Calculate Fatigue Damage from 6 DEL Outputs in NREL 5MW Simulation at 10m/s Wind Speed?

Wind & Water Structural Analysis
1

Dear Jason and all

I am currently working on fatigue damage analysis for the NREL 5MW wind turbine model, and I need some assistance with interpreting and calculating the fatigue damage based on the Damage Equivalent Load (DEL) values obtained from a 10-minute simulation.

Background:

  • We conducted a 10-minute simulation of the NREL 5MW wind turbine at a constant wind speed of 10 m/s.
  • The simulation was performed using OpenFAST, and we obtained the following 6 DEL outputs at the tower base section (TwrBs):
    • TwrBsFxt (kN) – Force in X-direction
    • TwrBsFyt (kN) – Force in Y-direction
    • TwrBsFzt (kN) – Force in Z-direction
    • TwrBsMxt (kN-m) – Moment in X-direction
    • TwrBsMyt (kN-m) – Moment in Y-direction
    • TwrBsMzt (kN-m) – Moment in Z-direction

The Problem:

Now, I want to calculate the fatigue damage over the 10-minute simulation using these 6 DEL values. I have already calculated the DELs using an S-N curve with a slope of 5, but I am unsure how to proceed with these DEL values to calculate fatigue damage.

I would appreciate it if someone could clarify.

Thank you in advance for any help or insights!

Best regards.

2

Dear @Tongzhou.Zhang,

Presumably you are calculating a short-term DEL using an equation like:

DEL = ( SUM( n_i * L_i^m, i = 1,I )/N_eq )^( 1 / m )

with an S-N curve like:

N_i = ( L_ult / ( 0.5 * L_i ) )^m

where
I = total number of cycles
L_i = load range for cycle i
L_ult = ultimate load causing failure at 1 cycle
m = Whoeler exponent (in your case, 5)
N_eq = equivalent number of cycles (for a 1-Hz DEL over 10 minutes, this would be 600)
n_i = number of cycles for cycle i
N_i = number of cycles to failure at load range i

The fatigue damage over this short-term simulation is then:

D = SUM( n_i / N_i, i = 1,I )

Combining terms and simplifying results in:

D = N_eq * ( DEL * 0.5 / L_ult )^m

Best regards,

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