Effectiveness of chemical dispersants used in broken ice conditions

A number of natural processes (collectively termed weathering) take place when oil is spilled a sea, including spreading, evaporation, dispersion, emulsification, biodegradation, and dissolution into the water column, all of which change the properties of the oil. These properties typically make the oil more difficult to recover or remediate. The efficiency of various oil spill countermeasures (e.g. mechanical recovery, in situ burning, and chemical dispersion) may therefore be reduced at a certain point after the release of oil, depending on the oil properties and weathering stage.

Ice-covered waters and Arctic conditions impose other challenges for oil spill response compared to open and more temperate waters, such as the remoteness of the area, the low temperatures, seasonal darkness, and the presence of ice. At the same time, several studies in Norway (e.g. Vefsmo and Johannesen (1994) and Sørstrøm et al. (2010)) and Canada (e.g. SL Ross and Dickins (1987) and Owens and Belore (2004)) have shown that ice can be an advantage in oil spill response operations. The ice slows down oil weathering, dampens the waves, prevents the oil from spreading over large distances, and allows for longer response time. The best response option will depend on site-specific conditions (such as near shore, shallow water, sensitivity of the environment, ice coverage, weather and drift forecast) and assessment of the environmental impact of the spill and the benefit from applying the different response operations.

In the event of an offshore oil spill, rapid decisions must be made regarding the best course of action to mitigate the effects. Chemical dispersants are one of the tools to be considered. Oil exploration activities are increasingly extending into regions where ice conditions of various types exist during the year (e.g. Barents Sea, Chukchi and Beaufort Seas). Oil spill response planners in these areas need to know whether oil spilled in these regions during periods when ice is present in various forms can be chemically dispersed. Several previous studies (summarized in Fingas and Ka’aihue, 2005) have shown that water salinity can significantly influence a dispersant’s performance. On average, seawater in the world’s oceans has a salinity of approximately 35 ppt, and the major commercially available dispersants have been formulated for use in normal marine salinities of 30 ppt or higher. Sea ice formation and melting may have a significant effect on the salinity of the seawater in areas where ice occurs, but also seasonal outflow of freshwater from rivers into the sea in cold regions will temporally and locally reduce the salinity.

The aim of using oil spill dispersants on spilled oil is to reduce the environmental impact of the oil by transferring it from the sea surface and into the water column. Mixing energy is required to create small oil droplets that will remain suspended in the water column, finally causing them to spread, dilute, and naturally biodegrade. A dispersant is a mixture of surface active components (or surfactants) in a solvent or a blend of solvents. Surfactants used in today’s dispersants are often the same as used in cosmetics and by the food industry due to their low toxicity and high biodegradability.

Early research in Canada on dispersant effectiveness in ice-infested waters included studies by Mackay et al. (1980), Cox and Schultz (1981), Byford et al. (1983), Brown et al. (1985), and Brown and Goodman (1996). These studies ranged from experiments in small containers, to medium-sized-tank experiments, to experiments in a large wave basin in Calgary. More recent work includes tests by SL Ross in their wave tank (e.g. SL Ross, 2006a) and at the Ohmsett facility (SL Ross (2002 and 2006b)). As a part of the SINTEF Oil in ice JIP (2006–2010), SINTEF completed dispersant studies with ice in their re-circulating flume (Brandvik et al., 2010a; 2010b). They have also performed mesoscale field tests in the fjord ice in Svalbard (Brandvik and Faksness, 2009) and large scale field tests of dispersant application on oil released in the Marginal Ice Zone (MIZ) in the Barents Sea in 2009 (Daling et al., 2010).

The results of these studies suggest that ice has a dual effect on the energy available for oil dispersion. The first effect is to dampen the wave motion, which serves to reduce energy available to form small droplets of oil and disperse the slick in ice. The second effect is a pumping action imparted to the oil by collisions between ice pieces, which serves to increase the formation of small droplets. The conclusion from the studies was that effectiveness of dispersant when used in an ice situation will strongly depend on the ice regime under consideration. The major issues are: i) dispersant accessibility to the oil; ii) the level of surface mixing energy that is available to first break up the oil into small droplets; and iii) the energy available to mix and suspend the dispersed oil droplets in the underlying body of water.

In this project, the focus has been to test the efficiency of using dispersants to disperse oil spilled between ice floes. As several oil types and dispersants were tested, a standardized test protocol was developed in order to be able to compare results and dispersant efficiencies between tests and between laboratories. The overall objective was to build on current knowledge to increase our understanding of the effect of oil types, degree of weathering and environmental conditions on dispersant effectiveness in ice-covered waters. The effects on dispersant efficiency due to changes in the test variables was measured using results from the flume-based experiments.

