156
STATISTICAL DESCRIPTIONS OF OFFSHORE WAVES
There are two general observations concerning significant wave height statistics. First, different wave time historiés can hâve approximately the same significant wave height, but still hâve widely different spectral properties. Ochi
and Whalen (1980) and Ochi (1981) offered a rational explanation and experimental documentation for such observations. Second, extensive global data for
significant wave height based on nine years of satellite measurements taken in
the 1990s were presented as contour maps by Young (1999). These data show
mean monthly values of Hst values which could be expected to be exceeded
10 percent, 20 percent, ... ,90 percent of the time. The highest value of H,
reported was 6 m with a probability of exceedence of 10 percent. Since confined
events such as hurricanes affect such monthly averages very little, these satellite
data hâve very limited use in the design of offshore structures.
Fetch-Limited Spectra. The JONSWAP spectra has the general form
S„(w) = aS2exp[-1.25(w/cvm)4] ■ 7«xpl-(‘-"~)a/2‘’a^l
(6.25)
where
7 =
3.3 for mean of selected JONSWAP data
7 =
7.0 for a very peaked spectrum
a —
0.07 for a? <
a =
0.09 for u > u>m
=
27r(3.5)(ÿ/V)(X)-0'33 peak frequency
a -
0.076(X)-°-22 or a = 0.0081
X =
gx/v2
X =
fetch length;
V = wind speed
6.4
SELECTION OF DESIGN WAVE SPECTRA
The preceding spectra are best-fit curves of a number of individual spectra,
each derived from actual wave records which are measured in generally similar
sites and environments. They represent the mean of the points of the farnily
of actual spectra and in no sense can be considered as theoretical spectra. For
instance, a comparison of many individual spectra with formula spectra showed
that the individual spectra height had a sigma variation of 30 percent or more
from applicable formula spectrum (Hoffman, 1974).
This observation, which is consistent with the observations of other investigations, was explained by Ochi and Whalen (1980), who introduced the concept
of familles of wave spectra and provided a meaningful insight into this behavior.
For example, using the two-parameter Bretschneider (1959) spectrum, équation
(6.24), Ochi and Whalen presented a farnily of this spectra along with their
probabilités of occurrence and their confidence limits. An example is shown
m Figure 6.5, which will be discussed further in Example Problem 6.1. The
important point is that each of the spectra représenta the same significant wave
STATISTICAL DESCRIPTIONS OF OFFSHORE WAVES
There are two general observations concerning significant wave height statistics. First, different wave time historiés can hâve approximately the same significant wave height, but still hâve widely different spectral properties. Ochi
and Whalen (1980) and Ochi (1981) offered a rational explanation and experimental documentation for such observations. Second, extensive global data for
significant wave height based on nine years of satellite measurements taken in
the 1990s were presented as contour maps by Young (1999). These data show
mean monthly values of Hst values which could be expected to be exceeded
10 percent, 20 percent, ... ,90 percent of the time. The highest value of H,
reported was 6 m with a probability of exceedence of 10 percent. Since confined
events such as hurricanes affect such monthly averages very little, these satellite
data hâve very limited use in the design of offshore structures.
Fetch-Limited Spectra. The JONSWAP spectra has the general form
S„(w) = aS2exp[-1.25(w/cvm)4] ■ 7«xpl-(‘-"~)a/2‘’a^l
(6.25)
where
7 =
3.3 for mean of selected JONSWAP data
7 =
7.0 for a very peaked spectrum
a —
0.07 for a? <
a =
0.09 for u > u>m
=
27r(3.5)(ÿ/V)(X)-0'33 peak frequency
a -
0.076(X)-°-22 or a = 0.0081
X =
gx/v2
X =
fetch length;
V = wind speed
6.4
SELECTION OF DESIGN WAVE SPECTRA
The preceding spectra are best-fit curves of a number of individual spectra,
each derived from actual wave records which are measured in generally similar
sites and environments. They represent the mean of the points of the farnily
of actual spectra and in no sense can be considered as theoretical spectra. For
instance, a comparison of many individual spectra with formula spectra showed
that the individual spectra height had a sigma variation of 30 percent or more
from applicable formula spectrum (Hoffman, 1974).
This observation, which is consistent with the observations of other investigations, was explained by Ochi and Whalen (1980), who introduced the concept
of familles of wave spectra and provided a meaningful insight into this behavior.
For example, using the two-parameter Bretschneider (1959) spectrum, équation
(6.24), Ochi and Whalen presented a farnily of this spectra along with their
probabilités of occurrence and their confidence limits. An example is shown
m Figure 6.5, which will be discussed further in Example Problem 6.1. The
important point is that each of the spectra représenta the same significant wave