World largest marine transport ships are owned by Dockwise. Dockwise Ltd. is a Bermuda-based holding company in the marine transport industry.
Dockwise was formed in September 1993 by the merger of two complementary companies, Wijsmuller Transport (a division of Heerema) and Dock Express Shipping (a division of Royal Vopak), becoming the world's largest seagoing heavy transport shipping company. They have several of largest transport ships. And some are semi-submersible vessels.
http://en.wikipedia.org/wiki/Dockwise
Below are some of those ships which are incredibly massive.