According to experts in the failure of oil and gas pipelines, there are a handful of factors that can contribute to a pipeline rupture, like the one on Exxon Mobil's Pegasus pipeline that spilled toxic diluted bitumen or dilbit from the Canadian tar sands into a Mayflower, Arkansas lake and subdivision.
Those factors include pressure swings within the pipe, reversing the direction of the flow of oil, the quality of the original pipe construction and a build-up of hydrogen atoms inside tiny cracks in the pipe.
Elizabeth Douglass reports at The Arkansas Times that all of these factors were in play in the Pegasus pipeline rupture.
Some operators may change their pumping pressures and their cycles to accommodate customers or to push more crude through the pipe faster, which generates more fees. Exxon, for example, increased the amount of dilbit flowing through the Pegasus by 50 percent in 2009. To accomplish that without installing larger pipe, Exxon had to send oil through the pipe faster, either by adding pumping stations or increasing the overall operating pressure, or a mix of the two.
Three years earlier, in 2006, Exxon also reversed the direction of the pipeline's flow, a move that would automatically alter the impact of pressure cycles by changing where the highest and lowest pressures hit along the pipeline.
Big changes in the internal pressure cause pipe to repeatedly flex, and that can cause special problems in crack-prone vintage pipes like the Pegasus. Exxon's pipe was doubly challenged, however, because its pipe was known to be exceptionally brittle around the seams. Brittleness can cause pipes to fracture instead of flex, just as the way wire will break after being bent back and forth repeatedly
And pipeline experts suspect the makeup of the tar sands oil or dilbit Exxon was transporting played a role, as well:
Excess hydrogen could also have played a role — or could even have been the primary factor — in the Pegasus failure, according to Pizzo and a failure analyst who did not want to be identified because of ongoing work with oil companies.
Hydrogen is found in and around oil and gas pipelines as a matter of course. The cathodic protection systems that operators use to prevent corrosion can give off hydrogen atoms if the systems are overcharged. The products that flow through pipelines also usually contain hydrogen in the form of hydrogen sulfide. "Sour" crude oil, which includes many forms of dilbit, tends to have more hydrogen sulfide than typical U.S. crude oils
The Pegasus, for example, was carrying a diluted bitumen called Wabasca Heavy. That variety has the second-highest sulfur content of the 29 kinds of Canadian crude oil and dilbit listed in a reference guide from Crude Quality Inc., which operates a website that tracks the chemical makeup of Canadian crudes.
Hydrogen sulfide becomes a problem only if it decomposes and the hydrogen atoms move into fragile areas of the pipe.
Pizzo said it works like this — An especially brittle area, like the seam weld region of the Pegasus, will draw hydrogen atoms into the steel pipe. The atoms congregate at the tips of cracks, where the internal stress is higher. Then they weaken the steel by creating larger gaps between the iron atoms that form the pipe. As the atomic hydrogen cluster grows, the pressure builds until the tip of the crack is extended. Then the hydrogen atoms move to the new tip, and the process repeats itself. When the crack grows large enough, the pipe breaks.
This phenomenon is well documented in the world of metals and has had catastrophic consequences for bridges, ships, liquid tanks, pipelines and other metal structures.
Studies have found that cracks in steel pipelines grow when higher levels of hydrogen sulfide are involved. One study found that cracks grew up to 10 times faster, Pizzo said. "If it's 10 times faster, then the consequence is that [the pipe] fails, but instead of in years, it fails in months."
Brittle pipe? Increased pressure? Chemically corrosive tar sands? The more we learn about this spill, the more it seems like it wasn't a matter of if this pipeline would rupture; it was just a matter of when and where the disaster would occur.