Last week's rig explosion caused this 600-square-mile oil leak off the shore of Louisiana. Now the slick's a major threat to the rest of the Gulf of Mexico, forcing the Coast Guard to begin setting it on fire. Here's how.
Because a 600-square-mile floating, burning oil slick is also dangerous, the Coast Guard and other response agencies aren't burning it all at once. Through a process called in situ, Latin for "in place," burning, the oil is burned off in sections.
Oil floats on water so the process requires surrounding small sections of the oil-and-water mixture and separating it from the main portion of the fire in large, waterproof booms in a "U-shape" towed by two boats. A control burn experiment (PDF) in Newfoundland, Canada determined oil has to be at least 1-2 mm thick above the water surface and no more than 30-to-50% water to actually burn so they've started with the thickest slicks.
Once the the oil is towed a safe distance, the 1-2 mm thick slick of oil is set on fire. The process for actually lighting the oil on fire varies, but the boats typically continue to tow the concentrated oil slick slightly ahead of the current so the oil continues to burn. This is why it sometimes appears in photos to be burning only at one end. Once the oil is burned it typically leaves a thick, tar-like substance skimmers can then collect.
The plan is to destroy 50% to 95% of the oil via fire and use conventional trapping techniques to grab the rest. Test burns from yesterday were considered successful, but poor weather conditions have stalled those efforts for the moment. The map below indicates the slick will reach the coast of Louisiana coast by tomorrow, which is why they're taking such drastic steps.
The Newfoundland study determined the amount of airborne emissions from a fire were less damaging to wildlife and coastal concerns than the oil slick itself. Unfortunately, this isn't the only problem. There are currently underwater ROVs (Remote Operated Vehicles) like the ones featured in the film Titanic trying to seal off the well on the bottom of the floor, which is leaking faster than reported at a rate of approximately 5,000 barrels a day.
The slide from BP below shows how they then seal off the leaking wells.
[NASA, BP and NOAA via US Coast Guard District 8]