# Overturning moment at tower base - 15MW turbine

Dear reader,

I disregarded the wind and wave loads to check the overturning moment at the tower base (since the CoG of the RNA mass is in front of the tower axis).
Here you can see the longitudinal bending moment at the tower base:

I assume the moment is oscillating since the gravity is internally ramped up, right?
But why is the deflection of the first moment positive? That would mean that the structure bends backwards first…

I found the total mass here -

but I’m not sure where to find the “RNA center of mass relative to the nacelle attachment point on the tower”…

I know it’s a basic question, maybe another beginner user can give me a hint

BR,
Heike

Dear @Heike.vonWaaden,

OpenFAST does not ramp up gravity at the start of the simulation. Rather, the transient dynamic response you are seeing is the start-up transient associated with the initial condition solution, which will depend on what initial conditions you have set (like initial rotor speed and initial blade and tower deflections).

ElastoDyn does not directly compute the center of mass of the RNA. But you can calculate that through an OpenFAST linearization analysis, e.g., as discussed in the following forum topics:

Best regards,

@Jason.Jonkman

Dear Jason,

thank you very much for your explanation and your hints!

BR,
Heike

Dear @Garrett.Barter,

I have a couple of questions regarding the IEA-15MW wind turbine:

Regarding the IEA-15MW-tabular excel, why the RNA mass does not have the same value as the sum between 3 blades and the nacelle? Is there any more term that makes the summation be the same as the RNA mass?

Since the 15 MW WT is CLASS IB, is it advisable to run a simulation with a reference speed of 50 m/s and reference turbulence intensity of 0,14 (These are the data associated with class IB WT)

I am comparing some graphs: Why YAWBRTDYP and Q_TFA1 are not the same??

Thank u very much
Alberto

Hi Alberto,

On the RNA mass summary, I think you are missing the inclusion of the Hub System, which includes not just the spherical shell of the hub, but also the pitch systems.

Perhaps I don’t understand your question about running a reference speed of 50m/s and reference TI. You are welcome to run any sort of DLC according to the IEC guidelines, and yes some of the DLC inputs are dependent on the turbine class.

On your last question, I’ll tag another to address that one.

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Dear @Alberto.Utrera,

`Q_TFA1` is the first fore-aft bending mode degree of freedom, which should be close to `YawBrTDxp` (not `YawBrTDyp`), except for the contribution of the second fore-aft bending mode degree of freedom.

Best regards,

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