Page 109 - Introduction to Naval Architecture

96 THE ENVIRONMENT

»?{ is the first moment of area of the energy spectrum about the axis
a) = 0, When only the significant wave height is known, S(o>) can be
represented approximately by:

WAVE STATISTICS

It has been seen how the wave surface can be characterized by a wave
spectrum. The designer still needs to know the severity of waves any
new design is likely to meet in service. For this, recourse is had to ocean
wave statistics. Over the years wave data have been obtained from
observations and measurements. Although they must be somewhat
subjective, visual observations are available for large ocean areas,
particularly the main shipping routes, and they have been successfully
integrated with measured data. Measurements can be at fixed points in
the ocean using buoys, taken by shipboard recorders or taken by
satellite. On board recorders need careful calibration to remove the
influence of the ship on the wave system being recorded. They tend to
be used only for special trials. Even then buoys deployed locally by the
trials ship are generally preferred. For one thing a suitably arranged
group of buoys can give information on the dominant wave direction as
well as on height and period.
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The concept of using a satellite radar altimeter was established by
Skylab in 1973. The satellite Seasat was operational for a few months in
1978 and was the first to give global coverage. The prospect now is for
two satellites to be operational at any one time. The higher the waves in
the footprint of the satellite radar, the more spread out is the time of
arrival of the return pulse. Adjusting the height of the return pulse to
a constant value, the slope of the leading edge gives a measure of the
significant wave height. Wind speed is indicated by the back scatter of
the signal. Early radars did not permit the wave period to be measured
but later synthetic aperture radars should fill this gap.
Statistical data on the probability of occurrence of various sea
conditions at different times of the year with the predominant wave
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direction are available. ' They are also available in PC form with wind
data added. The data, based on a million sets of observations are
presented for 50 sea areas covering the regularly sailed sea routes.
There are some 3000 tables arranged by area, season and wave
direction. Tables show, for instance, the number of observations within