ABSTRACT

This paper describes and analyzes a series of experiments conducted on the impact at a water surface of horizontal cylinders. The cylinders (0.2-, 0.4-, and 0.6-m diameter) were dropped from various heights, the maximum impact velocity being 3.88 m/second. Local pressures and accelerations were recorded during penetration in the water. A total of 185 drops were executed, five parameters being recorded for each of them.

Pressure measurements at different points of the cylinder circumference led to a precise description of the impact mechanism. Acceleration measurements furnished an estimation of impact forces intensity and duration.

The experiments were conducted in full scale; this is of importance considering the difficulties met in scaling down the phenomena involved: elastic vibrations, air-cushion effect, and transition between the impact and hydrodynamic flows.

Local impact pressures lasting less than 1 ms propagate along the cylinder circumference starting from the initial impact point, until the cylinder has penetrated a distance about half its radius. Evidence of an air-cushion effect is given by pressure recordings. Local pressure maxima are well fit by a linear function of impact velocity.

Combining pressure and acceleration results leads to a simple model of dynamic loading on cylinders during penetration. Values of impact forces thus calculated are comparable with those of Faltinsen et al.3 although obtained in a quite different way. Starting from that model, the design engineer can determine the response to wave impact for a given structure whose elastic characteristics are known.

INTRODUCTION

The horizontal cylindrical members of offshore platforms situated near the sea surface are submerged regularly by waves. In addition to hydrostatic and hydrodynamic forces thus generated, very brief and intense impact forces occur when the structure hits the water surface. These loadings generate very important and destructive vibrations in horizontal members, every time they cross the water surface. These vibrations often are attributed to impact forces whose main characteristics are not well known (duration, intensity, exposed area, etc.).

Several theoretical and experimental studies have been devoted to the encounter of cones, V shaped bodies, and flat plates with water, simulating the penetration of shells or ship stems. The slamming phenomenon, being a succession of different flow regimes, cannot be resolved as a whole by analytical methods. As for the experimental results, nothing indicates that they can be extended to different geometries. Only three studies have been found in the literature concerning the impact due to waves on horizontal cylindrical tubes. These studies1,3,4 insert the impact forces in a hydrodynamic-type formulation. Dalton and Nash, working at a reduced scale, furnish an experimental determination of an "impact coefficient" defined by the following relation.

(Mathematical equation available in full paper)

where Fs is the impact force and S the projected area on a plane normal to velocity.

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