Offshore Winds - Wind Energy Meteorology: Atmospheric Physics for Wind Power Generation - page 104

Fig. 5.10 Estimation of extreme wave heights for different wind sectors at FINO1. Capital
letters denote wind direction (see Table 5.1 ), subscripts ''s'' and ''u'' denote stable and unstable
thermal stratification. Slanted lines are approximations to the data points giving more weight to
lower wave heights. The y-axis refers to the data from the northern sector (full slanted line); data
from the other sectors (dotted slanted lines) have been shifted vertically accordingly to match the
plotted 1-50 year thresholds
northern sector because with 110, it is much wider than the western sector with
only 70. Curves from the other sectors, which are based on fewer values, have
50 year thresholds at y values lower than 11.5. These curves have therefore been
shifted vertically accordingly so that their thresholds match the horizontal lines in
Fig. 5.10 which indicate the thresholds for the northern sector. The highest 50 year
extreme significant wave heights which have to be expected will probably come
from the northern, eastern (under unstable conditions, i.e. cold air advection) and
from the western sector with 9-11 m. Extreme significant wave heights from the
southern sector and the eastern sector under stable conditions will only be between
4 and 7 m. The uncertainty of this extreme value estimation can be assessed from
the plots. The better the data fit to a Fisher-Tippett type 1 distribution, the more
the data points should arrange themselves along a straight line in the plot.
The uncertainty of the 50 year extreme value could be estimated from the
spread of the crossing points between possible straight lines through the data and
the 50 year line (y = 11.5). From this criterion, the uncertainty for the western
sector is the smallest (a few percent only). For the other sectors (except the
northern one), it may be up to about 10 %. For the northern sector, the reliability is
also good if values up to 6 m wave height are used, which has actually been done
in Fig. 5.10 . The data points for the northern sector in Fig. 5.10 for wave heights
above 6 m all come from the storm ''Britta''. The deviation of these data points to
the right from the straight line indicates that ''Britta'' must have been a rather rare
event. Taking the highest value (10 m) and going straight upright, we hit the
straight regression line for the Gumbel distribution at about the probability for the
20 year event. Thus, following the analysis given here, ''Britta'' has been a 20 year
event, whereas ''Erwin'' was not unusual and can be expected every 1-3 years.
This extreme value estimation technique is not devalued due to the fact that about