A crane vessel is a ship that is specialized in lifting heavy loads. The largest crane vessels are often used for offshore construction. The larger vessels are often semi-submersibles, but also conventional monohulls are used. One of the differences with a sheerleg is that the cranes can rotate.
In medieval Europe, crane vessel which could be flexibly deployed in the whole port basin were introduced as early as the 14th century. In 1920 the 1898 build battle ship USS Kearsarge (BB-5) was converted to crane ship. A crane with a capacity of 250 tons was placed. Later it was renamed Crane Ship No. 1. It was used amongst other things to place guns and other heavy lifts on battle ships under construction. Another remarkable feat was the raising of the USS Squalus (SS-192) in 1939.
In 1949 J. Ray McDermott had the Derrick Barge Four build, a barge that was outfitted with a 150 tons revolving crane. The appearance of this type of vessel changed the offshore construction. In stead of constructing oil platforms in parts, jackets and decks can be build onshore as modules. For use in the shallow part of the Gulf of Mexico, the cradle of the offshore industry, these barges sufficed.
In 1963 Heerema converted a Norwegian tanker, the Sunnaas, to a crane vessel with a capacity of 300 tons, the first one in the offshore industry that was ship shaped. It was renamed Global Adventurer. This type of crane vessel was better adapted to the harsh environment on the North sea.
In 1978 Heerema had two semi-submersible crane vessels build, the Hermod and the Balder, each with one 2000 short ton and one 3000 shortton crane. Later both were upgraded to a higher capacity. This type of crane vessel was much less sensitive for sea and swell, so that it was possible to operate on the Northsea during the winter months. The high stability also allows that heavier lifts can be lifted then with a monohull. The larger capacity of the cranes reduced the installation time of a platform from a whole season to a few weeks. Inspired by this success similar vessels were build. In 1985 the DB-102 was launched for McDermott, with two cranes with a capacity of 6000 tons each. Micoperi had the M7000 build in 1986 with two cranes of 7000 ton each.
However, in the mid 80s the boom in the offshore is over, resulting in collaborations. In 1988 a joint venture between Heerema and McDermott is formed, HeereMac. In 1990 Micoperi has to apply for bankruptcy. This enables Saipem - in the beginning of the 70s a large heavy lift contractor, but only a small player in this field at the end of the 80s - to take over the M7000 in 1995, renaming it Saipem 7000 later. In 1997 Heerema takes over the DB-102 from McDermott after discontinuation of their joint venture. The ship is renamed Thialf and after an un upgrade in 2000 to twice 7100 tons it is the largest crane vessel in the world.