Value of Dispersants for Offshore-Oil-Spill Response

Abstract

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper OTC 23359, ’The Value of Dispersants for Offshore-Oil-Spill Response,’ by Tim Nedwed, Tom Coolbaugh, and Greg Demarco, ExxonMobil, prepared for the 2012 Offshore Technology Conference, Houston, 30 April-3 May. The paper has not been peer reviewed. Each oil-spill-response option must be evaluated for operational limitations (e.g., sea state), potential effectiveness, environmental effects of the response option, applicability under various oil-spill scenarios (e.g., size and location of the spill), and health and safety of the responders. Alternative response tools are available to assist mechanical recovery in treating large offshore oil spills more effectively. There are misperceptions and misunderstandings about dispersants and their use; therefore, a description of dispersant use during the Deepwater Horizon incident is presented. Introduction The primary goal of any oil-spill response should be to minimize environmental harm. Although one expectation may be complete physical containment and removal of oil from the environment, this often is not possible (especially with large offshore spills) because of physical limitations of mechanical-recovery systems. Recovery operations during previous offshore spills collected only a small fraction of the spilled oil, even under ideal conditions. The Deepwater Horizon incident was no exception, with estimates indicating that only 3% of the oil was recovered mechanically. Recognizing significant limitations of mechanical recovery has led to developing alternative response tools—one of which is oil-spill dispersants. Oil-spill dispersants facilitate removal of oil from the environment by enhancing the natural biodegradation process. Dispersants break up a surface slick rapidly into micron-sized droplets that move into the water column. This action provides naturally occurring oil-degrading bacteria greater access to the oil by creating a dilute mixture of oil in water rather than a thick surface accumulation. Fortunately, oil-degrading bacteria exist in all marine environments, having evolved to degrade oil released by natural seeps. Dispersed oil dilutes rapidly, and concentrations above known toxicity thresholds persist for no more than a few hours after effective dispersant application. During the Deepwater Horizon incident, the amount of damage to the shorelines was far less than expected initially by the public. In many locations where oil did enter, marsh-grass recovery was apparent soon after the spill. This, and other evidence, suggests that a key reason for the limited shoreline effects during the incident was use of dispersants and, in particular, the subsea injection of dispersants at the wellhead. Oil Behavior During High-Flow Subsea Release The lower the viscosity of the oil, the higher the flow rate, and vice versa. Also, the ability to disperse oil is inversely related to the fluid’s viscosity, and oils remain readily dispersible with viscosities up to several thousand centipoises.