Irregular ocean waves are often characterised by a
"wave spectrum", this describes the distribution of wave energy
(height) with frequency.
Ocean waves are often characterised by statistical
analysis of the time history of the irregular waves. Typical parameters used to
classify irregular wave spectra are listed below:
Characterisation
of irregular wave time history
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|
mean
of many wave amplitude measurements
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mean
of many wave height measurements
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mean
of many wave period measurements between successive peaks
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mean
of many wave period measurements between successive troughs
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mean
of many wave period measurements between successive zero up-crossings
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mean
of many wave period measurements between successive zero down-crossings
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mean
of many wave period
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modal
wave period
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mean
of highest third amplitudes or significant amplitude
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mean
of highest third wave heights or significant wave height
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variance
of the surface elevation relative to the mean (mean square)
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standard
deviation of surface elevation to the mean (root mean square)
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In 1970 the World Meteorological Organisation agreed the
standard sea state code – see below. Each code represents a range of wave
heights but there is no indication of the corresponding wave periods.
World Meteorological Organisation sea state code
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Sea
State
Code
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Significant wave height [m]
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Description
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|
Range
|
Mean
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0
|
0
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0
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Calm (glassy)
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1
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0.0 - 0.1
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0.05
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Calm (rippled)
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2
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0.1 - 0.5
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0.3
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Smooth (wavelets)
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|
3
|
0.5 - 1.25
|
0.875
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Slight
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|
4
|
1.25 - 2.5
|
1.875
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Moderate
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|
5
|
2.5 - 4.0
|
3.25
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Rough
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|
6
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4.0 - 6.0
|
5.0
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Very rough
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|
7
|
6.0 - 9.0
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7.5
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High
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8
|
9.0 - 14.0
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11.5
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Very high
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|
9
|
over 14.0
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over 14.0
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Phenomenal
|
Sea state data may also be obtained for specific sea areas
and times of year. This can be useful routing information. One of the best
sources of this information is Hogben and Lumb (1967).
Irregular ocean waves are typically described in terms of
a wave spectrum. This describes a wave energy distribution as a function of
wave frequency. The continuous frequency domain representation shows the power
density variation of the waves with frequency and is known as the wave
amplitude energy density spectrum, or more commonly referred to as the wave
energy spectrum. The spectral ordinates (or wave spectral density) are given
the symbol: 
. (This is similar to the power spectral density, PSD, used
in electronics and communications analysis.) A typical wave spectrum is shown
below:
These spectral representations of sea conditions are
central to determining the response of a vessel in the seaway. This will be
discussed more fully in the following sections and also see: Wave
Spectra on page 56
for more in-depth information.
It is often useful to define idealised wave spectra which
broadly represent the characteristics of real wave energy spectra. Several such
idealised spectra are available in Seakeeper and are described below:
§
Bretschneider or ITTC two parameter spectrum
§
One parameter Bretschneider
§
JONSWAP
§
DNV Spectrum
§
Pierson Moskowitz
The spectra and their formulations are dealt with in more
detail in the Chapter
4 Theoretical Reference on page 55.
An important concept when calculating vessel motions is
that of the encountered wave spectrum. This is a transformation of the wave
spectrum which describes the waves encountered by a vessel travelling through
the ocean at a certain speed. This is effectively a Doppler shift of the
spectrum which smears the spectrum towards the higher frequencies in head seas
and towards the lower frequency in following seas.
A full explanation of this effect is given in Chapter
4 Theoretical Reference, section What the
Vessel Sees on page 60.