Effects of Sea-Surface Fluctuations on Response of Offshore Structures

The magnitudes of the effects of the sea-surface fluctuations on the response of wave-excited offshore structures are assessed. Such fluctuations are accounted for by using the linear wave theory in combination with two distinct approaches: “stretching” and “vertical extrapolation.” Structural responses are calculated for water-particle kinematics obtained by applications of the unmodified linear-wave theory, linear-wave theory in combination with the stretching approach, and linear-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 that responses obtained for the stretching approach are almost the same as those calculated for the unmodified wave-theory model. It is further shown that applications of the vertical-extrapolation technique lead to the evaluation of peak responses that are generally larger than those obtained for the unmodified wave model, particularly for drag-force dominated systems. These larger responses are primarily induced, under 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 improved 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.