ABSTRACT:

This paper presents the results from an experimental study of plunging wave action on a large horizontal cylinder in the splash zone. Impact pressures are found to range from localised impulsive pressures with time scales in the range of 0.001T to synchronous low frequency pressure oscillations with oscillation time scales around 0.01 T (T is the characteristic wave period). The highly impulsive cases, with peak pressures ranging from 4pC2 to 33pC2, is associated with minimal entrapped air at impact (p is the water density and C is the characteristic phase speed of the wave). The highest peak pressure is obtained when the profile of the incident wave front is concentric to the cylinder boundary. Peak pressures as high as S.4pC2 are obtained when a large pocket of air is entrapped by the plunging wave front at impact. The entrapped air also leads to synchronous pressure oscillations over a substantial region of the cylinder's surface, covering an angular zone of about 1050. The corresponding peak impact force is about 8.6pC2R where R is the radius of the cylinder. Overall, the incident wave profile trajectory and the entrapped air are found to influence the impact load significantly.

INTRODUCTION

In a hostile sea environment, the safe and economic design of a marine structure depends significantly on the prediction of a representative design wave load. For structural members located at elevations between the mean sea level and the crest elevation, the highest loadings are those associated with wave impacts or slamming. This is especially so for horizontal structures with large dimensions compared to the incident wave height Horizontal members such as the deck of an offshore structure are currently designed to be located well above the crest level to avoid slamming pressures.

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