Wave Impact Loads on Cylinders

Abstract

ABSTRACT The evolution of forces acting on horizontal cylinders subjected to impact by a sinusoidally oscillating free surface has been investigated both theoretically and experimentally. The experiments were carried out in a large U-shaped tunnel, with cylinders 3 to 8 inches in diameter. The results have been expressed in terms of two force coefficients. The first is the slamming coefficient which expresses the normalized force acting on the cylinder at the time of impact; and the second is the maximum drag coefficient which occurs when the cylinder is immersed approximately 1.8 diameters in water. The slamming-force coefficient was found to be equal to 3.2. It was also found that the force experienced by the cylinder cannot be considered independently of the dynamic response of the cylinder. In fact, the slamming-force coefficient may be amplified to a value as high as 6.3 through the dynamic response of the cylinder and its supports. INTRODUCTION Information concerning the forces acting on bluff bodies subjected to wave slamming is of significant importance in ocean engineering and naval architecture. The design of structures which must survive in a wave environment is dependent on knowledge of the forces which occur at impact, as well as on the dynamic response of the system. Two typical examples include the structural members of offshore drilling platforms at the splash zone and the often encountered slamming of ships. The general problem of hydrodynamic impact has been studied extensively motivated in part by its importance in ordnance and missile technology. Extensive mathematical models have been developed for cases of simple geometry such as spheres and wedges, and these models have been well supported by experiment. Unfortunately, the special case of wave impact 2as not been studied extensively. Kaplan and Silbert developed a solution for the forces acting on a cylinder from the instant of impact to full immersion. Dalton and Nash3 conducted slamming experiments with a 0.5 inch diameter cylinder with small amplitude waves generated in a laboratory tank. Their data exhibited large scatter and showed no particular correlation with either the predictions of the hydrodynamic theory or identifiable wave parameters. Miller presented the results of a series of wave-tank experiments to establish the magnitude of the wave-force slamming coefficient for a horizontal circular cylinder. He found an average slamming coefficient of 3.6 for those trials in which slamming was dominant. It appears that the evaluation of the slamming effects with wavy flows is extremely difficult partly because of the limited range of wave amplitudes that can be achieved and partly because of the difficulty of measuring the fluid velocities at the instant of impact. Faltinsen et al.5 investigated the load acting on rigid horizontal circular cylinders (with end plates and length-to-diameter ratios of about one) which were forced with constant velocity through an initially calm free surface. They found that the slamming coefficient ranged from 4.1 to 6.4. They have also carried out experiments with flexible horizontal cylinders, and found that the analytically predicted values were always lower (50% to 90%) than those found experimentally.