ssc-367 - Ship Structure Committee

0.3 EXTREME RESPONSE
The extreme response of an offshore structure may be determined in
two ways. An extreme environmental event may be selected, and the
responses to the extreme event then calculated. A set of
environmental spectra can be selected; the response spectra to each
environmental spectra calculated; and the extreme responses derived
by statistical analysis of the response spectra. The first method is
often called a “deterministic” method, and the second method is
referred to as a “probabilistic”method. In actual design practice
the two methods are often intermixed or combined in order to confirm
that the extreme response has been found.
B.3.1
Maximum Wave Height Method
In deterministic design, a set of extreme conditions is supplied by
oceanographers or meteorologists. The extreme conditions are of
course derived from statistical analyses of wave and weather records,
but the design engineer is usually not involved in that stage of the
calculations.
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The given extreme conditions are applied to the offshore structure to
determine the various responses. Unfortunately, the given extreme
conditions may not always produce the extreme responses. For
example, the prying and racking loads governing the design of many
structural members of semisubmersibles are typically maximized in
waves with lower heights and shorter lengths then the maximum height
wave. Tendon loads on tension leg platforms (TLPs) are also often
maximized in waves that are lower and shorter than the maximum wave.
Since the oceanographer or meteorologistwho produced the set of
extreme conditions does not have information about the
characteristics of the offshore structure, he/she is unable to select
an extreme or near extreme condition that will produce the greatest
response. Conversely, the design engineer usually has little or no
information about the wave and weather data that was used to derive
B-15