Deck cranes for efficient cargo handling on board cargo ships

Deck cranes for efficient cargo handling on board cargo ships

Cargo cranes – Shipboard cranes of various types and capacities are still required for multi-purpose cargo vessels, geared bulk carriers, feeders, reefers, heavy lift vessels and some forest product carriers. Manufactures offer crane designs and special handling attachments (container spreaders, rotators and grabs) to suit all dry cargo trades.

Computer-based cargo spotting systems enable even relatively unskilled operators to cope with the pendulum effects and centrifugal forces. They also help in keeping containers or other cargo units constantly aligned with a given axis, regardless of slewing motion and other external forces. Such electronic aids substantially improve productivity. Some owners report a doubling of the hourly container-handling rate. Other benefits include: reduced operator fatigue, improved safety and lower cargo and ship structure damage.

Cranes have replaced derricks on many modern ships. Generally they are considered as an alternative to the union purchase rig. Deck cranes have a number of advantages, the rigging time being negligible, and the crane is able to pick up and land permitted loads anywhere within its working radius. The safe working loads of cranes is generally of the order of 10 to 15 tonnes and larger cranes are available capable of lifts from 30 to 40 tonnes. As with the union purchase rig the crane is intended for rapid cargo loading and discharging duties with loads which only occasionally exceed, say, 3 tonnes. There is some controversy regarding the merits of cranes as opposed to the union purchase rig, but evidence is available to show that the crane is perhaps less efficient with very light loads.

Cranes may often be positioned on the ship’s centre line, but this may require an extremely long jib when the ship’s beam is large and a reasonable outreach is desired. Transverse positional cranes may then be fitted which, when not under load, can be moved port or starboard and secured to work the hatch and give the desired outreach. Alternatively fixed cranes, one at each end of the hatch, may be placed at opposite corners. This is an arrangement which is useful in discharging to port and starboard simultaneously. There is also a crane which is mounted on a hatch cover section capable of travelling under load along the hatch coaming in the longitudinal direction.

Deck cranes are available from specialist manufacturers and the shipbuilder would be responsible for installation, any local strengthening, and seatings. They are normally positioned between the holds, often on a platform which can be rotated through 360°, provides an immediately operational unit requiring only one man to operate it. Double gearing is a feature of most designs, providing a higher speed at lighter loads. Various types of crane exist for particular duties, for example a general duties crane using a hook and a grabbing crane for use with bulk cargoes.

Fig: General Cargo Ships deck crane

Three separate drives provide the principal movements: a hoisting motor for lifting the load, a luffing motor for raising or lowering the jib, and a slewing motor for rotating the crane. The operator's cab is designed to provide clear views of all the cargo working area so that the crane operator can function alone. The crane is usually mounted on a pedestal to offer adequate visibility to the operator. For occasional heavy loads arrangements for two cranes to work together, i.e. twinning, can be made with a single operator using a master and slave control system in the two cranes. A common revolving platform will be necessary for this arrangement. The operating medium for deck crane motors may be hydraulic or electric, utilising circuits referred to earlier.

Winches and cranes operation

Winches and cranes fitted for lifting cargo on board is required to be of adequate strength and stability for each load, having regard in particular to the stress induced at its mounting or fixing points , securely anchored, adequately ballasted or counterbalanced and supported by outriggers as necessary to ensure its stability when lifting.

Fig:General cargo ship BBC-Kusan mounted with deck cranes

The drum end of wire runners or falls should be secured to winch barrels or crane drums by proper clamps or U-bolts. The runner or fall should be long enough to leave at least three turns on the barrel or drum at maximum normal extension. Slack turns of wire or rope on a barrel or drum should be avoided as they are likely to pull out suddenly under load.

When a winch is changed from single to double gear or vice versa, any load should first be released and the clutch should be secured so that it cannot become disengaged when the winch is working.

Steam winches should be so maintained that the operator is not exposed to the risk of scalding by leaks of hot water and steam.

Before a steam winch is operated, the cylinders and steam pipes should be cleared of water by opening the appropriate drain cocks. The stop valve between winch and deck steam line should be kept unobstructed. Adequate measures should be taken to prevent steam obscuring the driver's vision in any part of a working area.

Ships' cranes should be properly operated and maintained in accordance with manufacturers' instructions. Companies, employers and masters, as appropriate, should ensure that sufficient technical information is available including the following information:-

(i) Length, size and safe working load of falls and topping lifts.
(ii) Safe working load of all fittings;
(iii) Boom limiting angles;
(iv) Manufacturers' instructions for replacing wires, topping up hydraulics and other maintenance as appropriate.

Power operated rail mounted cranes should have the following facilities incorporated in their control systems:-
(a) facilities to prevent unauthorised startup;
(b) an efficient braking mechanism which will arrest the motion along the rails, and where safety constraints require, emergency facilities operated by readily accessible controls or automatic systems should be available for braking or stopping equipment in the event of failure of the main facility;
(c) guards which reduce as far as possible the risk of the wheels running over persons' feet, and which will remove loose materials from the rails.

When a travelling crane is moved, any necessary holding bolts or clamps should be replaced before operations are resumed.
Access to a crane should be always by the proper means provided. Cranes should be stationary while accessing.

Deck crane maintenance

All deck machinery is exposed to the most severe aspects of the elements. Total enclosure of all working parts is usual with splash lubrication for gearing. The various bearings on the shafts will be greased by pressure grease points. Open gears and clutches are lubricated with open gear compound. Particular maintenance tasks will be associated with the type of motor drive employed.

On some vessels, it was reported that the ship's staff had carried out unauthorised repairs to crane jibs by cropping and welding inserts over damaged or wasted sections. Crane jibs are subject to heavy, fluctuating loads and must be periodically inspected, surveyed, load-tested and certified. They are often made of high-tensile, for which special procedures have to be observed during repairs. Therefore, repairs must be carried out only in consultation with the manufacturer and classification society concerned. Any damage noticed to crane jibs must be reported to the ship owner/manager and advice sought before carrying out any kind of repair.

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