Page 214 - Fluid Power Engineering

186 Chapter Nine

extreme coherent gust with direction change model (ECD), ex-
treme wind shear (EWS), and extreme operating gust (EOG).
The above wind conditions are paired with the appropriate
design situation. Two wind conditions are explained in more
detail below.
3. Two types of design analysis are performed, ultimate strength (U)
and fatigue (F). Ultimate strength analysis is static analysis
against the strength of materials of extreme loads because of
forces, moments, and deflections. Fatigue loads are computed
by: (a) dividing into discrete stress cycles each with a mean,
range, and frequency, (b) estimating duration of each stress
cycle for the 20-year life of the turbine, and (c) estimating
damage and summing up the damage using Palmgren-Miner
rule.
4. Partial safety factors. IEC 61400-1 defines three classes of com-
ponents depending on the consequences of failure. It also de-
fines three types of loads conditions: Normal (N), abnormal
(A), and transportation and erection (T). Partial safety factors
for ultimate strength analysis and fatigue analysis are defined
for three classes of components and the different load condi-
tions.

Table 9-4 shows the various groupings of design situation, wind con-
dition, type of design analysis, and partial safety factor. This is the
minimal set of design cases that must be analyzed. Turbine manufac-
turer must document the design analysis for each component and the
system as whole for at least the 12 design cases.

Design Wind Conditions
In order to illustrate the design process, two types of wind regimes are
considered: Normal operations and extreme conditions. The extreme
conditions fall into two categories with respect to the frequency of
wind events: 1- and 50-year events. Depending on the classification
of the turbine that is being designed (Class IA, . . . , IIIC) appropriate
wind conditions are used, as described in Table 8-3.

Normal Wind Profile Model (NWP)
For computing design loads under normal wind conditions the fol-
lowing parameters are used, per IEC 61400-1: 7

(9-1)
V ave = 0.2 V ref