Semi-submersible heavy lift

A semi-submersible heavy-lift ship transports very large floating cargo by float-on/float-off: it ballasts down until the cargo deck is submerged, the cargo is floated into position over the deck, then the ship deballasts to lift it clear and secures it for the voyage. This carries drilling rigs, jackups, semi-submersibles, port cranes, and damaged ships that cannot transit under their own power. The deck is long and unobstructed with buoyancy casings forward, and the largest units lift payloads above 50,000 tonnes, the best known being the Dockwise Blue Marlin and Black Marlin. The same ballasting principle underlies the offshore drilling and crane semi-submersibles, which hold position low in the water for motion stability.