Article: Container stack collapses – causes and solutions
18 January 2021
In 2019 the international liner shipping industry transported 226 million containers around the world with a cargo value of more than US$4tn. Many of these were carried on ships’ decks but – due to container stack collapses – not all arrived safely.
Despite various advances in standards and procedures, such collapses are still happening, putting vessels, their crews and the environment in danger. These incidents can often result in significant financial losses to the container industry and their marine insurers, sometimes with hefty fines for clean-up costs.
According to the World Shipping Council, an average of 1,382 containers were lost at sea each year between 2008 and 2019. Indeed, the frequency and value of container stack collapse claims experienced by Standard Club members has grown during the past five years, rising to a record US$1m from 13 incidents in 2019. While these figures are only a tiny proportion of the total number of containers carried, container stack collapses and their not insignificant costs are mostly preventable.
This article aims to remind ships’ officers and crews of the various factors that can contribute to container stack collapses, and how they can be avoided by taking greater care and attention during loading, securing and passage planning and when underway at sea.
Bigger, stiffer ships
Economies of scale have resulted in ever-larger container ships being built. Modern container ships have come a long way from the first vessels designed in the late 1950s, which had a capacity of about 600−800 TEU with less than 50 containers loaded on deck.
By contrast the 2020 Algeciras class container ships have a capacity of just under 24,000 TEU, with a length of 400m and a beam of 61m – over three times wider than the early vessels. With a deck capacity of 24 bays, 24 rows and up to 12 tiers, ultra-large container carriers can carry nearly 14,000 TEU above the holds.
But the large beams of these post-Panamax giants result in them having relatively large metacentric heights (GM), meaning the vessels are very stable and therefore stiff. This in turn can result in very high rolling accelerations when the weather deteriorates, generating similarly high loads in the container lashing and securing gear.
More powerful ship engines
Increasing commercial pressures means that container ships usually have to keep to very tight operating schedules, particularly in the liner trade, and they need to be as fully loaded as possible. As a result, they have increasingly powerful engines, not only to provide the high speeds required but also to enable speed to be maintained during bad weather.
The consequence is that, at times, container ships can be driven hard. When ships are driven hard in bad weather, the loads on the container lashing and securing gear can be severe.
Higher wind loading
Almost all container stack collapses at sea occur in rough weather with strong winds. When fully loaded, the deck stacks on modern container ships present additional windage areas over 25 m high. Combined with large freeboards, the stacks act like giant sails to amplify a ship’s motions as the weather deteriorates, further adding to lashing and securing loads.
Parametric rolling of ships
Parametric rolling is a phenomenon where sudden heavy rolling occurs in head or following seas. Although very rare, it tends to affect vessels such as containerships which have large bow and stern flares.
It is difficult for masters to predict when parametric rolling will occur, as it requires certain conditions to be met. These include larger waves with a wave length equal to the ship’s length, and a wave encounter period that is half the ship’s natural roll period.
The resulting variations in waterplane area can, at the right frequency, trigger violent rolling of over 30° in a very short period of time. Such violent rolling can lead to extreme loads on container lashing and securing gear.
Synchronous rolling of ships
For beam and quarter waves, if a container ship’s natural roll period synchronises with the experienced wave period, resonance can occur resulting in similarly violent rolling motions.
Larger, stiffer container vessels tend to have shorter natural roll periods that more closely match the periods of the wave spectrum. This in turn increases the risk of synchronous rolling and over-loaded container lashing and securing gear.
For example, following a large container stack collapse in January 2019, the Dutch Safety Board confirmed that large, wide container ships using the shipping routes north of the Wadden Islands in the North Sea are at risk from synchronous rolling during north-westerly winter storms.
Ship contact with seabed
In the same report by the Dutch Safety Board, it was concluded that on the shallower southern shipping route by the Wadden Islands, there is also a risk of container ships contacting the seabed as a result of violent motions caused by north-westerly storms.
Larger, deeper-drafted container ships are clearly at higher risk of contacting shallow seabeds during extreme roll and heave motions. Such contacts, even on a sandy seabed, can result in large additional loading in container lashing and securing gear. On rocky seabeds they can also severely damage the hull.
