Unitised cargo handling technics - general cargo ship procedure

Unitised cargo handling technics - general cargo ship procedure

A grouping together of two or more items (usually of a homogeneous nature) and securing them with banding, glue, shrinkwrap, slings (e.g. clover leaf), to form a unit which, together with a base (skids, pallets, etc.) or permanent slings, allows mechanical handling equipment (e.g. tynes of a fork lift truck) to lift and transport the unit.

The advantages of unitisation may include:

Ease of tallying.
Reduced breakages.
Reduced pilferage.
Faster speed of working between ship and shore (and on ship or shore).
May make more effective use of vertical storage space in sheds and holds by stacking units 4, 5 or 6 high (possibly with the need to incorporate shelves or racking). Reduced labour requirements when handling between interfaces.

The disadvantages of unitisation may include:

Loss of space below decks where the shape of the vessel is not compatible with the shape of the unit.
Loss of space caused by the shape of the package being unitised, e.g. drums on pallets.
Collapsed or crushed units require labour intensive efforts to rectify, handle and store.
An element of extra cost involved in the pallet; slings; skids; shrink-wrap; strapping, etc.

Pallet types

Pallet Sizes

The overall height of a pallet may be from 100mm (4) to 150mm (6), depending on the construction (single-deck, double-deck, etc.) and the use to which it will be put (single trip, pallet racking, etc.). The deck (or plan) size depends on a number of factors:

Size of cargo to be palletised.
Type of cargo to be palletised (e.g. drums, bags, etc.).
The use to which it will be put: e.g. closed circuit pool, one trip only, inland/ocean transport requirements, etc.

ISO have set some standard sizes:

1,000mm×800mm (40×32)
1,200mm×800mm (48×32)
1,200mm×1,000mm (48×40)
1,200mm×1,600mm (48×64)
1,200mm×1,800mm (48×72)

Slip Pads

Sometimes known as slip sheets. These may be made up of a single sheet of fibreboard or plastic, of the same plan dimensions as a pallet, but with an exposed "lip" which allows the special fitting on a fork lift truck to grip and pull the slip sheet onto a flat "spade" attachment. Thus the unitmade up on the slip sheetis handled in all respects as a pallet, except for the special FLT attachment necessary.

The advantages of this system include:

Saved space; as against pallets or skids. Sometimes particularly important, e.g. low `tween decks, containers, etc. Pads may be less expensive than pallets.
Less storage area required when not being used.

The disadvantages include:

Pads may be easily damaged; if the "lip" is ripped off then the mechanical handling equipment has nothing to grip.
Requires a flat surface without obstructions on which to operate.
Requires special equipment to operate and handle. May (usually) only be approached and handled from one side; i.e. the side on which the lip protrudes.

Making Up the Unit

The cargo should, as nearly as possible, exactly fit the pallet or slip sheet. Cargo overhanging the deck of a pallet may cause the following problems:

Put excessive pressure on the lowest tiers of cargo, particularly that which is in contact with the edge of the pallet. The g forces generated during transport can force the edge of the pallet deck to penetrate the cargoparticularly bags and cartonswith resultant damage to the contents.
Make the securing of the load to the pallet difficult or even impossible.
Risk the pallet load fouling adjacent cargo or units, with the load becoming wedged or jammed in the stow.

Similarly a unit module which is too small for the deck of the pallet will make the securing of the cargo to that pallet less effective, and reduce the ability of ship's bulkheads, container walls, or other cargo to support the pallet loads laterally in the stow. The height of a pallet load should be such that the pallet and cargo combined leaves a clearance of at least:

50mm (2) between top of cargo and door header (or ceiling) of a container.
150mm (6) between the top of the uppermost pallet and the lowest deck-head obstruction in a `tween deck or hold.

Method of Securing

The cargo should be secured onto the pallet by one or more of the following methods:

Strapping: Man-made fibre or steel. Banding should pass under the top deck of the pallet, to remain clear of mechanical handling equipment requirements. Some cargo may require protection from the point loading of the strapping, which may be achieved with scrap dunnage, waste cardboard, etc., or using proprietary equipment specially designed for the purpose. Steel strapping may not be suitable for securing cargo that may alter shape or contract, e.g. bags, telescopic cartons etc. Man-made fibres, particularly nylon, may stretch and allow rigid items to move in some instances.

