BP PLC has shut down an offshore oil field in the Arctic Ocean after a small leak was detected in a gas line, a company official said. The shutdown of the Northstar oil field has taken about 40,000 barrels of oil offline each day since Friday or Saturday. Company officials were not sure when the field would resume operations, but said that the facility had to be shut down in order to fix it. There was no environmental damage and no safety hazard associated with the leak, which was found in an 8-inch (20-centimeter) pipe that is used to handle extraneous gas from the field, reports indicated. The pinhole-sized leak was discovered during a routine inspection of Northstar's wells and processing facilities…