Four oils were used: Alaska North Slope, and three North Sea crudes Troll Blend, Oseberg Blend, and Grane. Properties of the oils are given in Table 1. The commercial dispersants Corexit 9500, Dasic NS, and Finasol OSR-52 were used in the experiments at a nominal dispersant-to-oil ratio (DOR) of 1 to 20 by volume. The test parameters applied are shown in Table 2; approximately 70 tests were performed using different combinations of the test parameters.

The oil weathering and dispersant effectiveness tests in ice were conducted in mesoscale recirculating flumes at SL Ross and SINTEF. The flumes at both locations are identical in size and construction, with an outer circumference of 16.6 m, a width of 0.5 m, and a height of 1.5 m. A schematic drawing of the flumes is presented in Figure 1.

Approximately 4.8 m³ of seawater at a depth of 1 m was circulating in the 10 m long flumes. The water temperature during the tests was maintained between 0 and −2 °C. The flumes are equipped with a wave maker and two fans, as shown, to produce wave action and wind during weathering. An 1 m² (2 m by 0.5 m) confinement area was set up with removable barriers to contain the oil in one section of the flumes during weathering and dispersant application. A propeller was installed at point A in Figure 1 (MinnKota Endura 30 electric trolling motor) to provide additional agitation when required by the dispersant test procedure. In addition, a LISST (Laser In Situ Scattering and Transmissiometry, Sequoia Scientific, Type C) particle size analyzer was positioned at point B (Figure 1) in the water column to detect and monitor the oil droplet size distribution during the test. Sub-surface water samples were collected using a siphon system placed next to the LISST at 50 cm depth.

Three different sizes of ice blocks (5 × 5 × 5 cm, 10 × 10 × 5 cm, and 20 × 20 × 5 cm) were prepared from 0.5% salinity water. The ice blocks were replenished in the confinement area during the weathering time as needed to maintain the target ice concentration.

A test protocol for the project was developed in order to obtain as comparable results as possible. As shown in Figure 1, a containment area of 1 m² created by placing removable barriers in the straight section of the flume on the opposite side of the wave generator. Ice pieces were added to the containment area only to achieve the desired coverage (50% or 80% ice). Then fresh oil (1 L) was applied to the open water between the ice blocks. The same quantity and size distribution of ice blocks was used for all tests for a given ice coverage. The oil was allowed to weather for 6 or 18 hrs with wind (1.2 m/s) and waves equivalent to gentle swells. After weathering, but prior to dispersant application, samples of the weathered oil were collected for physical and chemical analysis, to measure viscosity, water content, density (on broken emulsions), and to estimate evaporative loss. Ice blocks were placed in the rest of the flume outside the containment area to achieve the same ice coverage and size distribution. Dispersant was applied to the weathered oil in the containment area using a Wagner 450 sprayer with a 0.5 mm nozzle size applicator. Then the barriers for the containment were removed to allow free movement of oil and ice in the tank. Waves and wind were turned on and the oil and ice subjected to breaking waves roughly equivalent to Beaufort Sea State 3 for 30 minutes, followed by propeller (prop) wash for 10 minutes. Water grab samples were collected at a depth of 50 cm depth after the 30 minutes period, but prior to initiation of prop wash, and again after the 10 minute prop wash. The oil concentration was measured in the water to calculate the mass of the oil in the water column and to estimate the dispersant effectiveness.

The water grab samples were extracted with dichloro methane (DCM) followed by colorimetric analysis using a spectrophotometer (absorbance measured at 410 nm) to determine the concentration of oil in the sample. This concentration was multiplied by the total volume of water in the flume to quantify the mass of dispersed oil in the water column. Dispersant effectiveness (DE) was calculated as the percentage of the initial oil (corrected for evaporative loss and sampling) that was dispersed into the water column. The LISST particle size analyzer was used to detect and monitor the oil droplet size distribution during the test.

The results and discussion are based on the findings from the tests performed under the conditions tested in the SINTEF and SL Ross flumes and may not be directly transferable to realistic conditions in the Arctic. However, flume testing gives repeatable controlled comparisons of relative DE with different oils, dispersants, weathering times, and other “fixed” conditions, which cannot be done in the field.

Inter-calibration tests have been performed twice to compare methodology and settings for the two flumes used in this study. The results showed good correlation between the two laboratories (Faksness et al., 2014, Faksness and Belore, 2014).

The relative DE for four crude oils has been tested using three commercial oil spill dispersants applying different test parameters (given in Table 2). Usually no replicate tests were performed in order to maximize the number of different test conditions studied given the resources available for this project. Triplicate tests with Troll, Grane, and ANS were performed to evaluate the precision of these tests. The results indicated that some outliers might occur, but that the absolute standard deviation in DE, independent of oil type, seems to be 10% or lower (Faksness et al., 2016b).

