Crane vessel

A crane vessel, crane ship, crane barge, or floating crane is a ship with a crane specialized in lifting heavy loads, typically exceeding 1,500 t (1,476 long tons; 1,653 short tons) for modern ships. The largest crane vessels are used for offshore construction.

The cranes are fitted to conventional monohulls and barges, but the largest crane vessels are often catamaran or semi-submersible types which provide enhanced stability and reduced platform motion. Many crane vessels are fitted with one or more rotating cranes. Some of the largest crane vessels use fixed sheerlegs instead; in these designs, the crane cannot rotate relative to the ship, and the vessel must be manoeuvered to place loads. Other vessels use large gantry cranes and straddle the load.

There are several major configurations of crane vessel, usually with overlapping ranges of functionality, but each has at least one major advantage over the others in some circumstances, and consequently all these arrangements coexist.

Conventional seagoing self propelled monohulls with heavy lift crane equipment.

A Sheer-legs barge is a barge with sheer-legs mounted at one end, which can lift loads and luff the sheer-legs to adjust the reach, but cannot swing the load independently of the hull orientation. A typical arrangement has a substantial A-frame hinged at the stern, supported by stays to the bow. When the load has been lifted, the barge is maneuvered to the position where the load is to be lowered by onboard thrusters or tugs, and the load is lowered. A sheer-legs barge always keeps the load in the line of maximum static stability, and may use ballasting at the bow to increase longitudinal righting moment to compensate for the load. 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. It is usual to luff the sheer-legs before lifting to a position suitable for both lifting and setting the load, as luffing under load is generally slow, and there is seldom any need.

A heavy lift hammerhead crane barge has a fixed hammerhead crane, which neither slews nor luffs, but has a constant reach. They are operated in a similar way to sheer-legs barges. This arrangement may be mounted on a catamaran barge which allows it to straddle a bridge pier to lower a prefabricated section into place.

Catamaran heavy lift barges that consist of two barges connected by gantry cranes across the top have been used in sheltered water like harbours and rivers. 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 gantry trusses are usually fitted at both ends of the barges, allowing lifts of long loads.

Semi-submersible crane platforms have advantages where the water is too deep or the bottom composition unsuitable for a jack-up, and the water conditions are frequently too rough for efficient use of conventional hulls. 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. The gaps between the columns also allow waves to pass between then with little impact on the vessel. Disadvantages are lower inherent stability, and much greater cost and complexity.