Complex sentence: They were designed to transport very large semi-submersible drilling rigs which can weigh 30,000 tons and have a center of gravity around 30 meters (100 feet) above the transport ship's deck.
Split into simpler sentences: They were designed to transport very large semi-submersible drilling rigs. A typical rig of this kind weighs some 30,000 tons and has a center of gravity around 30 meters (100 feet) above the transport ship's deck.