1. Wind Turbine Broadband + Deterministic Fatigue
Nigel Barltrop
University of Strathclyde

2. Nature of Stress History
Broadband stochastic:
Low frequency gusting
Rotational sampling of gusts at multiples of rotor frequency
Natural frequency resonances
Deterministic:
Moment from blade weight reverses in plane (1P)
Blades sample high winds at height and low wind speeds near ground
Cyclic load from any yaw error
Tower shadow

3. Fatigue Analysis Options
Usual method for design
Time domain
Calculate all effects together
Rainflow count
Simplified check
Frequency domain for stochastic effect
Rainflow count using Dirlik distribution
Add in deterministic damage

4. Methods of adding Stochastic & Deterministic Damage
Assume Linear or Quadratic (SRSS) addition of proportion (p) of stresses at lowest frequency
Add additional damage from higher frequencies
(Stress to D1/m, m = 3 to 5, 3 assumed here (corrosive environment))
Linear suggests direct addition of stress ranges (likely when frequencies very different)
Quadratic suggests random addition (seems more reasonable when frequencies close)

5. Comparison of methods with rainflow count
Safe
Unsafe
Linear: safe but sometimes perhaps too safe,
Quadratic: generally unsafe but a closer estimate

6. Deterministic Stress: Phase Angle Sensitivity

7. Conclusion In this case:
Linear addition of stresses is as expected pessimistic (safe)
Quadratic (SRSS) addition is slightly unsafe