Semi-Submersible Heavy-Lift Vessel

A semi-submersible heavy-lift vessel is a float-on/float-off (FLO/FLO) transport ship that ballasts down to submerge its main deck, floats the cargo (a rig, jacket, damaged ship, or another vessel) over the deck, then deballasts to lift it clear of the water for dry carriage. Casting tugs and mooring winches position the floating cargo; the deck then carries indivisible loads far heavier than any shipboard crane could hoist, into the tens of thousands of tonnes. It contrasts with crane-type heavy-lift vessels, which load by lifting rather than by flotation.