Crane vessel
The sheer-legs arrangement is more economical to manufacture and maintain than a slewing crane, but may be less convenient as the whole vessel must be moved to precise position for lifting and lowering.
This arrangement may be mounted on a catamaran barge which allows it to straddle a bridge pier to lower a prefabricated section into place.
To reduce listing moments on the barges, the gantries may be attached to the hulls by pinned joints at the ends, allowing some independent roll.
The semi-submersible hull form has a lesser and slower response to waves and swell, due to reduced waterplane area, and stability and righting moment are adjustable by ballasting to suit the load.
[1] The low waterplane area causes a low heave response, and this can be utilised to function as a tension leg platform by using vertical mooring lines to anchor piles or clump weights on the seabed sufficient to prevent heave in the prevailing sea state.
Compared to land-based cranes, the additional dynamic loads and motion in a seaway complicate the operation and safety.
Once at working height the legs can be released one at a time and driven deeper with pile driving hammers for greater stability.
Removal is basically a similar procedure in reverse, with the legs being pulled out of the bottom ground one at a time after the hull is afloat, while the anchor spread limits side forces imposed by waves.
Other factors of importance are hull draught, depth to which the hook can be lowered (for offshore work), and sea state limits for transit and lifting.
[1] The interaction of the six degrees of freedom of the vessel, the response to the sea state and wind, and the position and motion of the upper block due to crane geometry and operational motion, can make the upper block describe a complex three-dimensional path in space.
Some of these responses are inherent to the combination of vessel, crane and load, and others depend on sea state and wind forces.
Similarly when setting the load down, it should be done as smoothly as possible, and once in contact should be allowed to settle as soon as possible to avoid re-lifting and pounding on the base structure.
[1] In medieval Europe, crane vessels which could be flexibly deployed in the whole port basin were introduced as early as the 14th century.
[2] During the age of sail, the sheer hulk was used extensively as a floating crane for tasks that required heavy lift.
At the time, the heaviest single components of ships were the main masts, and sheer hulks were essential for removing and replacing them, but they were also used for other purposes.
Their role was to enable the unloading of the Arctic convoys where port installations were either destroyed by German bombers or were non existent (as at Bakaritsa quay Archangel).
In 1963, Heerema converted a Norwegian tanker, Sunnaas, into a crane vessel with a capacity of 300 tons, the first one in the offshore industry that was ship-shaped.