Nets: Usually of polypropylene, and used to secure awkward shaped packages onto a pallet.

Glue: For securing regular stows of cartons or bags. Should be strong in sheer strength, but with little resistance to a vertical force when dismantling the pallet load. A 5% dextrine solution can be a suitable mixture for this purpose.

Shrinkwrap: A plastic (transparent) cover over the pallet load which is heat shrunk into place. Certain types of plastic wrapping may be put around the cargo without the need of heat. May be used for most goods and provide protection against dirt, pilferage, etc.

Locking Pattern: Unit loads may be built up on pallets with plan dimensions suited to the modules being handled. In this way layers may be built up each differing from its neighbours and providing restraint in a similar manner to a brick wall. Alternate layer patterns are reversed to "tie in" the layer beneath.

The Effect of the Pallet

The pallet, because it raises the cargo approximately 150mm (6) off the deck of the hold, has the effect of raising the centre of gravity of the cargo which may, in certain circumstances, have to be allowed for.

Pallets use more space than conventional dunnage, perhaps 10 to 14%. Some overhead space may be lost if the height of the pallet load does not form a module of the height between decks or in the holds of the shipor the internal height of the container. If the pallets are being made up on the quay this height may be adjusted (in some instances) to suit the particular height requirements of the ship. `Tween decks may be designed for "user friendly" palletised cargoes.

While pallet loads may be made up in such a way that the units are suitable for carriage under refrigeration, the following will need to be taken into consideration:

The air-flow through the cargo (if required) or around the cargo is not impeded by securing materials or the positions of the pallet loads.
If two-way entry pallets are being used, then the pallets must be so positioned that the correct air-flow is not blocked by the supporting members.
The deck of the pallet may have to be slatted or perforated to allow free movement of controlled temperature air. This is particularly important for cargoes (e.g. fruit) in packaging designed to allow vertical through-movement of air.

Handling and Stowage

A flat unobstructed area, e.g. deck space, is best suited for positioning and stowing pallets. They should never be dragged into nor out of the stowsuch treatment will collapse the pallet (particularly one-trip pallets) and make subsequent handling and storage difficult if not impossible.

Vessels which have not been built with pallet handling in mind, e.g. without flush `tween deck hatch coamings, etc., may need extensive preparation work carried out before loading commences, e.g.:

Temporary wooden ramps to surmount hatch coamings.
The removal of deck obstructions, e.g. ring-bolts, stanchions, etc.
The bridging and squaring off of rounded bilges, excessive shear, etc.
Temporary strengthening of hatch covers, tank tops, `tween decks, etc., with metal plates to allow for axle loading of fork lift trucks.
Point loadings may be very high when a fork lift truck is under load, and considerations should
be given to the Classification Societies' point loading figures (usually expressed in tonnes per square metre).

The stowage of the ship has to be planned with the strength and size of the units carefully considered. Two-way entry pallets and skidded units should be stowed so that the truck always has an entry to the cargo available. This is particularly important where the vessel is discharging at more than one port, and is often referred to as the "flowline" of the pallets. This "flowline" should be marked on the cargo plan with arrows.

If a solid stow is obtained immediately beneath the hatch square it is necessary to have key pallets clearly marked on the stowage plan to indicate which have to be lifted out first to allow access to the remainder of the stow and introduce the appropriate mechanical handling equipment. These pallets may with advantage, under most circumstances, be left with slings in place at time of loading to aid the process of discharging.

Any dunnage used with palletised (or other unit) loads should be limited, and lengths kept short to prevent obstructing or fouling mechanical equipment, cargo in the stow, and handling operations. Sometimes it is necessary to place sheets of plywood, chipboard or fibreboard on edge in the stow to prevent the cargo on one pallet "locking" with the next and so obstructing the unloading operation. It is important when operating below decks with mechanical handling equipment that the decks are free of obstructions: loose dunnage, rubbish, etc.

