Container stack collapse - Overweight and unfit containers

The collapse of a container stack on board a vessel is a significant and far too prevalent problem. According to Gard Services’ statistics for major claims for the five policy years from 1996 to 2000¹ the problem has accounted for the loss of 212 containers overboard. However, statistics do not tell of the grave consequences that can result from container stack collapse – loss of life or injury, damage to the ship, equipment, cargo and the environment. The consequences are likely to be graver where containers stuffed with dangerous goods are involved. Even where the consequences are fortunately minimal, the disruption to vessel operations alone can be very costly.

This photo illustrates the forces involved in a stack collapse.

Overweight and unfit containers
Whilst the causes of container stack collapse can be numerous and often difficult to determine, more recent cases suggest that overweight (actual weight, i.e., container and cargo, exceeds manifested/stowage plan weight) and unfit containers (not structurally sound) are causes that may not be fully appreciated or understood. It is also fair to say that the situation is likely to worsen in the absence of carriers taking preventative measures.²

Recent cases
Two recent cases which Gard Services has been involved with have certain similarities. Both cases involved heavy weather and the collapse of an on-deck container stack in way of the bottom container. In each case, the bottom container was of questionable fitness in terms of structural integrity. However, that was not the only factor. In each case the weight of certain containers within the stack was found to be in excess of the manifested weight. In one case, four containers (forty foot units) in the collapsed stack were found to have 18 MT or more undeclared cargo, which even resulted in the maximum operating gross weight for each container being exceeded.³ It is worth noting that an African load port was involved in all these cases, and although the problem of overweight and unfit containers is a world-wide one, it may well be greater in places where container fleets are generally older and where the enforcement of container related regulations is lacking.

The more obvious risks of stack collapse
Overweight and unfit containers give rise to some fairly obvious risks as far as stack collapse is concerned. For example, overweight containers may result in the overloading of securing systems, fittings or even decks on which they are loaded and an unfit container at the base of the stack may be unable to withstand the compression load from containers on top. A standard ISO container should be design-tested to withstand 192 MT of weight stacked on its corner posts when subject to 1.8 times the force of gravity. An unfit container may only be able to withstand a weight much less than that. In circumstances where containers on top exceed their maximum operating gross weights, as in the case mentioned above, even a sound container may become subject to a weight on top in excess of 192 MT.

Racking and transverse forces – the less obvious risk of container collapse
A standard ISO container is designed to withstand limited forces. One of the most important, but less obvious limitations to be aware of is that of transverse racking force. This is a force applied to the top container fittings (whilst the bottom fittings are assumed to be anchored) and which racks the end structures of the container sideways. For standard ISO containers this is typically 15,000 kilos, which means that the container is design-tested to withstand a racking force of 150 kN.⁴

It can be appreciated that the bottom fittings of a container properly secured to the deck of a vessel will generally have good resistance to the transverse forces acting on those fittings as a result of the vessel’s motion. However, the top fittings will be subject to greater transverse forces, particularly where the container is the bottom one in a stack. According to most classification society rules, it can be assumed that the top fittings of a base container in a stack will be subject to a proportion of the transverse forces acting on each container in the stack. The largest factor in determining the transverse force acting on a given container will be the container weight (with cargo). The vessel’s Cargo Securing Manual (CSM)⁵ will often include guidance to assist in determining transverse forces.

Racking forces will usually be greatest on containers stowed at the bottom of stacks and will be of greatest concern where the resistance to such forces is lowest, for example in stacks where there is no or little resistance to transverse forces from cell guides, lashings and shoring arrangements.

Container stowage plans
A vessel’s container stowage plan will often state limiting weights (container plus cargo) for each stack and sometimes each container position within a stack. These limits should take account of loading constraints on securing systems, fittings, decks and on the containers themselves.

Whilst lashings providing resistance to transverse forces will afford some margin of safety, this margin may well be lost if the container is not structurally sound or if the actual weight (container plus cargo) is in excess of the limiting weight.

The investigation of the UK’s Marine Accident and Investigation Branch into a container collapse incident on board the vessel DUTCH NAVIGATOR is worthy of mention. The limiting weights, as per the stowage plan, for containers within the stack were found to produce a transverse racking force on the base container (which contained dangerous goods) slightly in excess of the 150 kN limit. However, because the actual weights (container plus cargo) were in excess of the limiting weights, the actual racking force on the base container was calculated to be 278 kN. The problem was compounded by a lack of transverse securing and some questionable repairs to the frame of the base container.⁶

Recommendations
1. A careful watch should be kept for containers which may be unfit for carriage. Any container with suspect fitness should not be loaded, but put to one side for closer inspection ashore. If still considered unfit⁷ the container should be rejected for carriage until it has been certified fit by an approved surveyor.
2. The container inspection should include the Container Safety Convention (CSC) plate, which should evidence whether certified inspections are in or out of date.⁸ A classification society sticker does not mean that the container is in fact fit.
3. Container terminals often inspect containers at points of terminal exit/entry to avoid being held responsible for pre-existing container damage. Container lines may be able to make arrangements with terminals they use such that concerns as to container fitness are reported to the line.
4. Vessel staff should pay particular attention to the fitness of containers intended to be stowed at the base of stacks and follow similar steps to those suggested in 1 above, before other containers are loaded on top.
5. Spot checks on container weights are also advisable. Again, arrangements between lines and terminals might be possible. Some terminals will have their own policies. An alternative would be to request the shipper to provide evidence supporting the actual weight, like a weighbridge certificate. It is of course preferable to target checks, especially to containers with high density contents.
6. It would be a worthwhile exercise for owners to check what margin exists between the theoretical forces, based on the limiting weights in the stowage plan/CSM,⁹ and the maximum forces designed to prevent the overloading of containers (particularly the transverse racking force limit of 150kN), securing systems, fittings, decks and containers. If the margin is low, the vessel will be more at risk of stack collapse associated with overweight and unfit containers – unknowingly shipped, despite checks. In these circumstances it would also be worthwhile for owners to discuss with those designing/approving the stowage plan/CSM whether there is a case for reducing limiting weights. A more temporary solution may be to increase securing (to resist transverse forces) or to re-stow containers in less sensitive stacks.
7. A record of offending shippers (providing overweight/unfit containers) could be kept and information could be shared with other container lines calling at the same port.
8. Owners should seek the incorporation of appropriate charterparty provisions which, amongst other things, require charterers to:
a. Have in place procedures for preventing the shipment of overweight and unfit containers.
b. Provide full and accurate details (including gross weights) of goods and containers and a full and accurate stowage plan preferably before the ship’s arrival at the load port.
c. Warrant that all containers carried are constructed to an approved design, are properly maintained and are not loaded beyond their maximum operating gross weight.
d. Ensure that stowage is effected such that limiting weights are not exceeded.

However, it should be borne in mind that, if a stack collapse does occur, it will often be very difficult to determine the cause(s), particularly if evidence, such as an overweight container, is lost overboard. Therefore, far better than relying on charterparty provisions10 is to avoid the problem in the first place.

9. In the event of a container collapse incident Gard Services should be contacted to discuss the appointment of a surveyor to investigate.

This article has been produced with the kind assistance of John J Banister Ltd, Marine Surveyors and Consultants.