A floating crane is type of sea vessel that has a crane mounted on it. The earliest floating crane designs were no more than old ships converted to include a large crane mounted on the deck. Later, purpose built, catamaran and semi-submersible designs replaced these converted mono-hulls due mainly to increase lifting capacity and improved stability.
There are many uses for a floating crane. Offshore construction is the primary duty for a floating crane, which has proved extremely important in the drilling industry. With these vessels' ability to lift and maneuver extremely heavy and large sub-assemblies into position, floating cranes make it possible to assemble massive projects from many smaller assemblies in most weather conditions.
Beyond building platforms for drilling, projects such as retrieving sunken ships often involve the use of a floating crane. The operators are able to bring a retrieval vessel alongside of the sunken ship and raise the sunken ship to the surface by working in unison with each other. These types of missions are often made increasingly difficult by rough sea conditions. It requires an experienced operator and crew to successfully complete a lift in these conditions.
The semi-submersible floating crane allows projects in rougher waters to be completed with less danger to the project and crew. This type of vessel arrives on the work site and then pumps its on-board ballast tanks full of sea water. This allows the structure to essentially sink into the ocean, giving it much more stability and control as the seas get rougher. This also permits the crane to lift heavy items without having to raise them as high into the air.
The advent of the heavy lifting cranes also allows contractors to assemble an off-shore oil rig in a matter weeks. The same job would have taken months to construct without the large crane vessels. During the off-shore building boom of the 1970s, there were several companies operating these behemoth cranes around the globe. However, in the mid-1980s the offshore boom ended and many companies were forced to merge to avoid closing, meaning fewer floating cranes in the ocean.
Semi-submersible crane vessels remain relevant in the Gulf of Mexico, North Sea and pretty much any body of water that sees major construction projects, be them reclamation jobs or new drilling platforms. These floating cranes created on a semi-submersible platform are able to withstand much rougher seas and lift a greater amount of weight than previous designs.