Offshore Winds - Wind Energy Meteorology: Atmospheric Physics for Wind Power Generation - page 112

Fig. 5.18 Vertical profiles of extreme 3 s-gusts (left) and extreme 10 min-mean wind speeds for
50 year return period. Profiles with error bars denote extrapolations from observations at FINO1
using the Gumbel method (see Appendix A.3), the dashed curve extrapolations using the
independent storm method, the other two curves give the class I (right) and class II (left) limits
from the offshore IEC standard
( 3.10 ). Applying typical values for Sprogø (z = 70 m, z 0 = 0.0015 m) in Eq.
( 5.17 ) yields G = 1.195, which is again lower than 1.4. Although it should be paid
attention to the fact that ( 5.17 ) is simplified, because the wind speed dependence of
the roughness length is not considered in this relation, it turns out from relation
( 5.17 ) that a gust factor of 1.4 seems to be too high for marine conditions. We will
see in the next subsection that offshore turbulence intensity is typically between
0.05 and 0.10 which gives the gust factor from the first relation in ( 5.17 )tobe
approximately in the range between 1.105 and 1.21.
Figure 5.18 compares vertical profiles of the estimated extreme values for the
50 year return period from FINO1 observations with the offshore IEC standards
( 5.13 ) and ( 5.15 ). The estimations have been determined for each height separately
from 4 years of data. The independent storm method (Cook 1982 ; Palutikof et al.
1999 ) has been used to validate the Gumbel method (described in Appendix A.3)
employed for the estimation of the extreme values. The Gumbel method estima-
tions from the observations are well below the IEC limit curves for the 3 s-gust but
hit the limit curve for class II sites for 10 min-mean wind speeds.
5.4 Turbulence
This subchapter analyses several turbulence parameters which have relevance for
load calculations for wind turbines. Most of them are used in the definition of the
turbulence models and the extreme operating gust in the IEC standards (IEC
61400-1 Ed. 3 and IEC 61400-3 Ed. 1). These parameters comprise the turbulence
intensity, high-frequency wind speed variances, turbulence length scales and
inclination angles, and the wind speed variation with time during typical gust
events ('Mexican hat').