September 6, 2011
WHOI Studies Hydrocarbon Flow Rate Of Deepwater Horizon Spill
Researchers at the Woods Hole Oceanographic Institution (WHOI) have calculated the flow rate of the BP Macondo well to be about 57,000 barrels of oil per day, totaling close to 5 million barrels of oil and nearly 100 million standard cubic feet per day of natural gas released from April 2010 to July 2010.
The study results were published in the online issue of the Proceedings Of The National Academy Of Sciences (PNAS).
The estimates matched those made by the federal government and validate the estimation techniques employed by investigators, considering the investigators were under pressure and making quick estimates. The new study focuses on the quality and accuracy of these estimates. WHOI Scientist and lead author of the PNAS paper Richard Camilli said, “It provides a rigorous assessment of the statistical and systematic uncertainty in our earlier findings.”
Camilli testified to Congress that using the acoustic instruments would calculate the flow rate to within, “a factor of two,” the team found a 17% error with their calculations. Chris Reddy, of WHOI and a co-author of the study notes, “Considering all they [the WHOI team] had to do in such a short time frame, I´d be quite pleased with any uncertainty rate under 20 percent.”
The WHOI team used two acoustic instruments to read a cross-section of the hydrocarbon flow stream which were attached to the ROV (Remotely Operated Vehicle) called MAXX3. The first was an Acoustic Doppler Current Profiler that measures the Doppler shift in sound, much like how a siren sounds higher pitched when it is approaching you and lower pitched as it passes you.
The other instrument was an imaging multi-beam sonar that works similar to a medical ultrasound and allowed the team to distinguish between oil, gas, and seawater.
Camilli said, “By using the acoustic techniques, we were able to collect a tremendous amount of data in the limited time window that was available. We were able to see inside of the flow and make measurements of the velocities. With optical systems, you see only the outside. This was sort of like x-ray vision.”
The team also collected samples of the hydrocarbons escaping from the damaged well, using an isobaric gas-tight sampler. According to the study the captured sample showed that the well contained 77 percent oil, 22 percent natural gas and less than one percent other gases. This composition allowed the scientists to calculate how much oil was flowing out of the well.
Image 1: The scientists needed to distinguish how much oil versus gas was leaking from the well. To collect and analyze the well fluid itself, WHOI scientists used an isobaric gas-tight sampler, or IGT, a deep-sea device developed at WHOI to sample hydrothermal vent fluids. A pristine fluid sample from within the well was crucial to understand what fraction of the flow was oil. The manipulator arm of a robotic vehicle (upper right) moves the IGT sampler toward the jet of hot oil and gas shooting out of the broken drill pipe. (Photo courtesy of Oceaneering)
Image 2: 3D reconstruction of a hydrocarbon jet leaking from the Deepwater Horizon's broken riser. Each colored dot represents the location of an acoustic Doppler velocity measurement, with the dot color describing the velocity. The black ellipse indicates the size and location of the jet source; blue dashed ellipses indicate the jet flow perimeter. Black dots indicate the position of the ROV-mounted Doppler sonar during the measurement sequence. (Illustration courtesy of Rich Camilli, Woods Hole Oceanographic Institution)
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