The Value of Dispersants for Offshore Oil Spill Response
Authors: Nedwed Tim , Coolbaugh Tom , Demarco Greg 
Published: April 2012

Abstract
Oil spill response strategies are designed to minimize environmental impacts tothe extent possible. Each response option must be evaluated for operationallimitations (e.g., sea state), potential effectiveness, environmental impactsof the response option itself, and applicability under various oil spillscenarios (e.g., size and location of the spill) in addition to health andsafety of the responders.
Although mechanical recovery is often favored for its ability to directlyremove oil from the environment, it has long been recognized that for largeoffshore spills this technology has significant limitations. In addition toknown operational limits in the presence of currents and waves, thedynamic-nature of offshore oil slicks, i.e., rapid spreading and movement, hasresulted in mechanical recovery only treating a small fraction of spilled oilin the past. Because of these limitations, the oil and gas industry has workedto develop alternative response tools that can be used in addition tomechanical recovery to more effectively treat large offshore oil spills.
This paper provides a review of the primary oil spill response options, adetailed discussion that addresses misperceptions and misunderstandings aboutdispersants and their use, and a description of dispersant use during theDeepwater Horizon incident including the important health and safety aspects ofsubsea dispersant injection. The information provided will support the use ofdispersants as a primary response tool for large offshore oil spills when thegoal is to minimize environmental harm.

Introduction
The primary goal of any oil spill response operation should be to minimizeenvironmental harm. Although one expectation may be the complete physicalcontainment and removal of oil from the environment, this is often not possible(especially with large offshore spills) due to physical limitations ofmechanical recovery systems. In fact, recovery operations during previousoffshore spills only collected a small fraction of the spilled oil even underideal conditions (ITOPF Handbook, 2010). The Deepwater Horizon incident was noexception, with estimates indicating that only 3% of the oil was mechanicallyrecovered (NOAA Oil Budget, 2010). Relying solely on mechanical responsemeasures for large offshore spills may, therefore, result in less effectiveprotection of the environment.
Recognition of the significant limitations of mechanical recovery has led todevelopment of alternative response tools—one of which is oil spilldispersants. Oil spill dispersants facilitate removal of oil from theenvironment by enhancing the natural biodegradation process. Dispersantsrapidly break up a surface slick into micron-sized droplets that move into thewater column. This provides naturally occurring oil degrading bacteria greateraccess to the oil by creating a dilute mixture of oil-in-water rather than athick surface accumulation. Fortunately, oil degrading bacteria are present inall marine environments, having evolved to degrade oil released by naturalseeps (Margesin and Schinner, 2001; Prince and Clark, 2004).
Dispersed oil rapidly dilutes (French McCay and Payne, 2001; French McCay etal., 2006; McAuliffe et al., 1980; Cormack and Nichols, 1977; Daling andIndrebo, 1996), and concentrations above known toxicity thresholds do notpersist for more than a few hours after effective dispersant application. Thusthe potential for acute impacts to the environment from dispersed oil islimited in duration and space. In contrast, a surface slick has the potentialto impact marine mammals and birds for many days and strand on sensitiveshorelines. The most sensitive areas in many marine environments are marinemarshes and swamps. These areas can take years to decades to recover (Sell etal., 1993) once impacted by surface slicks.