly (RNA), “m” is the RNA mass, and “c” and “k” represent the tower damping and stiffness. Thus, acceleration is not enough, you need to know the applied force as well.

I hope that helps.

Best regards,