Research into controlled in situ burning (ISB) of an oil slick dates back to the late 1950s. Operational validation occurred during a trial burn following the Exxon Valdez incident. The first extensive long-term use of ISB as a principal oil spill response strategy occurred following the Deepwater Horizon incident in 2010. With the ability to effectively ignite and burn thick slicks naturally contained by ice, without the need for booms, ISB is an oil spill response option particularly suited to remote arctic areas.. However, in drift ice conditions with a mix of low ice concentrations and open water, oil spills can rapidly spread to become too thin to ignite. Fire-resistant booms can collect and keep slicks thick in open water; however, even light ice conditions make using booms challenging.
Herders rapidly spread across a water surface to create a surfactant monolayer that reduces the water surface tension. When the surfactants reach the boundary of an oil slick, they affect the balance of surface forces acting at the edge of the slick and cause the oil to contract to a new, thicker equilibrium state. The increased slick thickness produced by herders, 3 to 5 mm, provides favourable conditions for effective ignition and ISB without the need for containment booms.
Researchers began to study the use of herding agents in oil spill response in the 1970s. In 2004, ExxonMobil Upstream Research Company initiated a research programme to study oil-herding surfactants as an alternative to booms to thicken slicks for burning in light ice concentrations and relatively calm seas. Subsequently, industry and government agencies sponsored a series of successful herder tests utilizing small and large test tanks and an offshore field trial (2008). Herder research continues under the auspices of the JIP focusing on extending the use of herders in conjunction with burning, to higher sea states in open water.
With aerial application of both the herding agent and ignition source (igniter), the herder/burn combination becomes an extremely rapid and effective new response tool, independent from vessel support. The slower weathering of oil slicks in ice and cold water can also extend the window of opportunity for this new tool as an effective arctic response strategy.
Herders at a glance
- Herding agents were initially developed in the 1970’s as a method of thickening oil slicks prior to mechanical recovery.
- Herders to enable in situ burning have undergone more than 10 years of study.
- Herders use surface active agents to thicken slicks without the need to collect the oil in a physical boom and do not require a physical boundary to work. They are effective in the open sea, with or without the presence of ice, as long as there are few breaking waves present.
- Herding requires very small quantities of a low toxicity surfactant
- Herding agents cause the oil slick to contract, the same way a drop of dish soap in a wet, greasy pan forces the grease to the edges. As oil spills shrink in surface area, they get thicker, growing from <1 millimeter (0.04 inch) to as much as 6 millimeters (0.24 inch). This makes it possible to ignite the slick and achieve an efficient burn. The thicker a spill is before it’s burned, the more oil gets removed and the higher the overall response effectiveness.
- Herders are effective in fresh and marine waters.
- Herders use about 30 times less product compared to treating a slick with dispersant.
- The use of herders on an oil slick does not detract from the effectiveness of subsequent or concurrent chemical dispersant application or mechanical recovery.
- Field tests in 2008 in Norway demonstrated that herders work in cold open water with ice nearby.
- Two herding agents (ThickSlick 6535 and SilTech OP-40) are now on the U.S. Environmental Protection Agency, National Oil and Hazardous Substances Pollution Contingency Plan (NCP) Product Schedule for consideration for use in U.S. waters and both are commercially available as of June, 2012.
- A helicopter-based herder delivery system developed by ExxonMobil was tested through field trials funded through the JIP.
In situ burning at a glance
- ISB is a proven response technique that can rapidly eliminate more than 90 per cent of encountered oil in many situations.
- Compared to other response methods, greatly reduced demands on equipment and personnel often make ISB a desirable response method in remote Arctic environments where responders often have to work with minimal infrastructure including a lack of any approved oily waste disposal sites.
- The presence of colder temperatures and calmer sea states in the presence of ice may increase the window of opportunity for the effective use of ISB by increasing the equilibrium thickness and reducing the emulsification rate.
- The presence of slush and/or brash ice mixed with the oil affects the overall burn efficiency and actual response operations should consider these factors. Another JIP project is studying the effects of this type of ice on the window-of-opportunity for herder use (see Herders Fate, Effects and Window of Opportunity).
- ISB emissions are short lived and not likely to cause significant environmental effects or human health issues. Safety regulations and air quality monitoring requirements such as those in place for ISB in Alaska and elsewhere are designed to ensure the on-going safety of ISB for responders and the public.