Based on industry studies and ExxonMobil's own records for the Pegasus pipeline, the oil company knew or should have known the following six facts that help explain what went wrong in this year's oil spill in the North Woods neighborhood of Mayflower, Ark., a town of 2,300 people located about 20 miles northeast of Little Rock.

(NOTE: This slideshow is part of an article by Elizabeth Douglass, Exxon Knew Pipeline Was Old, Defective, and Brittle, and Still Added New Stresses. The article found that since at least 2006, ExxonMobil has known that its 1940s-era Pegasus pipeline had many manufacturing defects, and that the seams of the pipe have been identified by the industry as being especially brittle. Despite those risks, Exxon added new stresses to the line by using it to carry heavy oil from Canada, reversing the direction of the flow and increasing the amount of oil that surged through it.)

Credit: The Duncan Firm

Fact One: That about 650 miles of the 858-mile long Pegasus was made of pipe manufactured using a technique known to create hook cracks and other defects that can cause the seam welds to fail if operators don't take proper preventative measures.

Credit: The Duncan Firm

Fact Two: That that same 650-mile stretch of pipeline was built in 1947 and 1948 with pipe made by Youngstown Sheet & Tube Co., whose pipe from that era is known to be prone to brittleness and fractures.

Credit: Zahra Hirji, InsideClimate News

Fact Three: That the 2006 hydrotest—used to detect and eliminate perilous cracks on problematic pipelines—that Exxon performed on that 650-mile stretch of pipe was conducted at stress pressures appropriate for calibrating maximum operating pressures, but not at levels experts believe is necessary to rid a pipeline of seam crack threats. The stress pressure Exxon used in 2006 also was lower than the stress pressure it used in 1991 to test a newer segment of the Pegasus.

Credit: ExxonMobil Pipeline Company and the Pipeline and Hazardous Materials Safety Administration (PHMSA)

Fact Four: That 11 seam welds failed during the 2006 hydrotest, even though that test was conducted at relatively low pressure levels.

On the left, an example of a magnified cross section of a small hook crack near a low-frequency electric resistance welded seam.

Credit: Kiefner & Associates for PHMSA

Fact Five: That the new crack-detection technology used in the February 2013 probe of the Pegasus hasn't been proven effective in the field and anecdotal evidence suggests it has a spotty success rate.

Credit: ExxonMobil Pipeline Company and PHMSA

Fact Six: That 12 seam cracks were found during a 2010 in-line inspection of a segment of the pipeline outside the failure area.

Credit: The Duncan Firm

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