Final Report 4.1

UPWIND
turbines, as the differences in mean wind speed between the upwards and downwards moving
blades are low.
The wind speed distribution differs on- and offshore as well. It is typically described by a Weibull
distribution with
w
�V� Page 14 of 146
f
�
k
A
� V �
� � �
� A �
� �
� � V �
� exp � � � �
� �
� � A � �
k �1 k
For offshore sites the scale parameter A tends to higher values and thus higher probabilities of
higher wind speeds. Furthermore the shape of the distribution is defined by the parameter k and
here larger values tend to more pronounced shapes.
These differences in the wind speed distributions result in a higher power output and higher
mean wind load level. For the prediction of energy yield of a wind turbine, long-term variations of
the wind speed are significant, where in contrast for loads the short-term fluctuations are more
relevant. Here the stochastic effects in the wind speed, namely the turbulence, and transient
events like gusts are main contributors to fatigue and extreme loading.
Turbulence is the momentary deviation from the mean wind speed. The extent of turbulence
depends on several meteorological and geographical conditions like the atmospheric layering or
the terrain. A measure for turbulence is the so called turbulence intensity I, which is defined as
the ratio between of the standard deviation of the wind speed and the mean wind speed
σ
I �
V
1
hub
The turbulence intensity is correlated with the surface roughness of a turbine site and decreases
with the height, as the influence of the surface decreases as well with the height.
A further factor is that the turbulence intensity decreases with increasing wind speed. But this
assumption is not directly valid for offshore locations, as through the nature of the ocean surface
it is correlated with the wind conditions. Here the waves and therefore also the surface
roughness is connected with the existent wind speeds and duration of the wind impact.
Depending on the duration, a sea state can be fully or not fully developed. For higher wind
speeds the effects of the wind-wave-correlation lead to a slightlincrease in turbulence caused by
the increase in surface roughness.
Another aspect for fluctuating wind speeds is the turbulence induced in wake conditions in a
wind farm. Especially in dense wind park layouts wake effects play an important role. In a wind
farm, a turbine experiences a superimposed turbulent wind coming from the ambient and the
wake turbulence. Again, as offshore the ground roughness and thus also the ambient
turbulence is lower than onshore, the mixture of ambient and wake-induced turbulence is less
and therefore the wake fields remain longer in the atmosphere. This results in a higher loading
from wake effects at offshore sites than compared to onshore sites at a fixed turbine distance.
Here especially the partial wake operations can be critical. As the swept area of a turbine is only
partly affected by a wake the load fluctuations are higher.
In general, with respect to fatigue, ambient and wake-induced turbulence have a crucial
influence. Through the permanently fluctuating wind speeds and loads, the number of load
cycles is extremely large, which plays a major role in the operational stability.
In terms of extremes, the effect of turbulence is not that important. Here the occurrence of
certain transients is crucial. Offshore, the probability of extreme wind speeds, like gusts or wind
(2.2)
(2.3)