Actions on wind turbines - Concrete Structures for Wind Turbines - page 23

characteristic values of the wind climate may be determined by numerical methods as an
alternative.

2.3.2.3 Normal wind conditions
Wind speed distribution
The local distribution of the 10-min average of the wind speed at hub height (V hub )is
significant for the design of an offshore wind turbine because this determines the
frequency of occurrence of individual load components.
A Weibull distribution (P W ) must be derived from in situ measurements verified by
long-term measurements in the immediate vicinity:

h i

k

P W V V hub ð Þ ¼ 1 exp V hub = ð C Þ

where
C scale parameter [m/s]
k shape parameter (k ¼ 2 for designs in a standard wind turbine class)

When k ¼ 2, the Weibull distribution produces a Rayleigh distribution which can be
used for calculating the wind loads to [11] (Figure 2.9):

2

P R V V hub ð Þ ¼ 1 exp p= 4 V hub = ð V ave Þ

From this we get the probability density for the wind speeds:

h i

2

fV hub ð Þ¼p= 2 V hub = ð V ave Þ exp p= 4 V hub = ð V ave Þ

Normal wind profile model (NWP)
The following power law equation should be assumed for the wind profile V(z):

Þ a

V ðÞ¼ V hub ð z = z hub

Fig. 2.9 Rayleigh distribution (function P and density f) for wind speeds