leading to structural failure. The design of offshore structures must there-
fore not only take account of the forces exerted on them by currents and
waves, but also guard against fluid-induced vibration. Another important
mechanism to be wary of is the fatigue failure of structures which can
occur due to stress reversals from incessant exposure to wave-induced
forces. Information on forces exerted on the elements of offshore struc-
tures in the presence of irregular waves is thus important for design of
structures for fatigue. However, the treatment of such forces arising from
irregular waves is beyond the scope of this book.
The most noticeable feature of the sea is the oscillations of the free
surface. The nature of these oscillations depends on the way they have
been generated, the most important being the wind-generated waves that
are of a relatively short period. The understanding of wave motion and its
interaction with structure is of paramount importance in the design of off-
shore and coastal structures.
14.2 Wave motion
14.2.1 Solution for velocity potential
Wind waves generated on the oceans are random and short crested.
Although most wave theories have been developed for long-crested peri-
odic waves, they have been applied with satisfactory results to waves
which have travelled out of the generating areas.
Periodic waves are those in which a phase, for example the crest, is
observed by a stationary observer at equal intervals of time, T, called the
wave period. The still-water level (SWL) in Fig. 14.1 is the mean water
level about which the oscillations of the surface due to waves take place.
A sinusoidal wave and some of its properties are shown in Fig. 14.1. The
576 WAVES AND OFFSHORE ENGINEERING
Fig. 14.1 Definition sketch for a sinusoidal wave
