This oil tanker frequents the Portland Pipeline Corporation's oil terminals in South Portland, and was tied up to the Maine State Pier in downtown Portland for a few days this past winter for some repair work. She was last here on April 20th.
Shirley is also a felon, unfortunately.
According to MarineTraffic.com, the Overseas Shirley arrived in Portland early this morning after a stop in Halifax. Curiously, after departing from Halifax on May 14th, the Overseas Shirley took a northeasterly course towards Newfoundland, and was steaming towards Placentia Bay on the morning of May 16th, apparently destined to arrive at the Come-By-Chance oil refinery in Arnold's Cove. On the morning of May 17th, however, it was recorded steaming south out of the bay, about 10 miles away from the refinery - apparently it was either a very quick or a cancelled trip to Newfoundland's sole oil refinery.
Instead, the Overseas Shirley came here, to Portland, apparently to deliver crude oil into the Portland Pipeline and on to a larger refinery complex in Montreal. According to this Canadian history site, the Come-By-Chance refinery has had financial troubles in the past, probably owing to its geographic isolation. It seems likely that the Overseas Shirley filled its tanks with crude oil in Halifax (where, as in Portland, there are huge "tank farms" for oil storage), took some of it to Newfoundland, then delivered the rest to Montreal via the Portland Pipe Line.
Portland's harbor functions primarily as a foreign waystation for the Canadian oil industry. The United States is famously addicted to oil, but in this particular commerce, Maine plays the role of a junkie and of the cross-border mule.