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Why did the BP pipeline corrode?

Slate Magazine has an excellent series called The Explainer where the background behind a prominent news item is... well... explained. Here is their Explainer for the BP Pipeline corrosion scandal:

My Pipeline's Corroded
But I thought oil prevented rust.

Severely corroded pipelines will force BP to shut down its oil field at Prudhoe Bay, Alaska, company officials said on Sunday. After a major spill in March, the company discovered stretches where the pipe had lost more than 70 percent of its mass to corrosion. Why does oil corrode a pipeline?

It has water in it. The crude oil that flows from Prudhoe Bay isn't the pure stuff we might use to stave off rust in our cars. When an oil company pumps crude out of the ground, it can also pump out a hot mixture of water, carbon dioxide, sulfur, and microorganisms. (Crude oil with lots of sulfur in it is called "sour," as opposed to "sweet.") If enough of these contaminants collect in a steel pipe, they'll work together to eat away at its inner surface.

Water is the main concern. If there's only a little bit of water in the oil, or if the oil is flowing fast enough, you generally won't have a problem. That's because the water will be dispersed in tiny droplets that are suspended in the flow. These suspended droplets won't react much with the steel surface of the pipes.

Problems can arise in stretches of pipeline that run at lower pressures. In these spots, water droplets can coalesce and fall out of the oil flow. They might inundate globs of sand or dirt that have also fallen out of the crude-oil mix and form a watery sludge on the edge of the pipe. Once a watery muck forms in one part of the pipe, the natural process of corrosion speeds up. The crude can also serve as a breeding ground for anaerobic bacteria, which form slimy, sulfur-producing colonies on the inside of the pipe.

To make matters worse, crude oil comes out of the ground hot, and the pipelines are insulated to maintain those high temperatures. Oil flows better when it's hot, but heat also exacerbates corrosion within the pipes.

Sometimes the oil companies introduce corrosive bacteria, water, or gas into reservoirs themselves. When an oil field gets old, it starts to lose pressure, and it becomes harder to draw out the remaining crude. Engineers can try to add pressure underground by putting in seawater or carbon dioxide. While this increases the field's output, it also sours the product and leads to more pipeline corrosion.

The fact that British Petroleum didn't run the 'Smart Pig' through the pipe because there was too much sludge should have raised a big red flag. I am imagining that the person responsible for this decision is on their way to a filing/clerical job in Gary, Indiana for the rest of their professional career. Posted by DaveH at August 9, 2006 9:17 PM