A new independent report claims that the failure of ExxonMobil's Pegasus pipeline in Arkansas in late March this year was caused by a manufacturing defect, Exxon has announced.

The report was prepared by Hurst Metallurgical Research Laboratory Inc. and was sent to Exxon and the Pipeline and Hazardous Materials Safety Administration (PHMSA) but both organizations declined to release the report publicly. Investigators found that the rupture in the pipeline that sent 150,000 gallons of crude oil into the town of Mayflower was the result of cracks near a seam in the pipes, the Wall Street Journal reported.

The Pegasus pipeline runs 850 miles from Illinois to Texas, carrying 90,000 barrels per day from Alberta's tar sands. The fact that it has remained closed for four months now is causing serious financial loss to Exxon but the matter is expected to become even more costly with court proceedings launched by the Environmental Protection Agency (EPA), alleging Exxon of violations of the Clean Water Act.

According to a statement from Exxon, defects in the pipeline caused during the manufacturing process were the "root cause of the problem," but the company explained that it was still carrying out tests, looking into the possibility that other factors had also contributed to the incident. Results from these tests would help Exxon identify mitigation steps and preventive measures to be implemented in future, the company said. The Pegasus pipeline is still not operating, as cleanup work continues.

Results from the metallurgical analysis conducted by the independent laboratory identified hook-shaped cracks near the seam as the main cause of the rupture, but there were other factors that were likely to have added to the effect of the cracks, including atypical pipe properties, such as very low impact toughness and elongation properties across the seam. However, corrosion was not a contributing factor, according to the report, Exxon stated.

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Findings from the report sparked an outcry from environmental organizations and many residents in areas around pipelines, as concerns were raised that other pipelines across the United Sates might be at risk of similar ruptures and oil spills. According to Bloomberg, many pipes used across various states were produced with the same technology and are of a similar age to the Pegasus pipeline. This raises the question of whether Exxon had maintained its pipeline according to industry standards and, if so, are other pipelines in danger of failing?

Events like the Mayflower spill are easily preventable through proper maintenance and inspection, Mohammad Najafi, a pipeline construction expert and engineering professor at the University of Texas at Arlington, told Bloomberg. But as pipes come to the end of their expected life cycle, more extensive care should be taken of them, including more frequent and more detailed inspections, he explained.

Bloomberg reported that investigation of the 2010 gas line incident in San Bruno, Calif., which killed eight people and demolished 38 homes, had identified segments of the line produced through the same technology as the Pegasus pipeline. This resulted in a call to make the reports public as a preventive measure but the PHMSA denied access to the metallurgy paper, citing Exxon's ongoing investigation as a reason. The administration also failed to respond to a Freedom of Information Request made at the beginning of June.