Collection and Recovery of Chemically Treated, Undispersed Oil

It is known that an oil slick may not be fully dispersed using chemical dispersants if it is under-dosed, if sufficient surface mixing energy is not present, or if the oil has unfavorable properties. This may interfere with other oil spill response tactics, such as mechanical recovery. For example, during the Deepwater Horizon response dispersants were widely used and injected directly at the wellhead, while at the surface, reports indicated that surface oil was sometimes “slippery and not recoverable” by mechanical measures. This study conducted systematic testing to determine whether oil treated with low doses of dispersant behaved differently than untreated oil during mechanical collection and recovery operations. 

Oil response booms are used to gather surface oil into a thick slick, which is then recovered using oil recovery skimmers. An advancing containment boom was tested to measure the boom’s ability to contain untreated crude oil and crude oil treated with dispersant at dispersant to oil ratios (DORs) of 1:50 and 1:200. Two oleophilic skimmers, a smooth drum skimmer and a disc skimmer, were tested to measure their ability to recover untreated crude oil and crude oil treated with dispersant at DORs of 1:50 and 1:200.  Performance values were measured to compare collection and recovery performance.

The presence of dispersant had a significant effect on the containment boom’s ability to collect and contain surface oil, even at low DOR. The boom adequately collected the untreated oil at a tow speed that would typically be used in field operations, losing essentially no oil at 0.9 knot tow speed. However, for the oil treated with a small amount of dispersant (DOR 1:200) the boom at the same 0.9 knots lost an average of 15% of the oil. At the DOR 1:50 treatment, the full oil volume was unsuccessfully contained in the boom and was dispersed by the energy provided by the towing operation. The color and depth of the resulting dispersion indicated a relatively small median droplet size. No instrumentation was deployed in the tank to measure droplet size as that was beyond the scope of this project. 

Each skimmer reacted differently when recovering oil with increasing amounts of dispersant. The disc skimmer maintained a fairly consistent fluid recovery rate, although it picked up increasing amounts of water along with the oil which did not separate out during the standard settling time of 30 minutes. The drum skimmer was more significantly impacted by the presence of dispersants, exhibiting both a decrease of 10% in fluid recovery rate and a more significant decrease of 43% in oil recovery rate (ORR) with the increasing presence of dispersant.

For both skimmers, the presence of dispersant caused the recovered fluid to entrain increasing amounts of water that did not separate out within a standard settling time of 30 minutes. This effect increased with the increasing DOR and affected the recovery efficiency (RE) of both skimmers. This effect was more apparent with the drum skimmer, possibly because its larger surface area in contact with the water imparted greater mixing energy in the area of recovery. RE decreased for the disc skimmer from 99.5% to 81.0% and for the drum skimmer from 88.4% to 57.0%. 

This testing occurred with only one crude oil that is a relatively low viscosity crude oil and is readily dispersible. Testing with different crudes would likely provide different results, as other reviews have shown that oil properties have a significant impact on dispersibility. The same factors that influence the dispersibility of an oil may also influence the ability of that oil to be mechanically contained and recovered should it be under-dosed with dispersants. In addition, the properties of oil have a significant impact on recovery and emulsification absent of treatment by dispersants.