Post Panamax Carriers
During the course of development of containerships in the shipbuilding industry the need for the creation of `economies of scale resulted in the appearance of the 5th generation of container ships – the Post-Panamax in the 1980s.
A recent research in the container sector of the shipping industry indicates that the growth rate of post-Panamax containerships is the highest of all the containership sizes,
Although there were not many major casualties, in terms of loss of lives, resulting from accidents involving containerships, this particular ship type has more of its fair share of losses due to incidents involving cargo damage, personal injury, collision, ship structural failure and pollution. In terms of incident categories containerships differ from most other ship types in that shore error accounts for a high percentage of all major incidents. The result is an equally high percentage of cargo damage.
Although containerships follow the same pattern as the majority of cargo vessels, as far as the types of damages are concerned, they do differentiate in various aspects. The relative statistics available show that the percentage of incidents is higher in newer containerships, decreasing as they age, while in other cargo ship types, higher incident rates occurs at their middle age. The same statistics show that a high percentage of all incidents caused by human error were due to shore based personnel error, which is far higher than other cargo ship types. As far as ship size is concerned the smaller ships of this type are better placed with fewer incidents. Other operational characteristics of containerships, such as the fact that they very rarely travel in ballast condition and the few opportunities for overnight stay at ports, contribute to the overall performance of these vessels and their operators.
Some of the most outstanding accidents relating to container ships are.
- collisions,
- fires in containers,
- fatigue of the platings,
- damage caused by impacts on the bow,
- loss of containers,
- Parametric rolling and various complaints on the cargo, often relating to refrigerated containers.
The International Maritime Organisation passed a circular on the Formal Assessment of Safety (FSA) on container ships for reduction in hazards. FSA is a structured and systematic methodology, aimed at enhancing maritime safety, including protection of life, health, the marine environment and property, by using risk analysis and cost benefit assessment.
FSA can be used as a tool to help in the evaluation of new regulations for maritime safety and protection of the marine environment or in making a comparison between existing and possibly improved regulations, with a view to achieving a balance between the various technical and operational issues, including the human element, and between maritime safety or protection of the marine environment and costs.
FSA consists of five steps,
- identification of hazards (a list of all relevant accident scenarios with potential causes and outcomes);
- assessment of risks (evaluation of risk factors);
- risk control options (devising regulatory measures to control and reduce the identified risks);
- cost benefit assessment (determining cost effectiveness of each risk control option); and
- Recommendations for decision-making (information about the hazards, their associated risks and the cost effectiveness of alternative risk control options is provided).
Safety of Containers on Deck
The securing of containers on a vessel actually starts when the container is loaded (or stuffed) at the shipper’s facility. All securing plans aboard ship assume that the cargo within the container is properly stowed and secured. If a container is over loaded, improperly stowed or the cargo inadequately secured, this can lead to structural damage to the container which can result in a collapse of stow. This is particularly problematic in emerging industrial countries, where containers are often loaded at remote inland locations by shippers that are wholly unfamiliar with how to properly load and secure cargo within a container.
It is equally important that the container is in structurally sound condition, particularly the corner posts and fittings. While the global demand for containers fluctuates, during times of robust global shipping, shortages of containers will occur, which can result in containers being used that are in unsatisfactory condition. The securing scheme for a ship is incorporated into the design of the vessel, at the shipyard where it is built. While the containers in the holds are usually secured in cell guides, the resting points of the containers on the tank top are strengthened under the tank top. The same applies for the containers carried on deck.
In addition classification societies have established strict rules and regulations for welding of container bottom foundations, padeyes, D-rings and other securing devices attached to the vessel’s structure. Various types of Bottom Foundations are used,
- Deck Sockets – Cloverleaf
These can be flush or raised and can be single, double or quadruple sockets.
- Deck Sockets –Dovetails.
These are either single or double foundations.
On commercial cellular container vessels, mostly the flush dovetail foundations for sliding twist locks are observed, while the military often uses raised foundations. This is due to the fact that often other cargo than containers is carried on deck and D-Rings or other securing devices are welded on deck, which have to be overcome, when containers are stowed.
On commercial full container vessels these D-rings or lashing plates are positioned in accordance with the securing requirements adjacent to the container stacks. The containers themselves are in the stack secured directly to the deck by bottom stackers or bottom twist locks. Within the stacks, are located intermediate stackers or twist locks, bridge fittings or linkage plate. Bottom stackers are “no locking” devices, which hold the container in position. Bottom twist locks additionally secure the container in place by locking them at the four corners. It should be noted that there is a difference in the load ratings between bottom locking devices and container top locking devices and these should not be used interchangeably.
Turning a handle, which turns the locking device in the corner casting, mechanically operates conventional twist locks. These twist locks stay on the top of a container, as it is considered unsafe for the lashers to work on top of the container stacks to collect these twist locks. The intermediate twist locks, which currently have to be used on all ships calling in the United States, are “semi automatic” twist locks. These are spring-loaded twist locks, which secure both the containers in the stack. The semi automatic twist lock is attached to the bottom of the container that is going to be loaded. Upon lowering this container onto the container in the stack, the bottom lock will be activated and locks itself. Upon discharge, the lock at the bottom is opened by pulling at a wire with a knob at its end (the twist lock stays with the container on the top). The twist lock then travels with the container ashore, where it is removed by longshoremen.
