Dispersants

The adverse economic and environmental effects of offshore oil spills are greatest when the oil slick reaches the shoreline. For this reason, much effort is put on preventing offshore oil spills from reaching the shoreline. In calm seas, use of skimmers and booms to collect the oil at sea is the conventional method of cleanup and recovery. In-situ burning is also used in such a situation, but it has its limitations. In rough seas, skimming or burning the oil is not effective, and the use of chemical dispersants appears to be the promising approach for cleanup.

A dispersant is a mixture of surfactants and solvents that causes the oil slick to break into small droplets in a process known as dispersion. (The term "dispersion" used here is from the oil literature and is different from the spreading of chemicals due to the spatial variation of velocity). The generated small oil droplets get transported or transferred into the water column due to wave action and sea turbulence. They subsequently move away from the contaminated area due to prevailing currents. They could eventually adhere to suspended particulate matter and/or biodegrade.

Dispersion of oil droplets is enhanced by turbulence due to the mixing energy imposed by waves, especially breaking waves. This means that the artificial dispersion of oil is a chemical-physical process that depends both on the type of dispersant/oil pair and on the sea state. Typically, very light and very heavy oils are not easily dispersible. In the case of very light oils, the formed droplets have to be very small to overcome buoyancy. Hence, a high dosage of dispersant is required to cause the formation of such small droplets. Very heavy oils are much more resistant to dispersion because their high viscosity prevents the dispersant from penetrating them, which is a necessary condition to produce dispersed oil droplets. The use of dispersants in very calm or very rough sea is not effective. In very calm seas, the applied dispersant tends to run off the oil and gathers in small pools within the slick. The use of dispersants in very rough seas might not be needed because a high degree of dispersion occurs naturally due to the high energy at sea.

The evaluation of dispersant effectiveness used for oil spills is commonly done using tests conducted in laboratory flasks. The success of a test relies on replication of the conditions at sea. The Baffled Flask (BF) is being considered by the US EPA to replace the Swirling Flask (SF).

Our work aimed at quantifying the mixing in both flasks using instantaneous velocity measurements obtained from a Hot Wire Anemometer (HWA).

Tangential velocity in the SF rotating at 200 rpm at various times of the rotation. The red designates 20 cm/s

Tangential velocity in the BF rotating at 200 rpm at various times of the rotation. The red designates 70 cm/s

 

Current Work

The work has continued at the Bedford Institute of Oceanography in Halifax, Canada. There, a wave tank was constructed that allows the experiments to be performed on a considerably larger scale. The wave tank allows the generation of waves in controlled conditions. Deep water waves are created with a paddle and are allowed to develop over the length of the tank. Deep water waves are a misnomer; they are essentially waves that do not feel the sea floor (or tank bottom in this case). Computer operation of the paddle allows the creation of breaking waves by generating waves with different frequencies. Slower, high frequency waves are followed by faster, low frequency waves. The long waves overtake the short waves, and breaking occurs. This breaking is important as it transmits energy from the wave to the water column (and the oil and dispersant located in and on the water).

The rate of energy loss (energy dissipation rate, epsilon) is important as it quantifies how much energy can be input from the wave to the mixing of the oil and dispersant. This rate can be calculated by measuring the change in velocities in the wave. We used an acoustic doppler velocimeter (ADV) to obtain velocities in the wave. The ADV is a versatile tool that allows three dimensional sampling of the velocity in the water profile at sampling rates up to 50 Hz. The ADV was set up at increasing depths in the breaking zone and allowed to sample for a length of time. In addition, a capacitance wave gauge was used to measure the water level concurrently with the ADV sampling of velocity.

The velocity series obtained shows a regular wave pattern during non-breaking. During and after breaking, the velocity series exhibited "spikes" of velocity, which is indicative of turbulence. These spikes can be analyzed to obtain rates of energy dissipation over the wave series. We are currently using two methods (with similar results) to evaluate the energy dissipation rate. Preliminary results show that the rate of energy loss for breaking waves is of an order higher than for regular waves. In addition, energy dissipation rates as a function of depth follow exponential decay for breaking waves (which indicates that most energy is lost near the surface) and regular waves follow a linear decay. Deep in the column, the breaking waves follow a similar decay to the regular waves.