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An Enbridge pipeline spill in Michigan in July, 2010Paul Sancya/The Associated Press

U.S. environmental groups have added a new line of attack against Alberta's oil sands producers, warning that pipelines carrying bitumen are more prone to breaking, posing a serious risk to public safety.

The organizations are seizing on a study to enflame opposition to new pipelines - specifically TransCanada Corp.'s Keystone XL and Enbridge Inc.'s Gateway - which are critical if the industry is going to continue expanding oil sands production.

TransCanada is trying to win U.S. government approval for the next phase of its $13-billion Keystone pipeline, and has acknowledged that the heated political opposition to the project has delayed its original timelines.

Environmental groups are looking to capitalize on growing fears about pipeline safety, especially in light of Enbridge's major spill in Michigan last summer.

To decrease its viscosity and pipe the sludge-like bitumen to markets, the pipeline companies add chemical diluents, creating diluted bitumen, or "dilbit."

The study - authored by the Washington-based Natural Resources Defense Council - said the diluted bitumen is more corrosive than conventional oil and must be piped at higher pressure and greater temperatures, thereby increasing the risk of a pipeline break.

It claims Alberta's pipeline system has experienced 16 times the number of spills caused by internal corrosion per kilometre of pipe as the U.S. system.

Alberta's pipeline regulator quickly dismissed the NRDC study as flat-out wrong.

The Energy Resources Conservation Board said the environmental group included hazardous pipelines that were not carrying bitumen in its Alberta figures, artificially inflating the number of accidents in the province compared to the United States.

"Analysis of pipeline failure statistics in Alberta has not identified any significant differences in failure frequency between pipelines handling conventional crude versus pipelines carrying crude bitumen, crude oil or synthetic crude oil," the ERCB said.

The board said the diluted bitumen has essentially the same properties as other heavier-grades of oil that have been piped for years from Alberta to markets in the United States.

TransCanada also rejected the key contentions of the NRDC report, which was authored with support from the Sierra Club, National Wildlife Federation and Pipeline Safety Trust.

"While each crude oil differs slightly from another, the physical and chemical properties of the crude oils transported by the Keystone system are not unique and are similar to those already being transported and processed by other pipelines and refineries across the United States," TransCanada spokesman Terry Cunha said in an e-mail.

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