It's a jack-up installation vessel used in the construction of offshore windfarms.
The vessel manovers into position, the legs are lowered to the seabed, then the hull is jacked up until the hull is well clear of the water.
This provides a very stable platform for the crane to install the wind turbine towers nacelle and blades.
It would make a very good yacht maintenance vessel, it could lift boats out of the water, remove masts, very quickly and efficiently.
But no one would ever be able to afford the day rate!