'Extensive' ice plugs found in leaky BP line

* Not determined if ice plugs caused leak

* Lisburne oil production not affected

By Yereth Rosen

ANCHORAGE, Alaska, Dec 5 (Reuters) - The pipeline that leaked oily material onto the tundra at BP Plc's (BP.L) Lisburne oil field in Alaska was filled with extensive ice plugs, including one about 1,500 feet (457 metres) long, state officials said on Saturday.

The ice buildup within the line was detected by X-ray analysis launched by BP late on Thursday, the Alaska Department of Environmental Conservation said in a statement.

It has not been determined that the ice plugs caused the leak, said Weld Royal, a spokeswoman for the department. The cause remains under investigation, she said.

The spill and the cleanup work have not affected oil production at Lisburne, whose output is about 35,000 barrels of oil per day, BP spokesman Steve Rinehart said.

The leak in the pipeline, an 18-inch (46-cm) -diameter flow line, was discovered last Sunday. Although it had been taken off service weeks earlier because of suspected ice plugs, it was still pressurized and carrying material when the leak was found.

A mixture of crude oil, oil- and salt-laced produced water and natural gas flowed out of the leak and sprayed an estimated 8,400 square feet (780 square metres) of snow-covered tundra, according to state environmental officials and representatives from BP Exploration (Alaska) Inc. Produced water is a byproduct of the oil-drilling process.

By Saturday, officials overseeing the response had a tentative estimate of about 46,000 gallons (1,095 barrels) for the amount of liquids spilled, the Alaska Department of Environmental Conservation said in its situation report.

Cleanup workers now have access to the most heavily contaminated areas beneath the leak site, officials said. Managers lifted a rule that established a safety buffer around the leak, officials said.

About 160 cubic yards (122 cubic metres) of contaminated material had been scooped up from the site and carried to a storage site, the Department of Environmental Conservation said.

Lisburne is adjacent to the giant Prudhoe field, the biggest U.S. oil field. The affected line is used to carry oil, gas and produced water into a production center, where the materials are separated.

The leak presents potential legal problems for BP, which remains under a term of three years' probation as part of a 2007 federal criminal settlement concerning a major pipeline spill at Prudhoe Bay in 2006. (Reporting by Yereth Rosen; Editing by Peter Cooney)