E-Book Content
Calculating the Effect of Surface or Underwater Explosions on Submerged Equipment and Structures C. David Sulfredge, Robert H. Morris, and Robert L. Sanders Oak Ridge National Laboratory, Building 5700, MS-6085 P.O. Box 2008, Oak Ridge, Tennessee 37831-6085
Abstract−Since the terrorist attacks of September 11, 2001, the hazard to plant intake structures from waterborne explosives has become an area of particular concern. Both surface and underwater detonations are potential hazards to the water intakes or a plant spent fuel pool. The USS Cole incident shows the potential for terrorist attacks on our infrastructure involving waterborne bombs, yet little information on how to determine the potential damage from such scenarios is publicly available. This paper will present some techniques for calculating free-field blast parameters such as pressure and impulse in both surface and underwater explosions, as well as showing how these results can be applied to damage assessments for specific facility geometries.
I. INTRODUCTION In a free-field underwater detonation, the gas bubble from the explosion remains confined by water on all sides. During the initial expansion of the gas bubble after shock wave formation, the inertia of the outflowing water causes the expansion to persist until gas pressure inside the bubble drops below the corresponding hydrostatic pressure for that depth. The bubble then collapses to a high internal pressure condition and expands again. Thus the initial shock wave is followed by a further series of bubble oscillations that gradually diminish in intensity until they are damped out by viscous fluid friction. Each of these bubble oscillations transmits a secondary pressure pulse through the surrounding water. Bubble pulsation generates considerably lower peak overpressures than the primary explosion shock wave, but the time scale of the oscillations is much longer as well, so that the overall positive impulse delivered to a target may be comparable or even greater than that from the primary shock wave. The pressure and positive impulse generated by bubble oscillations vary as functions of charge weight, range, and depth. In addition, the primary shock wave intensity depends on both the charge and range and can also be affected by reflection from the bottom, other submerged structures, or the free surface of the water. Rigid surface reflections generate compression waves and free surface reflections generate rarefaction waves that superimpose on the original shock waveform.
This paper describes some methods that allow all these free-field blast parameters for underwater explosions to be estimated by scaling from available experimental data, including the effects of surface and bottom reflection. For a surface explosion the gas bubble from the charge immediately vents to the atmosphere, so there are no subsequent bubble oscillation pressure pulses. Thus the shock wave is the primary energy transmission mechanism through the water, and reflection of the shock wave from the free surface is not a major concern. There is also a substantial attenuation of both pressure and positive impulse compared to an explosion completely surrounded by water. Much less data are available for the case of surface explosions than for completely submerged bursts, but it is still possible to estimate both free-field shock overpressure and impulse as functions of range and depth. The objective for analyzing the surface explosion of a waterborne boat bomb is to determine the peak shock wave pressure and shock wave positive impulse as functions of target depth and radial distance from the detonation point. Calculated parameters needed for the underwater blasts include peak pressure and total positive impulse for both the shock wave and bubble oscillation phases of the explosions. These parameters need to be determined as functions of range