BANGKOK—A beach in eastern Thailand was declared a disaster area on Saturday as oil leaking from an underwater pipeline in the Gulf of Thailand continued to wash ashore and blacken the sand. The leak from the pipeline owned by Star Petroleum Refining Public Company Limited (SPRC) started late on Tuesday and was brought under control a day later after spilling an estimated 13,209 gallons of oil into the ocean 12 miles from the country’s industrialized eastern seaboard. Some of the oil reached the shoreline at Mae Ramphueng beach in Rayong province late on Friday after spreading over 18 sq miles of sea in the gulf. The navy is working with SPRC to contain the leak and said the main oil mass was still offshore with only a small amount washing up on at least two spots along the 12-km-long beach. About 150 SPRC workers and 200 navy personnel had been …