Completed the following sentence: NASSCO began building commercial cargo ships in 1959, eventually including large cargo ships and "Alaska"-class oil tankers. ️<unk>️<unk>️<unk>️<unk>️<unk>️<unk>️<unk>️<unk>️<unk>️
️<unk>️<unk>️<unk>️<unk>️<unk>️ 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.