The report also details the special risks of barges that carry dilbit, a thick oil produced from oil sands that is then thinned for transport.

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A report prepared for Friends of the Earth spotlights the oil-spill hazards posed by barges that move crude through Puget Sound to state refineries.

Barges pulled by tugs carry much smaller volumes of oil than self-propelled tankers. But Fred Felleman, a Seattle environmental consultant who prepared the report, said that “carrying oil by [cable] wire is the riskiest form of oil transportation on the water.”

Felleman, who last year was elected as a Port of Seattle commissioner, recommends that oil barges be accompanied by escort tugs, in event of an emergency. Currently, Washington state requires tug escorts only for crude-oil tankers in the Sound, while California also calls for barges to have escorts.

The report also details the special risks of barges that carry dilbit, a thick oil produced from oil sands that is then thinned for transport. Dilbit, during a spill, can pose special cleanup challenges because heavier elements may sink more quickly than conventional crude.

Dilbit is one of the Alberta oil products carried to tidewater through Kinder Morgan’s Trans Mountain pipeline to Burnaby, B.C.

Felleman’s report said some six barges a month of this oil are delivered to U.S. Oil & Co. in Tacoma. And he notes that a 2014 state transportation study cites the source of this crude as Canadian oil sands.

But Lisa Clements, a spokeswoman for Trans Mountain, said the company does not ship dilbit by barge — only by tankers. She said that oil shipped by barge to the Washington refineries was from conventional crude fields that also are carried to tidewater through the pipeline.

Marcia Nielsen, spokeswoman for the Tacoma refinery, said the Canadian oil is carried by a top-of-the-line barge that has carried the crude for years without incident.

“We think it’s safe,” Nielsen said.

In Washington in 1988, an oil barge spilled some 230,000 gallons of bunker oil off Grays Harbor.

The state Department of Ecology monitors the barges that carry oil through the Sound, and Scott Ferguson, a spill-response official, says operators have done a good job over the years.