Developments in Shore to Ship Container Cranes

10th February 1988

There are three types of crane for handling containers for sea cargo.
1. Ship mcunted, travelling the full length of the ship.
2. Quayside for shore/ship movements.
3. Ashore for stacking and transferring to or from road or rail vehicles.

The first is not now used because of expense, taking too much room, and many more international ports have quays with container cranes.

The second type is the subject of this lecture.
A large ship carries some 2,000 containers, and speed of loading is vital. Their holds are divided into cells by vertical angles into which containers are lowered. A 25 ton container is 30 feet long, and a 35 ton one, 40 ft. long. Nothing larger than this can be stowed. Direct stowage into the cells needs no further securing of the cargo.

Cranes need to be some 100 feet high to allow deck loading of additional containers up to 3 deep above hatches. They have a reach also of seme 100 feet to reach the far side of a berthed ship.

The legs and horizontal memoers of the main structure are usually box section, about 5 feet wide by 3 feet. This is needed to cater for the bending moments of the load and the accelerating forces to move along the dock and traversing across the ship.

Usually each sea-side corner has eight wheels to withstand a loading up to 300 tons at full reach. Land side, four or six wheels per corner are adequate. Only two boggies on each corner are driven, using an electric motor for each of some 15 h.p.

A storm anchor – a bolt that can be dropped into sockets on the quay side – is provided to prevent the crane from running away in storm conditions.

The hoist is from a trolley running on rails along the boom of the crane. The trolley carries the driver’s cab and pulleys, but the drive is from machinery in the main structure for lifting and traversing the trolley. This minimises the weight on the boom and so cuts down the overturning moment when at full stretch across the ship. Differential hauling of the multiple rope lift to adjust the load for insertion and withdrdawal from the cell is provided. This also provides for possible listing of the ship. The boom may be box section or open web girders.

A load factor of 7.5 is usually specified for heavy duty crane ropes for direct lift. However for these cranes also an allowance of 10% eccentricity is called for as the container may not have a truely vertical access to its cell. The multiple ropes may therefore not share the load equally, and the effective load factor could be as low as 4. In practise 10% eccentricity is rarely, if ever, required, and there is doubt about its origin; so that the usual load factor determination remains adequate.

A quick means of connecting the hoist to the containers is needed. The top of each container has steel corner castings, 6″ square by 4.5″ deep into which the rectangular spreader frame suspended from the crane is hooked. A T bolt at each spreader corner drops into the castings and is twisted by a hydraulic ram to lock into it. A vertical steel rod in each corner is pushed up as the spreader lands as an interlock to prevent the rams operating until the landing is complete. The crane hoist is also inhibited until all four corners are hooked.

The larger frames carry a hydraulic pump, driven by a 7.5 h.p. electric motor for all the operations of attachment and detachment, the power supply cable coming from the crane hoist system. They may be a fixed size for each size of container, but more often now they are “telescopic” to adjust for “universal” use. The box sections slide one within another, and are adjusted hydraulically as required. Those more complex frames are much heavier, and are made of high tensile steel to limit the weight. They weigh up to 12 tons compared to about 4 tons for a normal “straight” 40 ft. spreader. As a consequence the crane has to be made stronger.

To ensure accurate lowering of a spreader onto a container corner flaps are fitted to the spreader corners to guide it. Once located a hydraulic ram turns the spreaders 180° to enable the container to slot into its cel1.

Electrical control of the various traversing and hoisting motors is usually by the Ward Leonard system for smooth control. Two motor generators; one for hoist providing about 300 or 400 h.p. and one for travel and traverse of some 100 h.p., from a 3.3 kV supply.

The lecture was very much enhanced by clear drawings projected for us, all specially prepared for this lecture. Their non inclusion in this report is greatly regretted.