From the Duluth News Tribune: 
Rockets? Tanks? What were those cylinders?

Adele Yorde, spokeswoman for the Duluth Seaway Port Authority, said the strange-looking items were "pressure vessels" bound for the tar sands region of Alberta, Canada.

The vessels are used during the refining process as heavy crude oil is turned into gas, diesel and other products. Simply put, they're used to keep liquids or gases at a pressure significantly different than the surrounding atmosphere.