The magnitudes of the effects of the sea-surface fluctuations on the r
esponse of wave-excited offshore structures are assessed. Such fluctua
tions are accounted for by using the linear wave theory in combination
with two distinct approaches: ''stretching'' and ''vertical extrapola
tion.'' Structural responses are calculated for water-particle kinemat
ics obtained by applications of the unmodified linear-wave theory, lin
ear-wave theory in combination with the stretching approach, and linea
r-wave theory in combination with the vertical extrapolation technique
. These responses are examined by separating the associated static and
dynamic effects for wide ranges of natural frequencies. It is shown t
hat responses obtained for the stretching approach are almost the same
as those calculated for the unmodified wave-theory model. It is furth
er shown that applications of the vertical-extrapolation technique lea
d to the evaluation of peak responses that are generally larger than t
hose obtained for the unmodified wave model, particularly for drag-for
ce dominated systems. These larger responses are primarily induced, un
der conditions of practical importance, by increases in the associated
static effects. Additional relevant information is also reported that
is responsive to the need that exists at the present time for improve
d appreciations of the dynamics of offshore platforms. The results of
the study apply to offshore platforms operating in water depths in the
approximate range of 300-1,000 m.