TPT March 2012

G lobal M arketplace

Administration (NOAA) planes, the paper incorporates data from an ROV (remotely operated vehicle) equipped with a device to sample the leaking fluid at the well; as well as from a WHOI-designed and -built autonomous underwater vehicle outfitted with a miniaturised mass spectrometer developed by the same team. The samples were taken over six weeks in May-July 2010. When combined, the data enabled a new chemistry-based spill rate estimate of an average of 11,130 tons per day (tpd) of gas and oil compounds – close to the official average leak rate estimate of about 11,350 tpd (equal to about 59,200 barrels of liquid oil per day). In total, approximately 4.2 million barrels of oil were released from the well. Some surprises But a notable disclosure in the WHOI study is that 36% of the spill never rose out of the vast plume of microscopic oil droplets floating through the deep sea; and that the slick visible on the surface represented only a small percentage of the leaked gas and oil in total. “It makes perfectly legitimate sense that a large portion of the deep sea blowout wouldn’t always go to the surface,” David Hollander, a chemical oceanographer at the University of South Florida, told the Herald-Tribune . Dr Hollander was one of the first to discover the underwater plume of droplets estimated to lie 3,300 to 4,300 feet below the surface. The lead author of the paper, research chemist Thomas Ryerson of NOAA, a scientific agency within the US Department of Commerce, told the Florida daily, “The visible surface slick that people were

riveted by during the months of the spill was really only 15% of the total mass.” Dr Ryerson and his 13 colleagues determined that the airborne plume accounted for about 7% of the total. Another 17% was recovered at the surface through a marine riser. The location of the balance, about 25% of the total, is not accounted for by the chemical data. Tracing it, after two years, is considered unlikely. › The Deepwater Horizon tapped a well of light crude and gas. The riser rupture released a mix of hydrocarbons. Some of these substances were thick and buoyant; others dissolved readily. Measured in tons, gas made up nearly a quarter of the total release of 879,300 tons. Most of the gas – methane and ethane – dissolved before reaching the surface. All of the benzene, a carcinogenic liquid, also dissolved before reaching the surface, the study showed. Volatile compounds, such as propane and dozens of others, dissolved in the water or evaporated at the surface. Only the thick, more buoyant crude formed the surface slick. › The scientists were unable to determine whether chemical dispersants applied at the wellhead played a role in keeping most of the oil chemicals and gas from surfacing. “We can’t tease out any evidence that dispersant had any effect,” Dr Ryerson told Kate Spinner of the Sarasota Herald-Tribune . “Or, if it did, what effect.” Dorothy Fabian , Features Editor (USA)

145

www.read-tpt.com

M arch 2012