Two of the biggest oil companies in the world, BP and ConocoPhillips, have joined forces to try to break a longstanding deadlock over the vast reserves of natural gas in Alaska. The firms said they would spend billions to build a pipeline from the North Slope to feed markets in the United States and Canada. The proposal Tuesday won praise from Alaska's governor, Sarah Palin. "It's a good day," she said to reporters. The announcement comes at a time when the consumption of natural gas in the United States is increasing and conventional production is declining. Natural gas is cleaner than other power sources, like coal, and analysts say it is becoming increasingly critical to the nation's energy needs. BP and Conoco will initially spend $600 million in the next three years to drum up support for the project, seek state and federal approval, and secure gas supplies for the pipeline. BP and Conoco said the project would be the largest-ever private-sector construction project in North America. The project, which would include a $5 billion gas-processing facility on the North Slope, would cost about $30 billion and take at least 10 years to complete. At a time when energy prices and construction costs are soaring, the endeavor would dwarf the 800-mile, or 1,290 kilometer, trans-Alaska oil pipeline, a momentous project completed in 1977 that brought jobs and revenue to Alaska. As oil production from the Prudhoe Bay field declines, Alaskans are hoping that natural gas will take over from oil. An Alaska gas pipeline has long been sought as a critical component of the nation's energy security. The planned pipeline would have a daily capacity of 4 billion cubic feet, or 113 million cubic meters, of natural gas, or almost 7 percent of current U.S. consumption. But the companies will have to overcome some huge hurdles, said Christopher Ruppel, an energy analyst at Execution, a brokerage and research firm. "We've had a long record of Alaska pipeline projects coming out of Alaska and Canada, and they have consistently been delayed because of political opposition and rising costs," he said. "The United States and Canada desperately need the gas. But the question is, is it doable?" The companies will need to secure more than 1,000 permits from local, state and federal authorities in the United States and Canada, a process that will probably take years. They need to negotiate with native tribes along the pipeline's route to secure the right of passage. If the oil pipeline is any guide, the gas line will also require vast engineering feats. But with higher prices, and a growing appetite for natural gas, the economics of such a large project are starting to make sense for oil companies. The companies said the initial plan is to build a 2,000-mile pipeline from the North Slope to the Canadian province of Alberta; that would add to the total North American gas supply, freeing some Canadian gas for export to the United States. Eventually, the pipeline may be extended another 1,500 miles, to Chicago. "This will be a massive undertaking," said Doug Suttles, the president of BP Alaska. "It is going to take the big team to get this going." The plan to build a natural gas pipeline to export the vast gas resources in the state has been tangled in Alaskan politics for years. Today, gas reserves in Alaska are reinjected into oil fields or left dormant because of a lack of export facilities to bring them to consumers. When Palin took office in late 2006, she interrupted pipeline negotiations that her predecessor, Frank Murkowski, had been pursuing with the North Slope oil operators BP, Conoco and ExxonMobil. She started from scratch after criticizing the previous talks as not being competitive enough, and sought to bring in new operators to secure better terms for Alaska. Her administration is evaluating a proposal made by a Canadian pipeline operator, TransCanada. The oil companies protested the delays and said the governor's procedure was unrealistic. Eschewing $500 million in potential subsidies from the state, BP and ConocoPhillips declared Tuesday that the economics of natural gas have reached the point that they can finance the pipeline on their own. James Bowles, the president of Conoco Alaska, said that although the companies would seek no state subsidies, they would try to meet requirements outlined by Alaskan authorities, like offering local delivery points on the pipeline to meet the natural gas requirements of the state. "This project is moving forward on its own," he said.