Science & Environment

Gulf oil spill volume estimated from video

US Senate Committee on Environment and Public Works
Image caption The escaping oil volume was calculated by studying high-resolution video clips

US scientists who have analysed videos of the oil gushing from the Deepwater Horizon well say about 4.4 million barrels of crude escaped into the Gulf.

This new figure is in good agreement with calculations produced by federal government advisers who used different estimation techniques.

The Columbia University team reports its findings to the journal Science.

They indicate the volume leaking from the well at one stage could have filled four Olympic-sized pools every day.

Marine geophysicists Timothy Crone and Maya Tolstoy used a technique called optical plume velocimetry to study the oil escaping through the failed blowout preventer on the seafloor.

The technique analyses the images from video footage to gauge the velocity of flowing fluids.

It was originally devised by Dr Crone to study hydrothermal vents - volcanic areas in the deep ocean where super-heated waters are being spewed from cracks in the seabed.

Velocimetry converts what is a flat, two-dimensional image in the video pictures into a volumetric flow rate.

"Using this optical image analysis method we can apply essentially a conversion factor for what the apparent motion is [in the video] and obtain what the average velocity is at the opening," explained Dr Crone.

"And if you know the average velocity at the opening, you can then multiply that by the area of the opening and that gets you the volume. The work relies heavily on laboratory experiments which we have used to calibrate this method," he told BBC News.

One key assumption is the recognition that not all the plume seen in the video comprises oil. It is thought only 40% of it does, with the rest being gas and water.

For the Science study, the team assessed two short, high-resolution sequences of the BP Deepwater Horizon oil plume.

Image caption The impact on the economy and wildlife has yet to be fully assessed

One of these 20-30-second sequences was taken while the collapsed riser pipe was still connected to the well's blowout preventer.

The second sequence was taken after this riser pipe was cut and removed.

Crone's and Tolstoy's analysis suggests that the average flow rate of oil from the well between 22 April (the day the Deepwater Horizon rig sank) and 3 June was 56,000 barrels per day.

After the riser pipe was removed, the researchers say some 68,000 barrels of oil was flowing into the ocean each day, until the well was finally capped on 15 July.

Their total spill figure is 4.4 million barrels. This number assumes constant flow rate and the subtraction of the 800,000 or so barrels BP managed to recover at the disaster site.

The researchers stress their figures have a large uncertainty of perhaps 20%, but they compare well with the 4.1 million barrels estimate produced by the federal government's Flow Rate Technical Group (again, taking account of the 800,000 barrels recovered by BP).

Four-point-four million barrels is 185 million US gallons, or just over 700,000 cubic metres. To put it another way, the total volume is equivalent to about 280 Olympic-sized swimming pools (minimum specification).

Image caption The explosion on the Deepwater Horizon killed 11 workers and injured 17

By comparison, the volume lost from the Exxon Valdez tanker in Alaska in 1989, the largest previous US oil spill, amounted to 260,000 barrels.

"Our estimate should be viewed as preliminary," Dr Crone said.

"It was based on these two short clips. It's sort of a proof of concept. What we'd like to do in the future is apply this technique to more videos - if they can be made available - to get a better estimate of how the flow rate changed over time. This will help us narrow the uncertainties."

Commenting on the research, Dr Simon Boxall, from the UK's National Oceanography Centre, said the most important on-going issue remained not the size of the spill but its environmental impact.

"Whilst this may be the largest single marine spill of oil, it has not proven to be the most destructive," he explained.

"The depth of the spill, ocean conditions and the methods of coastal protection all helped to avoid catastrophic impact. Such research methods have an important application in identifying spill rates in determining best practice during an incident; but post-spill, it is the fate rather than volume of oil that still needs attention."

The federal government's Flow Rate Technical Group estimated some 4.9 million barrels (see chart) came out of the well, with BP managing to capture only 800,000 barrels.

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