Wave Impact Loads on Cylinders
Authors: Sarpkaya Turgut 
Published: December 2007

Sarpkaya, Turgut, Naval Postgraduate School, Monterey, Ca.

The evolution of forces acting on horizontal cylinders subjected to impact by a sinusoidally oscillating free surface was investigated both theoretically and experimentally. The experiments were conducted in a large U-shaped tunnel, with cylinders 3 to 8 in. (76 to 203 mm) in diameter. The results are expressed in terms of three force coefficients:the general slamming coefficient that expresses the normalized force acting on the cylinder at any time after the impact.the normalized impact force at the initial instants of slamming, andthe maximum drag coefficient that occurs when the cylinder is immersed in water about 1.8 diameters.

The slamming-force coefficient was found to equal 3.2. Also, the force experienced by the cylinder cannot be considered in dependently of the dynamic response of that 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 about the forces acting on bluff bodies subjected to wave slamming is of significant importance in ocean engineering and naval architecture. The design of structures that must survive in a wave environment depends on a knowledge of the forces that 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. These models have been well supported by experiment. Unfortunately, the special case of wave impact has 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 Nash conducted slamming experiments with a 0.5-in. (12.7-mm) diameter cylinder and small amplitude waves created 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.Evaluating 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 partly because of the difficulty of measuring the fluid velocities at the instant of impact.Faltinsen et al. investigated the load acting on rigid horizontal circular cylinders (with end plates and length-to-diameter ratios of about 1) that 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 also conducted experiments with flexible horizontal cylinders and found that the analytically predicted values were always lower (50 to 90%) than those found experimentally.This investigation was undertaken (1) to examine the existing theoretical models for determining wave slam forces on circular cylinders; (2) to furnish data, obtained under controlled laboratory conditions, about forces acting on circular cylinders subjected to impact with a sinusoidally oscillating water surface; (3) to determine the relative importance of the inertia- and drag-dominated forces during fluid impact; and (4) to correlate these data for identifiable wave parameters such as the Froude number (NFr); the Keulegan-Carpenter number (NK); and the Reynolds number (NRe).This investigation does not deal with the relatively more complex impact situations arising from the slamming of random ocean waves on the members of offshore structures.

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