State environmental officials are investigating why a pipeline on Alaska's North Slope operated by BP ruptured early Saturday during a pressure test.

Workers in nearby buildings felt the ground shake from the force of the underground line break, Tom DeRuyter, on-scene coordinator for the state Department of Environmental Conservation, said Sunday.

BP Exploration (Alaska) Inc. estimated the size of the spill at 2,100 to 4,200 gallons of fluid, mainly methanol and other fluids along with an undetermined amount of crude oil. It spilled onto a gravel pad and into a small tundra pond, DEC said.

The trans-Alaska oil pipeline is shut down for planned annual maintenance, and oil field operators were taking the opportunity to work on equipment as well, DeRuyter said.

BP was replacing corroded valves on an 8-inch pipeline used for testing oil pumped out of wells at the Lisburne Production Center. With the new valves in, crews began pressure testing the system, aiming to reach 2,000 pounds per square inch. But the line failed at 949 psi, according to the DEC.

The rupture occurred in a section of pipeline sleeved within a larger casing for protection as it went under a road. Fluids shot out of each end of the casing, the DEC said.

Most of the fluid was blown by heavy winds onto or near a gravel pad at the drill site, making cleanup easier, DeRuyter said. But some got in a small pond bordering the gravel. Cleanup crews pumped water out of the pond, leaving an oily residue behind. Crews then doused the tundra with water at a low pressure in an attempt to wash away the mess, DeRuyter said.

BP brought in Alaska Clean Seas, an environmental cleanup company based on the North Slope, but didn't have to hire crews from outside the area, he said.

By Sunday afternoon, 15 barrels -- 630 gallons -- of spilled liquid had been recovered, the DEC said.

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