Page 238 - Wind Energy Handbook

212 DESIGN LOADS FOR HORIZONTAL-AXIS WIND TURBINES

Extreme and normal wind conditions are generally defined in terms of the worst
condition occurring with a 50 year and 1 year return period respectively. It is
assumed that machine fault states arise only rarely and are uncorrelated with
extreme wind conditions, so that the occurrence of a machine fault in combination
with an extreme wind condition is an event with such a high return period that it
need not be considered as a load case. However, IEC 61400-1 wisely stipulates that
if there is some correlation between an extreme external condition and a fault state,
then the combination should be considered as a design case.

5.2.3 Fatigue loads

A typical wind turbine is subjected to a severe fatigue loading regime. The rotor of
8
a 600 kW machine will rotate some 2 3 10 times during a 20 year life, with each
revolution causing a complete gravity stress reversal in the low speed shaft and in
each blade, together with a cycle of blade out-of-plane loading due to the combined
effects of wind shear, yaw error, shaft tilt, tower shadow and turbulence. It is
therefore scarcely surprising that the design of many wind turbine components is
often governed by fatigue rather than by ultimate load.
The design fatigue load spectrum should be representative of the loading cycles
experienced during power production over the full operational wind speed range,
with the numbers of cycles weighted in accordance with the proportion of time
spent generating at each wind speed. For completeness, load cycles occurring at
start-up and shut-down and, if necessary, during shut-down, should also be
included. It is generally assumed that the extreme load cases occur so rarely that
they will not have a significant effect on fatigue life.

5.2.4 Partial safety factors for loads

Limit-state design requires the design load for a component to be calculated as the
sum of the products of each characteristic load and the appropriate partial load
factor. The partial safety factors for ultimate loads stipulated by IEC 61400-1, the GL
rules and DS 472 are given in Table 5.2:
Note that the GL rules treat machine fault conditions as abnormal cases with a
partial safety factor of unity. This implies that the fault conditions are considered to
be very rare events, which may be questionable in practice. On the other hand, IEC
61400-1 classifies load cases involving a machine fault as normal. IEC 61400-1
observes that in many cases, especially where varying loads result in dynamic load
effects, the loads from various sources cannot be evaluated separately. In these
situations, the standard requires the use of a single partial load factor equal to the
highest of the factors in the Table for the relevant design situation. The partial safety
factor for fatigue loads is taken as unity.