200885 Safe carriage of containers

Source: North of England P&I Club Signals issue 70

Editor's note: With the approaching northern winter, this report is a timely reminder of likely causes of container damage and stow collapses and for suitable precautions to be taken.

Some recent incidents of container losses and collapsed stows during heavy weather appear to have four principal factors as causes.

1. Lashing equipment

Investigations into a number of incidents indicated an apparently common feature of losses from, or collapsed stows on, large containerships fitted with fully automatic twistlocks of the latest design and manufacture.

Several advisories have been issued to operators, urging them to take note of these developments. They should contact their lashing equipment manufacturer and classification societies for advice and take appropriate action to reduce the risk of further incidents. Suggested actions include considering temporary reductions in container stack heights, revised weather routeing and replacement of suspect lashing equipment.

2. Cargo securing manual

The explanation sometimes offered after an incident is that 'the lashings broke'. However, this is unlikely to be the principal cause if the containers have been stowed and secured in accordance with the ship's cargo securing manual. If stowage, in terms of permitted stack weights and individual tier weights, is in accordance with the manual; if securing is carried out in accordance with the manual, using only the types of equipment specified; and if the ship's metacentric height (GM) is within the limits specified in the manual - then it is highly unlikely that the lashings will break in any reasonable circumstances, including heavy weather navigation.

What probably causes lashings to break are heavy containers stowed over lighter ones that exceed the individual tier position limits and/or the introduction of high-cube containers into a stack of containers, contrary to the cargo securing manual. This may raise the centre of gravity of the stack and the latter may also increase the securing angle of the long and short lashing beyond the designed angle of maximum effectiveness.

Consider a situation where an individual stack has a serious heavy-over-light mistake, including a high-cube (9'6Åh) container in a lower tier, but where the stack weight has not been exceeded. The ship's planning computer may default to stack weights and there will thus be no warning alarms. However, an experienced chief officer or master would also look at the 'lashing forces' function, where the errors would become immediately obvious. On the stack weights screen or the bay plan, the only clue indicating the presence of a high-cube container may be the letters HC (high-cube) instead of perhaps DC (dry container 8'6Åh).

Companies and mariners should thus check whether their ships' planning software includes a facility to check the effect of stowing of high-cube containers.

3. Mis-declared overweight containers

Examination of containers left on board after a stow of containers has collapsed sometimes reveals that the containers were over the declared weight: it is possible that containers lost overside were overweight.

Operationally, mis-declared overweight containers are a difficult problem to solve. The weights are declared by the shipper mainly on trust and small under-declarations may be undetectable. Gross under-declarations may be apparent during container handling by mobile equipment or by container gantry cranes fitted with strain gauges, provided of course that those involved in shore handling are aware of the potentially serious nature of the under-declaration.

The problem is perhaps best addressed by the carrier's shore organisations as an operational issue, sending representatives to observe suspect shippers stuffing containers, or as a commercial issue, identifying shippers from the manifest that are not known customers or have been identified previously with involvement in mis-declaring weights.

4. Navigation around heavy weather

Experienced mariners prefer to anticipate heavy weather and adjust the voyage plan to avoid it. Unfortunately, some ships heave-to only when they find that normal progress is no longer possible, even though the heavy weather was forecast. Consequently the ship is stressed, the potential for cargo damage or loss overboard is increased - and no time is saved over the ship that anticipated the heavy weather.
With the extent and increased accuracy of weather information available today, plus the weather routeing available from ashore or from on-board computer systems, it should be possible for mariners to anticipate and avoid heavy weather, including having a contingency in the voyage plan for a maximum-wave-height route or set parameters for a least-damage route.
Editor's note: In addition to the above, containership crews must verify, to the extent possible, that each container that is being loaded at the bottom of any stack, particularly those towards the ends and outboard, do not have any physical defects, such as cracked corner casting, bucked post or rail etc. In severe weather, these units are subject to very high acceleration forces which may, in extreme cases, exceed the strength of the box, and result in a stow collapse. Masters and shore managers should also fully understand the phenomenon of parametric rolling, and take effective preventive / avoiding actions.

See also pp 12-15 SEAWAYS December 2008.