The DE varied with both oil type and dispersant type applied, and the effectiveness increased when higher energy wave conditions were used. The waves provided sufficient mixing energy for small oil droplet formation, and applying prop wash after the waves usually did not significantly enhance the dispersant efficiency. Previous results from Faksness et al. (2016a) have shown that varying the ice cover between 50% and 80 % did not influence the results significantly (generally less than ±10% DE), and that gentle swells (low energy) did not provide enough energy to mix the oil and dispersant and produce an efficient dispersion.

The average evaporative loss after 18 hours weathering in the flume with 80% ice was 27% (±1) for Troll Blend, 31% (±6) for Oseberg Blend, 5% (±1) for Grane, and 23% (±2) for ANS. Tests with shorter weathering time (6 hours vs 18 hours) were performed for ANS and Grane. Evaporative loss after 6 hours was 17% (±1) for ANS and 5% for Grane. The DE after prop wash in 35 ppt water is given in Figure 2. The results show that the asphalthenic oil Grane is less dispersible after the longer weathering time, and indicated that shorter weathering time matters more for Grane than the more paraffinic oil ANS.

Dispersant efficiency for all oils is compared in Figure 3. The oils were weathered for 18 hours in 80% ice and 35 ppt salinity water, and DE was measured from water grab samples collected after prop wash. As expected, the DE varied with both oil type and dispersant type applied. Troll Blend (a mixture of naphthenic and paraffinic oil) was overall the most dispersible of the tested oils, independent of the dispersant applied. The highest DE for the Troll oil was obtained with Corexit 9500 and OSR-52 (> 80%), while Dasic NS was slightly lower (64%).

Previous results in the present project have shown that the salinity of the water is an important factor regarding dispersant effectiveness (Faksness et al., 2016a; 2016b). This is in agreement with observations done by e.g. Blondina et al. (1999) and Fingas et al. (2006). They used the so-called “Swirling flask test” to test dispersant effectiveness at different salinities, ranged from 0 to 35 ppt, and observed a decrease in dispersant effectiveness at lower salinities. Chandrasekar et al. (2006) made the same conclusions, using the “Baffled-flask test”. As it can be seen in Figure 4, the testing has shown that water salinity affects the performance of the dispersants depending on the oil type. Dasic NS and OSR-52 were less affected by lower salinity than Corexit 9500. For Troll Blend, Dasic’s performance was not reduced significantly over the range of salinities tested whereas Corexit 9500 and OSR-52 had slight reductions in DE with reduced water salinity. With the Oseberg Blend and Grane oils, both Dasic NS and OSR-52 had similar or better DE at 15 ppt compared to 35 ppt salinities, whereas the Corexit 9500 had a significant decrease in DE when the salinity dropped to 15 ppt. For ANS crude oil, both Dasic NS and Corexit 9500 showed similar reduction in DE when the salinity dropped to 15 ppt, but OSR-52 performed similarly at the two salinities. The asphaltenic oil Grane oils seem to be less dispersible in low salinity water (5ppt) then the other tested crudes. These variations in efficiency with commercial dispersants are linked to the fact that these products have been optimized for open water applications, with salinities close to 35 ppt. Salinity in itself is not a showstopper for promoting oil in water dispersions, as there exists dispersants formulated for low salinity water.

SINTEF and SL Ross completed a large number of flume tests that simulated ice conditions to evaluate the dispersibility of four crude oils using three commercially available dispersants. The findings provide information on the relative dispersibility of various oil types and dispersants in ice-covered water, but may not be directly transferable to all conditions that could be encountered in the Arctic.

Dispersant efficiency as a function of the different test variables was measured using results from the mesoscale recirculating flume experiments. As expected, shorter weathering times resulted in an increase in dispersant efficiency, and the effectiveness increased when higher mixing energy conditions were used. Low energy input (swells) did not provide enough energy effectively disperse the oil in the systems. Varying the ice cover did not influence the results significantly, but water salinity did, with the lowest dispersant efficiencies found at 5 ppt salinity. Overall, Dasic NS and OSR-52 were less affected by changing salinity than Corexit 9500, and the results indicated that they worked as well or better in 15 ppt versus 35 ppt water on some, but not all oils.

All of the crude oils used were found to have higher than 50% DE with at least one of the dispersants when tested in 80% ice cover and weathered for 18 hours. The dispersant effectiveness varied with both oil composition and dispersant type applied, and the naphthenic oil Troll was the most dispersible of the tested oils, independent of the dispersant applied or salinity.

This study indicates that dispersants can disperse oil in ice-covered environments, and that dispersants can be considered as a response option for spills in ice. However, the effectiveness needs to be validated in the field during an actual event and is dependent on the oil type, weathering, and environmental conditions.

This project was funded by the International Association of Oil and Gas Producers and is supported by the nine international oil and gas companies BP, Chevron, ConocoPhillips, Eni, ExxonMobil, North Caspian Operating Company, Shell, Statoil, and Total.