EFFECTS OF SEA-SURFACE FLUCTUATIONS ON RESPONSE OF OFFSHORE STRUCTURES

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.