24 September 2010

More accurate measurements makes oil spill much larger

By
Cosmos Online
After months of estimation, marine geophysicists have an accurate quantification of the BP oil spill in the Gulf of Mexico: 700 million litres - at least several orders of magnitude higher than initially reported.
Oil plume

The variation of velocities within the oil plume. Credit: Timothy Crone

SYDNEY: After months of estimation, an accurate quantification of the BP oil spill in the Gulf of Mexico: 700 million litres – at least several orders of magnitude higher than initially reported.

Described as the largest marine oil spill in the history of the petroleum industry, BP’s estimations of the scale of this disaster have ranged widely between 795, 000 litres to 9.5 million litres released per day.

Now, scientists from Columbia University in New York have used a new technique – optical plume velocimetry (OVP) – to film fluid escaping from the damaged rig, providing an independent measure.

Hydrothermal vent analysis used

“This technique is rather new. It was developed for measuring hydrothermal vent flows, and was suitable for also measuring the oil flow,” said Timothy Crone, the marine geophysicist at Columbia University. The work was published in Science.

This technique is an improvement on methods previously used, because it takes into account turbulent boundary layers as the oil spills out in the seawater.

Crone and his team have developed OVP to measure flow through high-temperature vents on the seafloor. “The oil leak flow is similar in many regards to the flows I study, and I was interested in providing an independent measure of the flow rate using this technique,” said Crone.

Two videos analysed

The analysis of two high-resolution video sequences of 20 to 30 seconds allowed for Crone and his team to account for differences in the average flow rate at the nozzle, and the image velocity – creating a more accurate measure of oil loss from the damaged rig.

Separate measurements were made for the initial flow that occurred from a jagged break in the riser, and following a cut made in the riser on June 3 temporarily allowing unimpeded oil flow into the ocean.

The video recorded in this study indicated that cutting the riser increased oil flow from 8.9 million litres per day to 10.8 million litres per day.

700 million litres still remain at sea

By assuming a constant flow rate, and subtracting the 127 million litres of oil that were collected from the ocean floor, the authors estimated that 700 million litres still remain suspended at sea.

While managing to produce that most accurate measurement of this BP blunder to date, the authors acknowledge an error margin of approximately 20%. Crone hopes to further refine his technique to allow for temporal variability and secondary leaks to be accounted for.

The more accurate estimate of the extent of this disaster will hopefully aid clean-up efforts. Andrew Ross, from the CSIRO, is currently leading two teams of scientists as they map the oil’s location and movement through the Gulf of Mexico.

The BP Oil spill took approximately 84 days to be controlled. It is now known that the oil released in this event exceeds that of the Exxon Valdez 119,000 litre spill by several orders of magnitude.

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