Green water and wave impacts
In heavy weather, waves and ship motions can become so large that water flows over the deck, known as 'green water loading'. On container ships this can cause high impulsive loading on container stacks and potentially trigger a collapse.
Steep waves with high horizontal speeds breaking against the side of a container ship can also generate additional forces in container lashing and securing gear.
Improper container stowage
The stack weight on a container ship is the total weight of all containers and their contents in the tiers of a particular stack added together. The ship’s cargo securing manual states the maximum permissible stack weight for each stack. Deck stack collapses often occur in those bays where the stack weight was exceeded.
Furthermore, the distribution of weights in a container stack directly affects a vessel’s stability. The cargo securing manual specifies a maximum permissible GM for the vessel to avoid excessively short rolling periods and high accelerations. It is therefore important to get the GM within the right range before a voyage starts to avoid overloading lashing and securing gear.
Cargo securing manuals generally advise that deck containers are stacked in weight order, with the heaviest in the bottom tier and the lightest at the top, to minimise loads on the lashing and securing gear. This relies on accurate knowledge of container weights. If heavy or overweight containers are inadvertently loaded into the upper tiers, it could result in catastrophically high forces on the lashing gear and collapse of the stack.
To tackle the problem of overweight containers, the International Maritime Organization (IMO) amended SOLAS chapter VI regulation 2 in 2016 to require mandatory verification of the gross mass of packed containers loaded on ships.
The shipper is responsible for providing the verified gross mass (VGM) by stating it in the shipping document. They must then submit it to the master or their representative and to the terminal representative in time for it to be used in preparing the ship stowage plan. Furthermore, a VGM declaration is a mandatory prerequisite for any containers loaded onto a ship subject to SOLAS.
In practice, the role of the ship planner and terminal representative in ensuring compliance with the regulations is critical. While some container ports in developed countries have created resilient systems to comply with the regulations, there are ports in lesser-developed jurisdictions which fail to implement them. Port authorities are often unable to afford spot checking or enforcement, which does little to encourage offending shippers to comply.
As stated above, overweight containers with incorrectly declared or deliberately misdeclared weights can, if loaded on the upper tiers of deck stack, lead to a stack collapse.
Poor packing of containers
Incorrect packing of containers can lead to both internal cargo damage and, more seriously, container stack collapse. Unlike breakbulk cargo, masters and officers do not have sight of or control over the contents of containers or the methods by which they are packed and secured. Carriers usually depend on third parties such as the shipper, freight forwarders or their sub-contractors for stuffing and securing cargo in containers.
If contents shift, they could potentially damage a container – and a stack of containers is only as strong as its weakest member. A container damaged due to shifting cargo could collapse and lead to a domino effect, resulting in an entire bay collapsing.
The IMO, the International Labour Organization (ILO) and the United Nations Economic Commission for Europe (UNECE) approved a Code of Practice for the Packing of Cargo Transport Units (CTU Code) in 2014 to help the container industry ensure safe stowage of cargo in containers. In summary, cargo should be packed evenly and solidly, and stowed securely within the container. Project or unusual cargo items should be adequately dunnaged and secured with adequate ratchet straps, wires or chains to secure fixing points. The side panels, end panels and roof panels of an ISO container should not be considered as structural members.
Structurally weak containers
Containers are essentially meant to contain cargo but can get seriously degraded with factors such as rough handling, forklift damage, inadequately secured contents, wear and tear, and overloading. These along with other factors could lead to structural failure of the container, which in turn could cause to the stack above it to collapse.
The strength of a container is provided principally by the outer framework, side rails and corner posts, together with the corner castings. The side, end panels and closed doors provide racking resistance only. Effective stacking of containers relies on the strength of the corner posts to support the weight of the containers above. Damage to a corner post, in particular buckling, can seriously degrade its compressive strength and lead to collapse of a container stack.