Making up Dockside Pallets

Where pallet loads are made up on the dockside for unit loading aboard ship, care must be taken that bills of lading are not mixed on the pallet. Labelling of the pallet load should be carried out on at least two sides. The mixed loading (or discharging) of breakbulk cargo and unitised cargo will completely negate the purpose of mechanised working, i.e. the fast handling of cargo with mechanical equipment and reduced numbers in each gang. It is also more likely to result in cargo becoming jammed and damaged.

Cargo into Units

When cargo is not suitable, because of size, shape or other reason, to be palletised, it may sometimes be made up into a unit load suitable for handling with mechanical equipment. This could be achieved in a number of ways, e.g.: Securing skids or bearers to the bottom of a suitably sized package or case. This would require at least two such "skids" spaced to allow fork lift tynes entry. The minimum depth of the skids should be 50mm (2), but more suitable 65mm (2½). Making up the packages that form the unit in such a way that space is left between the packages themselves for fork lift tynes to gain access. Typical commodities that are suitable for this treatment are ingots, bricks, etc.

Pre-Slinging Techniques

Pre-slinging is normally used to facilitate the speed of cargo loading and/or discharging. Also to open up the stow in way of the hatch square (see over).

It necessitates the positioning of sling loads of cargo in the stow at time of loading, leaving each unit with the sling still encompassing the load so that the port of discharge has only to hook on and lift out.

With open hatch ships and twin hatches, the advantages to be gained by using pre-slinging techniques can be great. Traditional hatch arrangements with deep wings are not so suitable since the sling load cannot be landed directly from the hook to its stowed position.

A certain amount of space is lost (perhaps 10 or even 15%), but this is usually more than made up for in the speed of cargo handling obtained. The space lost is less than that for palletised or skidded cargo, and the cost of slings is probably competitive with the cost of pallets.

Materials

Any normal sling material may be used for a pre-slinging operation:

Chain slings.
Fibre rope.
Steel wire rope.
Flat synthetic fibre.

Also, under certain circumstances, nets, baskets or trays. It is common for the following types of commodities to have the following types of sling:

Bagged Goods

Clover leaf slings, of natural or man-made fibre ropes and strops.

Reels of Paper and Newsprint

Flat synthetic strops, or endless slings often used in pairs. Sawn Timber

Wire or flat synthetic choker strops. N.B. Bearers and dunnage must be used between units when using flat synthetic slings to avoid severe chafing damage in bad weather.

Cartons and Cased Goods, Bales and Pulp

Flat synthetic slingsalso rope or wire slings.

Steel Products

Various, depending on weight, unit shape, and number of potential lifting points. Round steel piping is often pre-slung using two flat choker strops in a bridle. Steel plate, on the other hand, will usually require wire or chain slings.

Handling

When loading it must be ensured that the sling eyes are left in an accessible position on top of the load ready for immediate hooking on at time of discharging.

When discharging care must be taken that eyes from two adjacent units are not hooked on together in the belief that they are from the same unit.

It is important that during handling operations a pre-slung load remains intact and does not become broken up due to careless handling. If pre-slung loads do break up then there is immediately a labour intensive operation required to move that cargo, and much of the advantage gained from pre-slinging is lost due to higher costs in labour and time.

It is important that a series of pre-slung units be made up of the same number of bags, cartons or whatever, so that tallying is made easier. Bills of lading should not be mixed in one pre-slung load.

The drift of the sling, when handling fragile items such as cartoned goods and small crates, etc., should be sufficient to prevent pinching and crushing of the sling loads.

Slings should be of adequate strength and colour codes checked so that safe working loads and limitations may be observed.

Small bags, e.g. of salt, may be made up into loads contained within large specially constructed bags. This pre-slinging technique is practised widely in the West African trade. It should not be confused with mini-bulk (but see also that section).

Summarized below some more details on general cargo ship cargo handling procedure and operational info:

Cargo handling procedure for general cargo ship

Unitised cargo handling technics - general cargo ship procedure

Machinery system main info pages