Heavy lift vessels can be divided into two types, ie semi-submersible and conventional heavy lift
vessels.
The semi-submersible ships can lower their main cargo deck below the water line and large
floating objects such as oil drilling rigs or damaged ships can be floated on or off the vessel.
The conventional carriers are loaded and discharged by onboard and dock side cranes. The
configuration of the vessels is determined by the nature of the cargo. The typical cargo would be such
items as very large electrical transformers, locomotives, rockets (space exploration vehicles), ore
crushing equipment, gantry cranes, etc. Some conventional carriers have ro-ro capabilities where the
cargo is carried on multi-wheel trailers and driven onboard.
Characteristics of a semi-submersible vessel.
The following characteristics are based on those of the “Blue Marlin” which is typical of the genre:
- Overall length is between 200 and 230 metres.
- Beam is 63 metres.
- Gross tonnage is 52000 tons.
- The Deadweight tonnage is 76000 tons.
- Free deck space is 66 x 178.2 metres.
- One bow thruster is fitted up forward.
- Two retractable azimuth motors are fitted.
- It has a single screw.
- It has four ballast pumps.
- The superstructure is situated forward and contains the bridge, accommodation, workshops, stores and a machinery space.
- At the after end of the cargo deck are two casings. The one on the starboard side is fixed and contains the exhaust and air intakes for the main engine. The casing on the port side is moveable and only contains ballast tanks.
- The engine room is situated at the after end of the vessel.
- Between the casings and the superstructure is the main cargo deck. The space between decks comprises ballast tanks.
In order to load cargo, the ballast tanks are flooded with water so that the cargo deck is sunk below the sea surface. The floating cargo is then “floated” on. And the tanks are pumped dry, lifting the cargo and cargo deck above the sea surface.
Heavy lift vessel Mighty Servant 3 carrying a jack-up rig.