NOAA's Response and Restoration Blog


Furlongs per Fortnight: Units of Measure and the Deepwater Horizon/BP Oil Spill

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It’s been almost a year since I started writing for this blog. Shortly after the first entry, the Deepwater Horizon oil rig caught fire in the Gulf of Mexico, taking the lives of eleven men and creating the biggest spill in U.S. history. As the Incident Operations Coordinator for NOAA, there were days when I barely got an hour of sleep. The past year was hectic and exhausting, so as much as I wanted to, keeping up a blog about oil spills was one of many activities that fell through the cracks when up against actually responding to a massive oil spill.

One of my roles in the response to the spill was outreach to the media and Congress on oil spill science. During the last year, I gave hundreds of interviews, dozens of briefings, and was a witness before both House and Senate hearings. Trying to convey the science of spills in a way in which everyone could relate was challenging, especially when trying to communicate the varied units and scales of spills.

And it seems that every area of science and business has their own units of measure. Many of the stories about the nuclear radiation leaks in Japan right now talk about units such as sieverts and becquerels, which my spell checker doesn’t even recognize.

Understanding the Scale of Oil Released into the Gulf of Mexico

I really wanted everyone to understand the magnitude of the Deepwater Horizon/BP spill, so I tried to provide some of the following examples that everyone could relate to and visualize themselves.

The Deepwater Horizon oil rig was located in about 5,000 feet of seawater, and the depth of the well it was drilling into was approximately 18,000 feet below sea level.  The tallest building in the U.S. is the Willis Tower (formerly the Sears Tower) in Chicago, standing at 1,451 feet. Sport divers can’t swim much below 200 feet, and past 1,000 feet down in the ocean, everything has to be done by remotely operated submarine robots.

Many misunderstandings arise because crude oil in the U.S. is typically measured in barrels. Barrels are used in many industries to store bulk liquids, and there is no universal measure. A barrel of beer is 31 gallons. A barrel of wine is 60 gallons.

An oil barrel is 42 U.S. gallons. Elsewhere in the world, oil is commonly measured using cubic meters (m3), also known as metric tons or tonnes. There are approximately 300 gallons per ton of oil, depending on the density of the particular oil. Early reports about a spill incident may use any of these units, so it is important to clarify whether the reports are using gallons, barrels, or tonnes.  We created a desktop conversion program that converts units unique to oil spill response. Try it out:

Right now, our best estimate is that about 4.9 million barrels of oil were released into the Gulf of Mexico over about 87 days. At 42 gallons per barrel, that adds up to 205 million gallons total. To give you some perspective, a typical bathtub is about 40 to60 gallons, and an Olympic-sized swimming pool holds about 660,000 gallons. So imagine 4 million bathtubs or 340 swimming pools filled with oil.

When oil is spilled, it quickly spreads with winds and currents, creating a very thin layer on the sea surface.  Like oil in a puddle after a rainstorm, the thinnest layers of oil can be rainbow-colored or nearly transparent. These thin layers are typically measured in microns, and there are 1,000 microns in 1 millimeter.  Rainbow sheen is typically less than 1 to 5 microns thick.  It is hard to visualize these small units, but standard copy paper is about 100 microns thick. More information on how oil spreads and appears at sea can be found in our “Open Water Oil Identification Job Aid” at [PDF, 4.6 M].

Oil dispersed in water is also hard to visualize. Scientists generally use “parts per million” (ppm) or “parts per billion” (ppb) to describe very low concentrations of contaminants. I tried to use a number of examples to illustrate these low concentrations (a ppm is like one inch in 16 miles, or one second in 11.5 days), but one of the more useful tools was a simple photograph that one of our scientists took. You can see that 1 ppm or 10 ppm still looks like clear water.

Six vials of water showing differences in concentration from 0 parts per million to 10,000 parts per millionEnvironmental sampling and chemical testing can detect some chemicals in extremely small amounts—sometimes creating excessive public concern. We as scientists need to improve how we explain our technical results so that they are meaningful to non-scientists.

And by the way, a furlong per fortnight, as I referenced in the title of this post, is a tongue-in-cheek measure of speed. A furlong is 1/8 of a mile and a fortnight is 14 days, meaning one furlong per fortnight would be about 0.0004 miles per hour, or 1 centimeter per minute.

Author: doughelton


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