This section picks up important issues of safety that have been experienced with containerized cargo and to the related subject of cargo securing:
There have been considerable incidents over the years of containers being loaded above their stipulated payload. In many cases the manifests declare cargo within payload limits when it is over. This is often exposed by the suspicions of a container handler or even an accident, resulting in a weigh-bridge check.
Overloading is something which shipping lines cannot entertain and in accepting cargo the following should be obtained or checked:
- The number of pieces, size, weight and cubic of any commodity must be obtained.
- The payload and cubic of the container selected / requested must be compared with the cargo
- particulars to establish whether a weight or cubic restriction applies. (A higher rated container may be appropriate).
- Where a weight restriction applies the point loading of the cargo must also be checked in order that the tonnes per square metre loading limit are not breached.
- Cargos such as metal ingots come into this category and timber is often required to spread the weight.
Road and Rail Limits :
Apart from not overloading the container, the gross weight of the container (cargo plus container tare) must not breach the road or rail limits appropriate on all legs of the transit journey. This information can be obtained in DRS records or from the appropriate trade. The importance of observing these limits cannot be overstressed particularly where liability occurs in the case of an accident.
Shifting Cargo :
The incidence of cargo moving inside a container during transit is considerable. It is usually because the cargo has not been secured properly or the packaging is defective. In particular we have recorded several cases where road vehicles have turned over due to cargo moving when negotiating a bend. The key issue is to secure cargo effectively to prevent the initial movement, because once loose the game is lost.
Securing & Reference to the International Maritime Organization (IMO) :
Though containerized cargo is well protected, it is still subject to the constant movement and stress of transport. In heavy seas, the cargo is exposed to compressive forces due to pitching and rolling. These forces may increase the normal strain on lashings, struts and other securing devices as much as 100%. Effective securing of the load throughout the entire transport process is of absolute importance.
Reference books of note for detailed information include:
IMO Code of Safe Practice for Cargo Stowage and Securing. Vessel Cargo Securing Manuals - available onboard all merchant vessels and approved by the vessels Flag State Maritime Authority. Thomas' Stowage (ISBN 0 85174 625 X)
Securing Fundamentals :
Contained in the IMO code above is a simple "Rule of Thumb" to apply to securing loads aboard ship. (Refer to the code for more detailed information).
The total of the MSL values of the securing devices on each side of a unit of cargo (port as well as starboard) should equal the weight of the unit.
MSL or Maximum Securing Load is to securing devices as safe working load (SWL) is to lifting tackle. It is a term used to define the load capacity for a securing device.
Lashing and securing is a matter of know-how and experience and is normally performed either by terminal staff or specialized riggers. Essential information required when considering any piece of sizeable cargo is:
Mass in metric tons Principle dimensions (drawing if possible). Location of centre of gravity. Bedding area and particular bedding precautions, sometimes referred to as the "Footprint".
Lifting points or slinging positions. Lifting gear if any accompanying the cargo, complete with test certificates. Securing arrangements aboard ship. Landside transport arrangements.
When considering the shipboard stowage location of cargo items, acceleration forces should be borne in mind:
Lower accelerations forces occur in the midship sections, lowest cell position underdeck and as close to the centerline as possible.
Higher accelerations forces occur at ship's extremities, high on deck and in the outrigger slots by the ship's side.
When the lashing material is not specified (size of wire etc.) by the forwarder the lashing company will lash the cargo according to their own rules and experiences.
Not all cargo necessarily needs to be lashed. The best way of stowing cargo is often to stow one piece of cargo tight against the other cargo so they can support each other.
Securing Gear :
A basic securing lashing may comprise the pieces listed below and its strength is that of the weakest link.
Wire of suitable strength (MSL). Wire measured in diameter, parts, and strands. Wire / bulldog clips to tie the wire must be fitted correctly. Turnbuckles / bottle-screws to tension the lashing shackles to fasten the ends.
A securing lashing will always break at the weakest point or at the part with the lowest breaking strength so make sure you know the breaking strength of all lashing materials used. Protect the lashing material from additional weakening factors such as sharp edges, bending of bottle screws and shackles.
Where bottle-screws or turnbuckles are introduced the rating of this equipment must be equal to the maximum weight each lashing is expected to bear.
Securing in Containers : Cargo inside the containers needs to be stowed in such a way that the cargo cannot move. The container itself is designed to permit tight, secure stowage of cargo.
These facilities include:
- Floor of wood or plywood which permit blocks, stays and wedges to be anchored with nails or screws.
- Internal walls, for the support of light cargo only.
- Corner posts which are suitable for bracing to with timbers and lashing from via lugs.
- Lashing lugs are located along top and bottom rails of the container at regular intervals.
An example of a typical 20 ft GP's lashing facilities are:
- Sidewalls - Base - 5 lashing points rated at 2000 kgs Safe Working Load (SWL).
- Sidewalls - Top - 5 lashing points rated at 500 kgs SWL.
- Front end - Base - 1 lashing point rated at 500 kgs SWL.
- Rear end - Door area - 5 lashing points rated at 500 kgs SWL.
The walls, doors and roof of the container are merely a protective shell which can not withstand concentrated stress. If the walls or ceiling are used for lashing purposes, make sure the stress on the walls or ceiling is evenly distributed.
Most types of cargo can be secured using the following materials:
- Timber beams, struts, chocks, planks for shoring, bracing and relieving pressure.
- Adjustable wooden battens, rods or strap belts for securing the load in sections, facilitating mechanical discharge.
- Plywood and dunnage to separate several layers of cargo or to segregate different types of cargo into separate sections.
- Foam-rubber cushions and air bags to reduce vibration and prevent the load from shifting.
- Second hand tyres or bags with paper waste or saw dust to fill empty spaces soften the impact and prevent shifting.
- Nets to secure fragile goods.
- Rope (hemp, manila, sisal etc.), wire, steel bands and terylene straps for lashing.
- Nylon span sets.
- Bolt clips into T section flooring in Insulated containers.
- Bulkhead bars.
Securing Calculations Aboard Ship :
As per the previous diagram forces acting in a seaway are:
Rotational: Rolling, Pitching, Sheering / Yawing.
Linear Movements: Swaying, Surging, Heaving.
Calculations to secure against the above forces follow a prescribed discipline and it is important to refer to this procedure when performing calculations. The recognized calculations are contained in "IMO Code of Safe Practice for Cargo Stowage and Securing".