Page 314 - Wind Energy Handbook

288 DESIGN LOADS FOR HORIZONTAL-AXIS WIND TURBINES

(4) Sum the fatigue damage numbers, n i =N i , according to Miner’s rule, for each
moment range ‘bin’ in the fatigue load spectrum, according to the appropriate
S N curve for the material. S N curves for different blade materials are
considered in Section 7.1.6 and 7.1.7, together with the allowance necessary for
mean stress.

Sections 5.9.2 and 5.9.3 are concerned with the first step of the sequence above. For
a given mean wind speed, the periodic component of blade loading will be invariant
over time, and the stochastic component will be stationary. As noted in Section 5.7.5,
the stochastic component can be analysed either in the frequency domain (provided
that a linear relationship between incident wind speed and blade loadings can be
assumed) or in the time domain, i.e., by using wind simulation. Section 5.9.2
considers how the deterministic and stochastic components may be combined if the
latter have been analysed in the frequency domain, while Section 5.9.3 looks in detail
at the option of assessing fatigue damage completely in the frequency domain.
If the fatigue damage resulting from both in-plane and out-of-plane loading is to
be computed, it is necessary to revise the ordering of the steps above, in order to
derive the periodic and stochastic components of the stress variation for each point
under consideration and for each mean wind speed. For a chosen point, the
procedure becomes as shown below.

(A1) For a given mean wind speed, calculate the time histories of the bending
moments about the principal axes resulting from the periodic load compo-
nents over one blade rotation. The derivation of aerodynamic moments from
blade element loads is illustrated in Figure 5.37.
(A2) Convert these bending moment time histories to stress time histories by
dividing by the appropriate section modulus, and adding them together.

(B) For the same mean wind speed, convert the power spectrum of the stochastic
bending moment component (which, because of the linearity assumption,
arises from fluctuating lift only) to a power spectrum of stress at the chosen
point.
(C) Calculate the fatigue damage resulting from the combined periodic and
stochastic stress components, using the methods of Sections 5.9.2 and 5.9.3.

(D) Repeat the above steps for the other mean wind speeds.

(E) Add together the fatigue damages arising at each mean wind speed to obtain
the total fatigue damage during normal running.

5.9.2 Combination of deterministic and stochastic components

Previous sections have shown how the deterministic (i.e., periodic) and stochastic
components of blade bending moments can be characterized in terms of time