Experimental Characterization of the Influence of Dispersant Addition on Rising Oil Droplets in Water Column

Abstract

The accident of the Deepwater Horizon platform in the Gulf of Mexico in April 2010 clearly shows the importance of fully understanding the oil and gas leaks phenomena in the water column and their consequences at the sea surface. While the modelling of the rise of oil droplets in the water column is widely documented, in particular through analytical model using slip velocity concept, there is a lack of information on the effect of dispersants addition on rising oil droplet. Yet dispersants, given their surfactant capacity, will necessarily modify the oil droplets behavior in the water column in terms of buoyancy due to the oil shape variations. This paper contributes to the characterization of dispersant effect on accidental oil release especially in term of physical modification for oil droplets. To investigate the study, experimental tests were performed in the Cedre Experimental Column (CEC) which is a five meter high hexagonal column with internal diameter of 1 m and total capacity of 4.50 m3 (Le Floch, 2009). This experimental device is equipped with an injection system, equipable with different nozzles diameters, and two high speed video recording systems based on shadowgraphy. It is designed to observe the shape of the droplets, including rise trail, and the dissolution rate during the ascension to the surface. This paper presents the physical modifications undergone by an oil droplet when surface active molecules are positioned at its interface with seawater. The oil droplets observation shows the addition of dispersant decreases the average droplet volume and conversely increases the number of droplets observed in the water column. Thus, the water-oil contact surface area is increased, enhancing the exchanges between these two phases, in particular in terms of the dissolution of the lightest hydrocarbon molecules. In terms of physical appearance, oil droplets showing spherical shape modify to elliptical shape from which filaments of varying sizes are fluttering. Changing shape implies a variation in the oil droplets slip velocity throughout the seawater column as well as the definition of a more complex trajectory. These two changes tend to decrease the buoyancy of oil droplets in the water column. The formation of a plume instead of surface oil slick occurs and is carried by marine currents.