When large, heavy items need to be transported, a specialised heavylift ship can be used. These items could be a large oil rig, platforms used in the offshore oil industry, large pieces of machinery, cranes, and sometimes, even some small ships need to be transported.

For floating objects (e.g. oil rigs, floating platforms), the specialised heavylift vessel will be semi-submerged through flooding special ballast tanks in the heavylift ship. The object which is the cargo (e.g. an oil rig or platform) will be floated onto the ship, water will be pumped from the special ballast tanks, and the ship will revert to her normal draught. In the process, the cargo deck rises above the water, and the cargo (the rig or platform) is lifted clear of the water. Once the cargo has been secured properly – often by welding it to the deck of the heavylift ship – the ship can then proceed to its destination.

At the destination, the reverse procedure is followed : securing welds or other securing measures are removed, the heavylift ship’s special tanks are flooded to semi-submerge her. The cargo deck is fully submerged and the cargo (the rig or platform) is then floated off. The water is pumped from the ballast tanks to restore normal buoyancy in the heavylift ship.

Heavylift Ships Slide 1 of 4

The heavylift ship Blue Marlin with two floating platforms as her cargo. These are circled in orange (A;B). To judge the size of these platforms, look at the size of the men (C) on the deck. To load the platforms, specials tanks in the ship wil have been flooded until the ship is semi-submerged to a level when the ship is down in the water to where the orange part meets the white part. The platforms are then floated onto the ship, and the water is pumped out so that the ship is at her normal level in the water (as shown in the photograph.) The platforms are then secured to the deck. Once that has happened, the ship then goes to the place where the platforms are to be offloaded. The securing devices are removed. The tanks are flooded until the ship is down in the water to where the orange meets the white. The platforms are floated off the ship, the water is pumped out of the ship’s tanks and she resumes her normal draught.

Heavylift Ships Slide 2 of 4

The Chinese heavylift ship Zhen Hua 20 carrying a cargo of four large red container gantry cranes and four rubber-tyred mobile gantries for operation in a container terminal. To prevent the ship from capsizing, she is equipped with special tanks for ballast water. Photograph: Aad Norland

Heavylift Ships Slide 3 of 4

The Deepwater Construction Vessel (DCV) Balder Balder has two large cranes that together can lift 6300 tons. She is designed to install offshore oil facilities such as “topsides” or even the foundations and pipes for offshore oil operations. Source of photograph unknown.

Heavylift Ships Slide 4 of 4

BBC Nordland can carry smaller, but heavy items. Her cranes can lift these heavy items that are stowed either on deck or in the holds. Photograph: Aad Norland