ABSTRACT

Recent results of development and testing of the free-vortex approach to floating oil recovery are presented. This work has been directed toward high-seas oil-spill recovery-a much-needed capability in offshore oil operations. The free-vortex technique, in contrast to typical skimming methods, is not sensitive to surface disturbance and provides for capturing oil at a distance from the skimmer. Free-vortex skimming operates through induced flow fields--a surface inflow that gathers oil and a central vortex that concentrates the oil for removal by conventional pumping. The: principles and hardware for maintaining the free-vortex flows on an open surface are described. Testing was conducted with different oil types, oil films 1 in. thick or less, in calm water and waves. A good capability for operation in high-seas conditions is indicated.

INTRODUCTION

Technological advances in marine oil pollution control have necessarily included the development of new equipment for recovering floating oil. Most mechanical recovery devices or skimmers involve picking off surface oil by means of a weir or a sorption system. Typical recovery devices work adequately in calm water, but performance degrades in waves over 1 or 2 ft high, making high-seas operation impractical. Another typical limitation of skimmers is that thick oil films and/or relative motion between skimmer and surface are needed to obtain oil influx. If the oil film is too thin or discontinuous, film spreading cannot provide a useful oil flow toward the recovery device. If surface currents are absent and the skimmer cannot move freely, as in the vicinity of an oil-spill containment boom, the floating oil cannot be captured.

In the free-vortex approach to skimming, originated by Scientific Associates, Inc., oil recovery is uniquely achieved through water flows induced by the skimmer itself. As shown schematically in Fig. 1, two distinct flows are produced on and near the surface of the water:

  1. inward or converging radial water inflow is created by the submerged axial impeller, and

  2. rotation or vortex flow in a central water column is induced by the submerged rotator hardware.Under the action of these water flows, surface oil is first brought inward to the vortex and then concentrated by the vortex into a central zone, which is idealized as an oil pocket in Fig. 1. A pump intake within this pocket can thus recover oil with relatively little water.

Since free-vortex skimming operates through induced water flows, it is not critically dependent on the placement and movement of hardware relative to a surface oil film. The approach is thus applicable even if the water surface is disturbed by wave action. The energy and momentum of the induced flows also provide an inherent resistance to process disruption. Free-vortex skimming therefore remains practicable for operations with strong ambient disturbances, as in high-seas oil-spill recovery. Moreover, the process does not require any particular flow behavior within the oil film or even a continuous oil film. Hence, free-vortex skimming is applicable to thin or fragmented surface films of any oil.

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