Heavy-Lift Ship

Overview

A heavy-lift ship is a cargo vessel built to carry its own loading equipment so it can pick up indivisible pieces that no shore crane at the destination could handle. Typical cargo includes power-plant transformers, dock and harbour cranes, refinery columns, offshore jackets and modules, rail locomotives, large yachts and pre-assembled bridge spans. The ship is box-shaped with a long flush open deck, a removable tween deck and two or more high-capacity mast cranes that can work singly or paired in tandem.

The structural core is the Strengthened Hull, a strengthened double-bottom box whose side tanks hold the ballast that keeps the ship upright under crane loads. Above it the Cargo Deck & Tween Deck carries cargo on the weather deck and on a strengthened Tween-Deck Panel arrangement that can be lifted out to make a tall single hold. Each Heavy-Lift Mast Crane stands on a Crane Foundation built into the hull. The Ballast & Anti-Heel System counters heeling, the Propulsion Train train drives the ship, the Electrical Power Plant feeds the crane motors, and the crew live and navigate from the Deckhouse.

The hull and cargo deck

The hull is wide and shallow with a large flat deck, which gives the deck area and the stability margin needed to set down a single piece weighing more than a thousand tonnes. The bottom is strengthened so the ship can take the ground on a falling tide for ro-ro or float-on loading, and the deck is built up over closely spaced web frames so concentrated loads do not dent the plating. Where a heavy piece sits, the crew build up a Load Grillage of beams and mats to spread its point load into the deck framing. Cargo is tied down through hundreds of flush Deck Lashing Pot fittings, and tall pieces are braced with stanchions dropped into deck sockets. The Hatch Cover panels close the holds and can themselves carry deck cargo when sealed.

The mast cranes

Each Heavy-Lift Mast Crane is a fixed-pedestal revolving crane. The load is hung from a Hook Block reeved with many parts of Hoist Wire Rope running over Rope Sheave wheels at the head of the Crane Boom. The whole crane rotates on a Slewing Ring Bearing, a large toothed ring driven by slew pinions, mounted on the Crane Pedestal. Hoisting, luffing the boom and slewing are all done by winches in the Crane Winch Pack, each held by a spring-applied Winch Brake so the load cannot run away if power is lost. The operator works from the Crane Operator Cabin, watching a moment limiter that compares the hook reading from the Load Cell against the rated capacity for the current radius. When a piece is heavier than one crane can lift, the two cranes are slung to a common spreader and lifted in tandem, which roughly doubles the capacity but demands tight coordination of hoist speeds.

Stability and the ballast system

The hardest part of a heavy lift afloat is not raising the weight but keeping the ship level while the load swings outboard. As a crane luffs a load over the side, the combined centre of gravity moves toward that side and the ship begins to heel, which moves the load further out and threatens a runaway. The Ballast & Anti-Heel System answers this with high-flow Anti-Heel Pump units that shift water between port and starboard tanks faster than the heel can build, plus the main Ballast Pump set that adjusts overall draught and trim. Remotely operated Ballast Valve fittings and many Pressure Sensor level transmitters let one operator manage the whole tank system from a console during the lift.

Propulsion and power

The ship is driven by the Propulsion Train train: a medium-speed Main Engine turning a Controllable-Pitch Propeller through a reduction gearbox, with a Bow Thruster in the bow for berthing and station-keeping. A controllable-pitch propeller is used because it gives full thrust control at constant shaft speed, which matters when the ship must hold position alongside while loading. Electrical power for the crane drives comes from the Electrical Power Plant, where several Auxiliary Genset sets feed a Main Switchboard and then a bank of Crane VFD Cabinet drives that control the hoist and slew motors. The drives are sized for the surge of power a heavy hoist draws, separate from the steady house load.

Materials and construction

The hull is welded from grade A and AH36 shipbuilding plate, with thickened strakes in the bottom and deck and crack-tolerant steel around the crane foundations. The cranes use high-tensile structural steel in the boom and forged alloy steel for the Boom Pin joints and the hook. Wire ropes are high-strength steel reeved many parts to bring the line pull within winch limits. The propeller blades are nickel-aluminium-bronze for seawater corrosion resistance.

Variants and use

Heavy-lift ships range from open-deck multipurpose carriers with two 250 t cranes up to dedicated project carriers with paired cranes over 1,000 t and tandem capacity beyond 3,000 t. A related class, the semi-submersible heavy-transport ship, dispenses with cranes entirely and instead ballasts down so its deck submerges, letting an offshore rig or another ship float on, then deballasts to lift it clear. The crane-fitted type covered here is the workhorse of the project-cargo trade, moving the single largest pieces of industrial plant between fabrication yards and installation sites worldwide.