* Isaac could shutter 85 pct of U.S. offshore oil-Weather Insight
* Storm's westward shift brings greater US energy impacts
* BP shuts down all U.S. GOM operations on storm threat
By Kristen Hays
HOUSTON, Aug 26 (Reuters) - Gulf of Mexico operators on Sunday braced for the first hurricane in 2012 to affect the U.S. oilpatch, which is also expected to disrupt the majority of U.S. offshore oil output.
After initially calling for Tropical Storm Isaac to pass well east of the U.S. offshore production zone, forecasters are now predicting a more westward track which could bring Isaac as a powerful Category 2 hurricane over the heart of the U.S. offshore oilpatch, which produces about 23 percent of U.S. oil output and 7 percent of its natural gas output.
Isaac could be the biggest test to U.S. energy infrastructure since 2008, when Hurricanes Gustav and Ike disrupted offshore oil output for months as well as damaged onshore natural gas processing plants and pipelines.
London-based BP Plc, the biggest U.S. Gulf producer, on Sunday evacuated all of its Gulf of Mexico rigs, after earlier shutting down its giant Thunder Horse platform, the world's largest, which can process 250,000 barrels of oil and 200 million cubic of natural gas per day (mmcfd), and three others.
BP's evacuations and shutdowns initiated Sunday included Atlantis, the world's second-largest offshore platform, which can produce up to 200,000 bpd of oil and 189 mmcf per day of gas.
Other offshore operators like Chevron Corp. and Royal Dutch/Shell have evacuated some workers, though they have not yet shut down any production. (For a FACTBOX on oil, gas operations affected by the storm, see: )
Isaac has so far shut about 9 percent of offshore oil output, according to U.S. government figures released on Saturday. The U.S. Bureau of Safety and Environmental Enforcement will issue updated figures later on Sunday.
Outage figures will rise in the days ahead, according to forecasters at Weather Insight, an arm of Thomson Reuters, who predict the storm will spur short-term shutdowns of 85 percent of U.S. offshore oil capacity and 68 percent of its natural gas output. Isaac has a 95 percent chance of entering the heart of the oil and gas producing region, Weather Insight said on Sunday.
Isaac could also impact the U.S. refinery row along the Gulf Coast, which stretches from Mississippi to south Texas and accounts from more than 40 percents of U.S. fuel output.
Chevron's 330,000 barrel per day (bpd) Pascagoula, Mississippi, refinery is near the storm's expected landfall.
The Louisiana Offshore Oil Port (LOOP), which can offload about 1 million barrels per day of foreign crude for delivery to Gulf Coast refiners, remained in operation on Sunday, a spokeswoman said.