BOSTON — Three specially designed tugboats yank the hulking pale green liquefied natural gas tanker from its dock near Boston Harbor. More than 1,000 feet long, with an odd color and sets of pipes sticking out the top, it’s apparent that this is no ordinary ship. And with three Coast Guard boats, and four others from state, city and port authority law enforcement agencies surrounding it, this is no ordinary departure. Bringing up the rear is the Pendant, a 65-year-old, 65-foot-long ocean-going tugboat, and one of the oldest platforms in the service’s inventory that pulls double duty in the winter...