A federal judge has nullified Massachusetts’ landmark Oil Spill Prevention Act, striking down regulations on oil shipping in Buzzards Bay that included mandatory tug escorts and navigational routes as well as minimum staffing. U.S. District Judge Joseph L. Tauro ruled the state violated federal law because those areas fall under Coast Guard jurisdiction. He said the state regulations ''are pre-empted, invalid and unconstitutional under the Supremacy Clause of the United States Constitution.'' The Massachusetts Legislature passed the Oil Spill Act in 2004, amid warnings from the oil industry that it was stepping on federal jurisdiction. Lawmakers moved quickly to respond to a 98,000-gallon oil spill in Buzzards Bay on April 27, 2003, when a Bouchard Transportation Co.