Environmental Engineering Reference
In-Depth Information
Extreme Winds
Design of a wind turbine for structural integrity requires that the support structures,
(both above and below ground) and the rotor, pitch, and yaw assemblies be of sufficient
strength to withstand the
extreme wind
loading that is likely to be encountered during its
design life. Generally, the turbine is shut down under extreme wind conditions, so dynamic
loading of the structure caused by rotor rotation is not usually an additive factor. Thus, the
discussion in this section pertains to
maximum static wind loading
of the same type as that
experienced by buildings, towers, water tanks, and signs.
Measuring Extreme Winds
Two measures of extreme wind speed are currently used in wind engineering, namely
the
fastest-mile wind speed
and the
peak gust.
The first of these is recorded by a special
anemometer called a
1-mile contact anemometer
,
in which an electrical contact causes a
transverse mark to be made on a rotating drum chart at the completion of each whole mile
of wind movement past the instrument. The shortest time interval between two consecutive
marks in a wind record gives the fastest-mile wind speed. This measure of extreme wind
is therefore the maximum one-hour steady wind speed.
Peak wind statistics
(like the peak gust) have an advantage over mean wind statistics
(like the fastest-mile wind speed) in that they do not depend upon an averaging operation
that can vary from day to day and from observer to observer. Hourly peak wind speed
readings avoid this sometimes subjective averaging process. The time duration of a peak
wind speed, however, is an important factor in wind loading, and this duration is a function
of the anemometer sensitivity. Standard weather service anemometers measure gusts with
approximately 3-second durations, whereas speeds recorded by research anemometers can
normally be resolved to 0.1 second.
Recurrence Intervals
The general concept of determining an extreme design wind speed is to select the most
extreme value of the wind that the structure will probably experience in a given number of
years of exposure. Extreme wind values are therefore tabulated according to percent
probability of occurring at least once in a given
recurrence interval.
Thorn [1961, 1968]
has published
isotach maps
(lines of constant wind speed) of the fastest-mile wind speed,
based on U.S. Weather Bureau measurements over a 21-year period. Typical mean
recurrence intervals graphed on these maps are 2, 10, 25, 50, and 100 years. The accuracy
of the speeds on these maps is given as approximately ± 15 percent. Extreme wind speed val-
ues are normally reported at a given reference elevation (usually 9.1 m) over smooth, uniform
terrain that is typical of airports. These speeds must be corrected for elevation, including
the effects of surface roughness (see Figure 8-13), and to account for the dynamic response
time of the structure.
Risk of Exceedance
Since there is a finite probability that the actual extreme wind speed will be higher than
the selected design value during the turbine's design life, the design engineer must select
an acceptable degree of
risk of exceedance.
The relationship between risk of exceed-
ance, mean recurrence interval, and design life is relatively straightforward, and is shown
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