* Oily water, methanol spill from ruptured pipeline
* Comes as BP tries to rebuild public, investor confidence
* 2,100-4,200 gallons spilled
* 30,000 bpd field remains shut in
* BP shares down (Adds details and background in paragraphs 4, 7-8)
By Yereth Rosen and Tom Bergin
ANCHORAGE/LONDON, July 18 BP (BP.L) reported yet another pipeline leak at its Alaskan oilfields, frustrating the oil giant's attempts to rebuild its reputation after the Gulf of Mexico oil spill.
BP said on Monday that a pipeline at its 30,000 barrel-per-day Lisburne field, which is currently closed for maintenance, ruptured during testing and spilled a mixture of methanol and oily water onto the tundra.
The London-based company has a long history of oil spills at its Alaskan pipelines, including lines servicing Lisburne. The accidents have hurt BP's public image in the United States, where around 40 percent of its assets are based.
The Alaska Department of Environmental Conservation said the spill occurred on Saturday and amounted to 2,100 to 4,200 gallons, affecting 4,960 square feet of gravel pad and about 2,040 square feet of wet and aquatic tundra.
Production from the entire Lisburne field remains shut off while the spill is addressed, Alaska officials said.
A BP spokesman said the cleanup was under way and the company would determine the cause "in due course."
Immediate efforts are focused on containment and cleanup, said Tom DeRuyter, state on-scene coordinator for the Department of Environmental Conservation. Cleanup must be completed before the pipe section is excavated, he added.
"BP has got a goal to have the investigation completed and the line repaired by fall freeze-up," he said. "I do not anticipate the cleanup will take that long."
The pipeline will also have to be dug up to allow for an investigation into why it failed, DeRuyter said.
Lisburne, managed as part of the Greater Prudhoe Bay Unit, has produced no oil since June 18, according to Alaska Oil and Gas Conservation Commission records, suggesting maintenance work requiring a prolonged shutdown.
Dawn Patience, BP Alaska spokeswoman, said there is not yet an estimated time for the Lisburne field to be back in production.
"The planned maintenance is ongoing," she said, adding the leak happened when BP was testing recently installed valves in the piping.
The affected pipeline is an eight-inch diameter line, and the area that leaked runs underground beneath a road, according to the Alaska Department of Environmental Conservation.
BP's blown out Macondo well caused the worst offshore oil spill in U.S. history, spewing almost 5 million barrels of oil into the Gulf and putting BP's future in the U.S. at risk.
Previous problems including leaks from corroded pipelines in Alaska and the fatal Texas City refinery blast in 2005 had already earned the company a poor reputation for safety, something analysts say it needs to address if it is to continue to grow in North America.
In 2009, a crack in a flow line that serves Lisburne spilled around 46,000 gallons of a mixture of oil and water on to the snowy tundra. BP in May agreed to pay a $25 million civil penalty and spend $60 million on enhanced safety measures, to settle a federal probe of a pipeline oil spill on Alaska's North Slope in 2006.
New York-listed shares in BP were down 0.2 percent, or 9 cents, to $44.20 per share in late afternoon trading. (Editing by David Gregorio)
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