Air Barriers as Oil-Spill Containment Devices

Abstract

Abstract As a part of a program to evaluate oils pill containment devices, an experimental investigation of air barriers was performed. Previously published relationships for air barriers operating in still water were verified. Tests involving a current were conducted in a recirculating test tank in water 7 ft deep. The structure of the bubble plume was found to change with increasing current, and a phenomenological explanation was developed. phenomenological explanation was developed. During all of these tests, measurements of the magnitude of the surface current showed it to be very turbulent. Tests with oil on the water surface showed this turbulence to be the major factor in the failure of an air barrier completely to retain oil in the presence of a current. The combination of turbulence and the inherent instability of flowing, stratified fluids tears drops off the bottom of the oil slick in the stagnation region. The downward flow in this region then transports the oil drops below the surface current produced by the air barrier, and the main current carries the drops through the barrier. On the downstream side, the drops gradually rise to the surface. The main body of the slick is retained upstream of the barrier, and observation from above the surface of murky waters would not reveal these drops surfacing far downstream of the barrier. Varying orifice size and spacing had no effect on the surface current turbulence or mean velocity. Large quantities of spilled oil have been recovered at a prototype installation by placing the barrier at an angle to the current. The final phase of the investigation was concerned with the use of an air barrier across the mouth of an enclosed body of still water a slip, cove, or marina. Thickness of oil retained by the barrier in still water as a function of operating conditions has been obtained experimentally, and tentative limits on the use of this formulation for prototype spills are given. Introduction Air barriers have been used as breakwaters, to guard against salt water intrusion into fresh water regions, to keep harbors free of ice during winter, and as a means of preventing sediment deposition in critical areas. In recent years they have been proposed and installed as oil containment devices. An air barrier is basically a pipe with holes spaced along its length that is laid under water on the bottom (Fig. 1). Air is supplied to the pipe (manifold), exits through the holes (orifices) pipe (manifold), exits through the holes (orifices) and rises to the water surface. Water is entrained by the upward flow of air bubbles and moves vertically with the air. At the surface, the air bubbles escape and the water flow becomes two horizontal surface currents moving in opposite directions away from the manifold location. The object of this program was to give the operating personnel, who might conceivably be involved with an oil spill, some idea of the capabilities of containment and removal equipment currently available. A review of this program has been given by Milz. Air barriers were included in the equipment surveyed and evaluated; experiments were conducted to evaluate the effects of a current on an air barrier and, if possible, to determine the conditions under which an air barrier would fail to retain oil.