(Repeats story originally published overnight)

CALGARY, Alberta/WASHINGTON, Dec 4 (Reuters) - A U.S. regulator’s preliminary investigation into the biggest oil pipeline spill this year has raised a red flag that could trigger an extensive and costly inspection of tens of thousands of miles of underground energy lines.

The 5,000-barrel leak on TransCanada Corp’s Keystone pipeline on Nov. 16 in South Dakota might have stemmed from damage caused by a weight put in place when it was built in 2008, the Pipeline and Hazardous Materials Safety Administration said in a report published on Tuesday.

Weights are used to prevent pipelines from moving and reduce the risk of damage or ruptures when water tables rise.

The regulator’s finding has implications for the 2,687-mile (4,324 km) pipeline and others throughout the world. The weights, which tip the scales at 7,000 pounds (3,175 kg) or more, are commonly used, but only the pipeline operators know where they are located.

Damage from weights “could happen on other segments of this pipeline and other pipelines,” said Najmedin Meshkati, professor of civil and environmental engineering at the University of Southern California.

The Keystone pipeline carries 590,000 barrels per day from Alberta’s oil sands to U.S. refineries. TransCanada’s proposed Keystone XL line would add another 830,000 bpd of capacity.

Nebraska officials approved the construction of that line even after the leak, although it is still unclear if TransCanada will build it.

Depending on the results of the full investigation, construction plans for new lines such as the Keystone XL may need modification. Existing lines may also have to be checked, a difficult and potentially expensive undertaking.

U.S. regulators do not have specific information on the types of weights or their locations because pipeline companies are not required to submit data, said Carl Weimer, executive director of the non-profit Pipeline Safety Trust.

PHMSA did not respond to requests for comment on this question.

The Canadian Energy Pipeline Association also said operators, not regulators, keep tabs on this information. “We would not have an inventory; that would need to come from the individual companies,” said Carla Beynon, a spokeswoman for the industry group.

On Tuesday, PHMSA ordered TransCanada to clean up the site and analyze data on the location of other weights on the Keystone line where the land may have similar characteristics as where the leak occurred. TransCanada would not say how many weights were placed along the pipeline, which runs through several states and Canadian provinces, during construction.

In one of those states, the North Dakota Public Service Commission, which regulates pipelines, has asked for briefings with TransCanada on its monitoring procedures. Commissioners are also waiting to see the full PHMSA report and results of testing on the damaged section of pipeline.

“If there are issues on how this pipeline was designed and constructed, we will certainly be concerned,” said commission Chairman Randy Christmann.

STANDARD PROCEDURES

Using weights made of sand, gravel or concrete is standard, pipeline companies and industry representatives said.

“This is the first time I’ve heard of this type of issue causing an incident,” said Association of Oil Pipe Lines spokesman John Stoody.

A handful of companies, including PipeSak Pipeline Products & Services and Keymay Industries, manufacture bagged weights filled with gravel or permeable textiles, but pipelines built before 2009 like Keystone would probably have 7,000-to-9,000-pound concrete weights.

The Keystone line’s coating may have been scratched during installation, which could have led to corrosion, said PipeSak President Geoff Connors.

Weimer, the Pipeline Safety Trust director, said problems during Keystone’s construction, when ditches filled up with water, signaled that weighting was needed in those places.

“We are hoping more information gets released about how prevalent the weights are,” he said.

The 5,000-barrel leak came just days before regulators in neighboring Nebraska approved a route for the long-delayed and controversial Keystone XL pipeline.

XL opponents, who have campaigned against the pipeline on concerns that a spill could pollute areas vital to Nebraska’s agriculture industry, said the PHMSA report reinforced those worries.

“This has implications for XL, which crosses over the Ogallala aquifer and would require similar construction,” said Anthony Swift, director of the Natural Resources Defense Council environmental group’s Canada Project.

TransCanada did not say whether it would use a similar practice of weighting for the XL.

Crystal Rhoades, one of two Nebraska Public Service commissioners who voted against the last permit needed for Keystone XL, said the commission had no jurisdiction over safety issues and therefore probably could not revoke TransCanada’s permit or request additional conditions on the pipeline.

Former TransCanada engineer Evan Vokes, a whistleblower, said spills occurred due to shoddy or outmoded construction techniques. He said little could be done to prevent leaks once weights are installed if they are not built in the right places.

“The time to address this is when you put it in the ditch,” he said.

“It’s a pointless exercise to fix it afterwards. It’s like putting on a helmet and running through a shooting range.” (Additional reporting by Catherine Ngai in New York; Editing by David Gaffen, Simon Webb and Lisa Von Ahn)