Diluted bitumen, a controversial form of heavy Canadian oil, poses no more risks to pipelines than conventional oil, according to a long-awaited report released Tuesday by the National Academy of Sciences.

But environmentalists and pipeline watchdogs said the study's scope was so narrow and its methodology so flawed that it does little to settle the controversy over whether diluted bitumen, or dilbit, is more dangerous to humans and the environment than the light, conventional crude oil that most U.S. pipelines were built to handle.

The report examined the potential for pipeline leaks but did not address the consequences of a spill, the key concern for environmentalists and people who live near pipelines. And the conclusions were based not on new research but primarily on self-reported industry data, scientific research that was funded or conducted by the oil industry, and government databases that even federal regulators admit are incomplete and sometimes inaccurate.

Supporters of the proposed Keystone XL pipeline—which would carry up to 830,000 barrels of dilbit per day from Canada's oil sands to Gulf Coast refineries—welcomed the study's results.

"Oil sands-derived crude is safe for delivery—this confirmation is good news for pipelines, the environment and American consumers," Andy Black, president of the Association of Oil Pipe Lines, said in a news release.

Some of the challenges the academy faced in preparing the report are mentioned in the body of the report. But the study's executive summary, where the authors presented their main findings, reflects none of the uncertainties behind the data.

"The committee does not find any causes of pipeline failure unique to the transportation of diluted bitumen," says the first sentence in the section of the executive summary labeled "results."

Carl Weimer, executive director of the nonprofit, nonpartisan Pipeline Safety Trust, said the report's conclusions aren't surprising, given its narrow scope.

The report "only tells us that the probability of a failure of a pipeline carrying dilbit is no different than the probability of the failure of an oil pipeline carrying other types of heavy oils," Weimer said in a statement. Regulators have "so far failed to analyze whether the consequences of dilbit pipeline failures are greater than those of conventional oil spills."

The study was commissioned by the Pipeline and Hazardous Materials Safety Administration (PHMSA), the federal agency that regulates interstate pipelines, after Congress passed a pipeline safety law in 2011 that asked for a "comprehensive review of hazardous liquid pipeline facility regulations" to determine if they're "sufficient" to regulate pipelines that transport dilbit.

PHMSA hired the National Academy of Sciences (NAS) to conduct the study. But it specifically limited the scope of the report to an examination of whether pipelines carrying dilbit are more likely to leak than pipelines carrying conventional crude oil.

Mark Barteau, chair of the 12-member NAS committee that conducted the study, told reporters during a Tuesday press call that the committee did not consider the consequences of a spill—and that the committee members didn't have the expertise to discuss that issue.

Barteau is a chemical engineering professor at the University of Michigan. The other committee members have similar expertise in pipeline engineering and metallurgy.

Committee member George Tenley, Jr., a former president of the industry group Pipeline Research Council International, said during the call that the committee was told to review existing pipeline regulations only if it found that dilbit was more likely to cause a pipeline spill. Because the committee did not reach that conclusion, it didn't examine whether current regulations are adequate.

InsideClimate called PHMSA to ask when the agency intends to commission a separate report on the impacts of dilbit spills, but PHMSA did not respond before deadline.

The issue is important because dilbit behaves differently from conventional crude oil when it spills into water. A 2010 dilbit spill in Michigan's Kalamazoo River is still being cleaned up nearly three years later. Unlike conventional oil, which usually floats on water, dilbit is composed of bitumen—a heavy crude oil—and light hydrocarbons used to thin the bitumen so it can flow through pipelines. During the Kalamazoo spill, the light chemicals gradually evaporated, leaving the bitumen to sink into the riverbed.

Despite the rapid growth of the oil sands industry, and plans to build or expand more than 10,000 miles of pipelines in the next few years, federal pipeline regulations don't distinguish between dilbit and conventional crude oil.

The NAS report says dilbit does not pose any "unique" risks to pipelines, and that dilbit has chemical and physical properties within "the range of other crude oils."

Anthony Swift, an attorney at the Natural Resources Defense Council who has spent years studying the chemistry of dilbit, said these findings are misleading because the committee didn't compare dilbit with light conventional crude oil—which is what most of the nation's pipelines were built to handle. Instead, the study focused on comparing dilbit with heavy conventional Canadian crudes, which like dilbit must be thinned with light chemicals before they can flow through pipelines. Some of these crudes are almost as heavy as some of the bitumens, which means they, too, will sink more easily than light crude after a pipeline spill.

"It appears that the NAS has concluded dilbit and heavy crude present similar risks," said Swift. "That's a conclusion we [at NRDC] reached before the NAS study came out."

The committee met five times over the past year and invited more than a dozen industry and government representatives to present data and concerns during a series of public meetings. Swift was the only speaker from an environmental group.

Swift said he wants more attention paid to the differences between dilbit and light crude oils. Until the late 90's, U.S. pipelines transported almost exclusively light or medium crude oil from the Middle East or the United States, he said. Those crude oils are light enough to flow through pipelines without dilution, and they float when spilled into water.

"The more relevant and useful question is, does the influx of Canadian heavy crude and dilbit flooding the U.S. pipeline system present an increased risk to the nation's waters, communities and lands? That's a question that PHMSA did not ask the NAS to address," Swift said.

Barteau, the NAS committee chair, said on the press call that the report compared dilbit to the entire range of conventional crude oils. But a close look at numerous charts in the 114-page report shows that most of the comparisons involved heavy Canadian oils. Only one chart compared the chemical properties of dilbit and light crude oil.

Strong Ties to Industry

Weimer, the Pipeline Safety Trust director, said the scientists were hampered by the lack of good data. Because there has been little independent research on dilbit, the committee relied largely on industry studies and conference papers, along with some peer-reviewed research and a questionnaire it sent to several oil sands producers.

At least four of the 12 committee members have strong ties to the industry. Weimer said such ties are almost inevitable, because most of the funding for pipeline corrosion studies comes from the industry, including work conducted by university scientists. He said that situation could change if PHMSA pushes for more research in this area.

The committee members also analyzed pipeline spill data maintained by PHMSA and the Canadian federal government. But those databases don't specify the exact cause of each spill, and they don't track whether the pipeline carried dilbit or conventional crude oil at the time of the spill.

During the press call, Barteau said the committee was also challenged by the fact that some pipelines carry both dilbit and conventional oil and that the contents vary over time. "There are not separate pipeline systems for diluted bitumen and other crudes," he said. "So one can't really do a side-by-side comparison."

The NAS study was peer reviewed before its release, but neither the previous version nor the reviewers' comments are available to the public.