Why Do Containership Stacks Collapse?

 Caption: Containers falling off a ship

 This article is an overview of the typical causes of stack collapse as well as the legal implications when dealing with the resulting liability claims.

Causes

Understanding causation is key to preventing incidents, but also to determine liability in individual cases.

Heavy weather has been one of the fundamental challenges for carriers since the dawn of shipping. Advanced technology for voyage planning and weather routing helps the Master, but his judgment will be questioned if an incident occurs. Containers, the securing mechanisms and container stacks are exposed to great forces when container ships move in heavy weather. Parametric- and synchronous roll resonance phenomena have caused several serious accidents to container ships during the last years.

Parametric rolling describes large spontaneous rolling motions occurring in head or stern seas and has to do with dynamics of length of ship and waves as well as the vessel’s wave encounter period. A vessel’s roll angle can increase from comfortable rolling motions to over 30 degrees in only a few cycles causing excessive acceleration on the container stacks. Synchronous rolling is caused by the ship’s rolling period becoming synchronous with the wave period. The waves may then cause resonance, meaning that the ship may lose control over the roll angels as the action of the wave rolls the vessel increasingly over.

Size matters as bigger vessels move differently in the sea compared with smaller vessels. For example, investigations following the APL China incident in 1998 revealed that large box ships with large bow flares are particularly exposed to parametric rolling. Furthermore, the containers on board the largest container vessels are stowed up to 40 meters above the waterline and 60 meters wide across the deck. When ships and container stacks of these dimensions start rolling, you do not have to be a physicist to understand that container stacks will be subject to great forces when the vessel starts to move with the motions of the sea.

Ship stowage plays an important factor because weight distribution on-board also influences the vessel’s motions at sea. The GM (see box) is a measurement of the initial static stability of the vessel. It is of the utmost importance to get the GM within the right range before the voyage. This represents challenges in terms of correct cargo planning both ashore and on-board. In practice, advanced software will do most of the job, but computer programs depend on correct software development, correct data entered as well as human interaction and, ultimately, human decisions.

The metacentric height (GM) is calculated as the distance between the center of gravity of a ship and its metacenter. The metacentric height influences the natural period of rolling of a hull: A low GM will cause the vessel to roll excessively with too large movements. A high GM implies greater initial stability against overturning, but high GM is also associated with shorter periods of roll which will cause rapid movements and greater forces on the cargo stowage. Hence, the GM will have to be correct: not too high, not to low.

Cargo stowage inside containers causes problems as a container stack is only as strong as its weakest container. If cargo inside one container starts to shift, it may have a domino effect on the stack. We have seen severe cases where one piece of cargo has damaged its container structure resulting in the collapse of a complete row of containers. Therefore, the Container Securing Manual (CSM) must be followed accurately, and further stowage guidelines should be sought for problematic cargoes. One of the challenges is that container carriers largely depend on shippers, freight forwarders or their sub-contractors to pack and secure cargoes adequately. Errors are inevitable.

The container is designed to fit the purpose of containing cargo, but if exposed to excessive weight pressure from excessive loads, containers may suffer structural failure. Container shells are exposed to wear and tear, rough handling and operations which may weaken their structure. If one container fails, the rest of the stow above and around will follow.

The weight of cargo will be declared by the shippers. Mis-declaration of weight is an industry problem and may cause considerable difficulty for cargo stowage planners as they rely on cargo details as declared by the shippers. If numbers are inaccurate, or even deliberately mis-declared, the integrity of container stacks may be jeopardized.

Lashing and securing of thousands of containers in large stacks onboard is a major challenge. Failure to do it correctly may have serious consequences. In simple terms: containers on deck are attached to each other with twist locks in the four corners of the container. Further lashing rods are attached between the container stack and lashing bridges or hatch covers. Eachtwist lock and lashing rod needs to be in its right place, work and be able to withhold required forces. Inadequate securing, missing or failing twist locks and lashings that become loose are probably among the more common causes of containers lost at sea. Failures in securing have caused severe incidents.

Multiple causes often make cases complex, not least when working with liability. In most cases there are elements of several of the abovementioned causes which lead lawyers deep into legal considerations about issues such as proximate causes, intervening causes, independent sufficient causes and foreseeability.

 

 

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