Inadequate container securing
The lashing of many containers in a large deck stack can prove challenging and difficult. Containers are basically secured to each other with twistlocks fitted at their four corners. Lashing rods and turnbuckles are then used to secure the container stacks to the deck by connecting them to the hatch covers, deck posts or lashing bridges if fitted.
However, lashing rods are only able to reach to the bottom of the third tier of containers loaded on hatch covers or deck posts, or to the bottom of the fourth or fifth tier of containers where a lashing bridge is fitted. This means that on large modern container ships, several upper tiers are secured by twistlocks only.
For the deck stowage system to be effective, the lashing and securing gear needs to be fitted correctly. Missing twistlocks, unlocked twistlocks, damaged lashing gear and lashings becoming lose in a seaway are examples of inadequate securing which can lead to a container stack collapse.
While lashing and securing gear is class approved, it is not usually inspected by a classification society. Replacement of sub-standard equipment is the responsibility of a ship’s crew, who must keep a watchful eye out for damaged or worn components and arrange for them to be replaced without delay.
Adjacent container stack clashing
Each cargo stack will experience slightly different lateral and vertical forces during a ship’s motions at sea such that, in the event of large motions, adjacent stacks can clash. As a result, a stack of containers could collapse, either falling overboard or against another stack. Stack collapses due to clashing are often progressive as, when one stack begins knocking into adjacent ones, the forces can be much higher.
Conclusions and solutions
Proper packing, stowage and securing of containers, and reporting of correct weights, are of key importance to the safety of container ships, their crews and cargoes; of shore-based workers and equipment; and of the environment. However, despite proper packing of the cargo into containers, correct weight declarations, and proper stowage and securing on ships, factors ranging from severe weather and rough seas to more catastrophic and rare events like groundings, structural failures and collisions can result in containers being lost at sea.
All of the factors discussed in this handout could contribute towards a catastrophic stack collapse which, besides causing large monetary losses, could potentially lead to serious crew injury and damage to the vessel and the environment. Understanding the cause of such collapses is the key to preventing them from occurring again and to appreciate who is liable for the incident.
As container ships have become larger, beamier and thus stiffer, the only significant enhancement in deck lashing and securing systems has been the provision of lashing bridges. While larger container ships provide commercial advantage to shipowners, these are often being staffed with fewer and fewer crewmembers. Given the highly commercial and systems-driven nature of the container trade, crewmembers might sometimes think their role is reduced to that of passive bystanders. This must not be allowed to happen: they must always be able to react quickly and make the correct decisions.
Crewmembers need to be mindful at all times of all the factors which could contribute to a container stack collapse. Indeed, proper training given to crewmembers could enhance their nuanced understanding and therefore enhance situational awareness on board container vessels. A proper understanding of the loading and lashing software and its limitations will go a long way to preventing such losses from occurring. Similarly, a thorough understanding of the trim and stability booklet and the cargo securing manual, and the limitations stipulated within them, must be considered and strictly adhered to by ships’ crews and officers.
However, they need to bear in mind that while the cargo securing manual may only state one permissible GM value, this might not account for different wind exposures or consider if high cube containers (2.9 m high) are being loaded. There are many variables and officers and crew need to appreciate the limitations of the cargo securing manual and interpret its content. A correct stow requires innovative planning both ashore and on board. While approved software and advanced programs can be used, it is ultimately the crewmembers and cargo planners who need to make their own considered and informed decisions on loading.
Crewmembers must also not let commercial pressure dictate their actions; a sharp eye on cargo operations should be kept at all times to ensure that errors are prevented. Damaged, leaking and overweight containers must be spotted and rejected from being loaded on board.
Similarly, a sharp eye should be kept on the condition of the lashing and securing gear on board, which should be regularly evaluated for damage and deterioration in quality; and should be removed and replaced as necessary. While at sea, regular checks and tightening of the lashing gear, including turnbuckles and associated check nuts, will help keep the containers safely stowed.
Finally, since heavy weather is always a causal factor for stack collapses, a sound and well considered passage plan, an understanding of the dynamic forces affecting the vessel, and proactive and effective weather routing for container vessels will go a long way to preventing such incidents from occurring in the future.