NASSCO began building commercial cargo ships in 1959, eventually including large cargo ships and "Alaska"-class oil tankers. Its most famous commercial ship was the "Exxon Valdez" tanker, which completed construction at NASSCO in 1986, and in 1989 returned to NASSCO for repairs after its accident and oil spill in Alaska. In December 2012 the company signed a contract to build two container ships powered by liquefied natural gas (LNG). When completed they will be the largest LNG-powered ships of any kind in the world.