Offshore Winds - Wind Energy Meteorology: Atmospheric Physics for Wind Power Generation - page 115

Fig. 5.21 Vertical profiles of
turbulence intensity [see Eq.
( 3.10 ) ] as function of gradient
Richardson number at FINO1
in the German Bight for the
period October 2004 to Jan-
uary 2005 for the wind
direction sector 210-250

distributions of turbulence intensity within different wind speed classes at FINO1
at 90 m height from the 4 years of data displayed in Fig. 5.19 .
Most measurement heights of the 100 m high FINO1 mast in the German Bight
are usually above the well-mixed surface or Prandtl layer. Therefore, we observe a
considerable decrease of turbulence with height. This decrease is depicted in
Fig. 5.21 . This decrease is largest for very stable thermal stratification and is
smallest for very unstable stratification.
For the calculation of loads the 90th percentile of the turbulence intensity for a
given wind speed bin is important. The normal turbulence model (NTM) of the
IEC standards (IEC 61400-1 Ed. 3 and IEC 61400-3 Ed. 1) describe the 90th
percentile of the offshore turbulence intensity using the following parameterization
for the 90th percentile of the standard deviation of the horizontal wind speed, r u90 :

u h
ln ð z h = z 0 Þ þ 1 : 28 1 : 44 m/s

r u90 ¼ ð Þ I 15 ð 5 : 18 Þ

where u h is the wind speed at hub height of the wind turbines, z h is the height of the
hub above sea level and I 15 is the average turbulence intensity at hub height at