One answer, from the BBC:
By Giancarlo Rinaldi
Most boats do not have legs. But a jack-up barge has six, protruding high into the air when the ship is in transit.
Extending to a length of 48m from the bottom of the ship, and penetrating up to 5m into the sea bed, the "legs" of these ships provide a stable "ground" in a place where there is only roiling water.
As the legs push down, the ship is lifted above the waves. Purpose-built at a Chinese shipyard, the £60m jack-up barge MPIO Resolution is an extraordinary piece of engineering in itself....MORE