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graphically in Figure 8-14. Two different design criteria for selecting an acceptable risk of
exceedance have been used: An aerospace vehicle criterion , which is a 10 percent risk of
exceedance [Kaufman 1977]; and a building code criterion , which is a 63 percent risk for a
design life of 50 years [British Standard Code of Practice CP3 1972]. The latter criterion is
equivalent to specifying a mean recurrence interval of 50 years.
The aerospace criterion has been used for experimental large-scale HAWTs ( e.g. , the
2.5-MW Mod-2 turbines), but it is considered to be too conservative for commercial wind
power stations. Risk of exceedance for these is usually determined by building design prac-
tices and state and local building codes. Lower (more conservative) risks of exceedance
may be selected for commercial turbines on the basis of economic trade-off studies.
The following example will illustrate the selection of a design extreme wind speed:
-- design life ....................30 yr
-- acceptable risk of exceedance ...10 percent (aerospace criterion)
-- site location ...................150 km east of Portland, Oregon
-- surface roughness length ........0.05 m
-- reference elevation .............10.0 m
-- elevation of center of swept area . 61.0 m
The first two specifications locate point A in Figure 8-14, from which the minimum mean
recurrence interval is determined to be 280 years. Using isotach map data for Portland
Figure 8-14. Relationship between risk of exceedance, mean recurrence interval, and
design life used to determine the design extreme wind speed. [Frost et al. 1978]
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