Assessment of Existing Structures and Repairs - Offshore Structures: Design, Construction and Maintenance - page 462

As discussed in Chapter 3 , significant wave height was defined as the mean
of the third highest waves. From the mid to late 1970s, analogue magnetic tapes
were used more widely, and the data were processed to derive significant wave
heights defined using the root mean square (rms) of the sea-state wave heights
(H =4H nns ). In the early 1980s, the definition of significant wave height was
based on the spectral moment (H = 4(mo) 05 ).
Methods of data reduction and statistical analysis have improved consider-
ably from the manual methods of the early 1970s to the sophisticated computa-
tions undertaken today. In the early years, it was not unusual for only one or
two distribution types to be considered and a best-fit line fitted by eye; now
a wide range of distribution types are considered, a variety of data reduction
methods are used, a number of sophisticated fitting methods are employed
and significance or goodness-of-fit tests are undertaken.
Wave history is usually available from the government authority concerned
with metocean data. In the UK, data recording environmental parameters for
25 years of winter storms (covering the period 1964 1989) are available on a

1-km grid covering the North Sea and the northeast Atlantic Ocean.

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7.4.2 Fluid Loading Analysis
In the early years of North Sea development, only Airy and Stokes third- and
fifth-order wave theories were available to designers for practical use. All the-
ories were used for their simplicity, particularly for appurtenance design, but
as design became more sophisticated, Stokes theories were used more widely,
and today Stokes fifth-order wave theory is the most widely used theory for
design.
Dean (1965) followed work primarily by Airy, and Stokes wave theories
were shown to inaccurately predict wave particle kinematics for some combina-
tions of water depth and wave period. The selection of the most suitable wave
theory was improved following work by Le Mehaute (1976) and Barltrop et al.
(1990) . With advances in computing power, stream theory has become more
popular, particularly in shallower water and for waves close to breaking. Stream
function theory up to the eleventh order, as developed by Dean in 1965 ,or
Chappelear velocity potential is now used.
All of the above are regular wave theories. Tromans et al. (1994) proved a
theory based on random waves. A new wave theory has recently been devel-
oped by, and is being used by, the Shell company. It is said to model the
irregular nature of real seas better.
Along with advances in wave theories, research and development have
increased the speed of the computer software used to assess structural loading.
It is very important to emphasize that marine growth was not usually con-
sidered in design before the late 1970s. While the effects of marine growth
on platform loading have usually been considered in the design of North Sea