Fully Automatic Twist locks (FAT) were introduced in recent years. These are Twist locks that are able to release from the deck stow simply by making a vertical lift with a slight twist. (This eliminates the need which was present with the semi automatic twist locks to have the longshoremen take the extra step on the dock to remove the twist locks before placing a container on a chassis.) These new twist locks tend to be smaller and are suspect in their reliability. A common factor in several incidents of loss of containers overboard has been the use of FAT’s.
If containers are stowed so close to one another (e.g. 20’ containers on 40’ stow places) that lashing cannot be done at both ends, linkage plates have to be inserted between the two stacks longitudinally and the accessible ends are lashed with bars crosswise. The uppermost containers have to be connected with tension-pressure bridge fittings.
The most common lashing units are the lashing bars, which are hooked into the container corner castings and tightened by turnbuckles. The locking devices (heads) are either fixed or adjustable. Chain assemblies with turnbuckles and/or chain tensioners are also in use.
The container lashing gear is usually designed to be stronger than the container; problems may arise not from the container lashing gear, but the collapse of container frames due to racking or other influence.
Stowage & Collapse of Stow Issues
Stowage issues must be properly planned and addressed, in order for the standard container securing schemes to be effective. The heavier containers must be placed on the bottom tier and the lighter containers on the upper tiers; otherwise, the loads on the lower tier can become excessive and the container may collapse. There also will be greater accelerations and forces on the securing gear, when heavier containers are stacked in the higher tiers, although most of the force must be restrained by the lower tier. Unfortunately, there is no universal definition of what constitutes a heavy or light container, and this can become a judgment issue.
Container ship operators employ vessel planners at their terminals. The role of the planner is to calculate and determine at what position each container is to be loaded aboard the ship. The two critical pieces of information are the port of discharge and the weight of the loaded container. Planners determine the position for each container, while balancing ship stability, stack loading, and operational (discharge) needs.
A standard 20’ ISO rated container has a gross weight of approximately 24 Metric Tons(MT) and 32 MT for a standard ISO 40’ container. A standard ISO container is designed to withstand 192 MT of weight stacked on its corner posts, when subject to dynamics that impart a G force of 1.8. This suggests that a bottom container can support a stack of 6 fully loaded 40’ containers and 8 fully loaded 20’ containers.
With the worldwide shortage of containers, shippers sometimes use containers that are not built to ISO standards and keeping containers in service that are in marginal condition. The condition of a container’s corner posts and fittings is particularly important.
Another problem today is the practice of some carriers still accepting cargo, while the vessel is already in port and loading. The previous cargo plan then becomes obsolete and in many cases, late arriving heavy boxes end up on top of all other containers, changing considerably the vessel’s pre-calculated stability and stow load pressures.
Oversized containers can also present securing challenges because they will preclude the use of bridge fittings, which are normally used to tie the containers together, across the tops.
The farther a container is placed away from amidships, the greater the accelerations will be on the container. Containers stacked on deck, near the stern, have generally had a greater incidence of loss than those carried in other locations.
Collapse of stow on the stern, where accelerations & forces are the greatest. To compensate for this, some companies have built “Bird Cages” or buttresses on the after deck, for the purpose of stacking and securing containers that are stowed on the stern.
Another area that can be problematic is the securing of cargo itself, within the container. If heavy equipment is loaded within a container, and not adequately secured, the cargo can damage the container wall, or break through the side of the container, during the voyage. If this container is loaded in a lower tier (where heavier containers should be), then the damaged container can cause a collapse of the stow of those containers loaded above it.
The condition of the container itself, and its critical structural members, can also be a contributing factor in the collapse of the stow. When a container stow collapses, it can also create a domino effect, by damaging containers in the adjacent rows. However, in the aftermath of an incident with all containers lost, little evidence usually remains to establish the true cause of loss.
The over loading of containers by shippers is a problem that seems to be growing, due to the increasing types of cargo now being loaded in containers. Today, it is routinely observed that containers are stuffed with scrap steel, grain and even large coils of steel. These are sometimes loaded by shippers that simply do not adhere to the weight limitations of marine containers.
While containers entering terminals in the US, and many terminals around the world, by truck are passed over certified weight scales, this is not the case for containers that arrive by rail. Therefore, the ship must often accept the weight of a container to be as declared by the shipper in their shipping documents.
Cargo Securing Manuals
Due to the dramatic increase in frequency of accidental discharge of containers overboard in the past decade, with the consequent increased hazard to navigation in open water and environmental concerns; the IMO elected to amend the Safety of Life at Sea (SOLAS) VI/5 and VII/5 to require a vessel customized Cargo Securing Manual, for vessels over 500 gross tons, engaged in international voyages. The purpose is to provide guidance for the Master and crew on board with respect to the proper stowage and securing of cargo units. These are specific manuals, made for specific ships.
It is the responsibility of the Master to ensure that cargo units are at all times stowed and secured in an efficient manner taking into accounts the prevailing conditions and general conditions of safe stowage.
It is the responsibility of the Master to ensure that all securing equipment is adequate for the load as calculated within the manual. IMO has published certain guidelines, which are to be incorporated into the manual. These manuals are usually approved by the vessel’s classification society. Cargo Securing Manuals are a valuable reference tool, for all personnel in securing cargo in a safe manner.
