CONTAINER LOADING AND STOWAGE
The preferred method of loading container vessels is with a rail mounted gantry crane. The main arm can be raised when the vessel departs so as to clear the mast and superstructure.
Loading with mobile crane. Used at ports that don’t have the cargo throughput to justify a rail mounted gantry crane installation. It also has the advantage that it is not restricted to container cargoes.
Containers are secured by cross bracing, with turnbuckles and lashing bars, and anchored to slots or fitting on the deck.
The majority of reefer cargo is now transported by container. Containers with their own integral cooling system can be plugged into the ships electricity supply. Cell guides on the sides of the hold ensure that the containers stack properly.
One of the most persistent problems experienced onboard containerships is bad stowage. This can take many different forms, but the most potentially damaging example occurs when heavyweight containers find their way into the upper tiers of container stacks on deck.
Collapse of container stack due deformation of bottom container.
Careful monitoring of the ships stability during loading operations is required or else the ship might capsize.
Out of gauge cargo, that is cargo which is slightly higher or wider than will fit standard containers, can still be carried in open top, openside or flatrack containers. The latter type has higher payload ratings which is often important. When such cargo is shipped on a flatrack it is essential that accurate positioning is achieved and particularly for cargo destined for underdeck stowage. Underdeck cargo must clear the cell protrusions and cargo in adjacent cells.
CONTAINER LASHING
The securing and lashing of containers on ship's decks is a difficult operation in terms of the work environment. There are great problems during loading and discharge of containers. The stevedores who carry out this work, known as riggers, have to work on container stacks which often are 13 metres high or more above the ship's deck. Safety arrangements are in some ports poor and the work frequently has to be performed in the dark, under windy and rainy or sometimes icy conditions. The difficulties are to a large extent due to the lashing equipment. The immense diversity of the devices used gives rise to great problems. Securing of containers is the responsibility of the ship's master, which can mean that there are large differences in the manner in which the operation is effected between individual vessels and shipping companies.
In the early years of containerisation, existing general cargo vessels were converted with the removal of tween decks and the addition of cell guides into the cargo holds. On deck, the hatch covers were strengthened and fittings added for lashings. However, the containers on deck were seldom stowed above one high and so were secured to the vessel by 'traditional' cargo ship methods. Often seen still trading today, are a few of the 'first generation' vessels built during the late sixties and early seventies. These ships were the first to be designed and built as pure container carriers. The holds and hatch covers were as wide as possible, and container posts were fitted on deck to facilitate loading of deck-stowed containers out to the ship's side. For this generation of vessel, two systems of securing the cargo were common. One relied on the use of twistlocks in conjunction with lashing bars or chains, and the second relied on the use of stacking cones and bridge pieces in conjunction with lashing bars or chains. Gradually, due to the increased utilisation of differing height containers, the second method became redundant and it became common practice to use twistlocks throughout the stow. This method normally allowed containers to be stacked three high and, in some cases, four high if the fourth tier was light in weight or empty. For first generation vessels, computer technology was not available onboard to speedily calculate dynamic loads acting on container lashings and frames. The shipboard computer (if any) was only used to calculate stresses and stability for the ship itself. Therefore, the shipboard staff would ensure the vessel was lashed according to a lashing plan taken from the lashing equipment manufacturer's manual, which appeared to assume an ideal stow with respect to the distribution of weight in each stack (the homogenous stack).
Containers are locked together using twistlocks. They come in many variations but their purpose is to lock the container stack together at the corner posts. The above animation (just click along the numbers) shows the operation of a typical twistlock.
With further development in the industry during the 1970s and 80s, the size of containerships continued to grow, with 9-high stowage in holds and 4-high stowage on deck becoming commonplace and the industry began to wake up to the fact that standards in lashing were required. Ships were, at this stage, still supplied with loading computers that continued to calculate a ship's stability, shear forces, bending and, occasionally, torsion moments. Very few had the capability to calculate dynamic loads on container frames and lashing systems caused by ship motions and wind forces. And so the lashings were still applied throughout the stow in accordance with the manufacturer's manual. Cargo was being lost overboard even though a properly designed securing system was in place and the cargo was correctly stowed. It became apparent that there was a great deal of ignorance concerning the combined static and dynamic loads acting on a securing system when adverse weather was causing severe ship motions, particularly rolling. Today, large container ships are being built - known as the 'post-panamax' class (too large to transit the Panama Canal) - capable of carrying up to 8,500 TEUs, and small container ships down to coaster/feeder vessels of a few hundred TEUs. But in general terms, by a process of evolution, the lashing systems in use on both types of vessels are very similar. Both have adopted the twistlock and lashing bar/turnbuckle system.
| |
Lashing Bridge
Turnbuckles and lashing rods. |
On post-panamax vessels - where among other features the vessel's large beam results in an unavoidable, relatively large GM (metacentric height), and 6-high stowage on deck is common - the modern practice is for the vessel to be fitted with a lashing bridge; a substantial steel structure running athwartships between each forty foot container bay. This allows the second and third tiers of containers to be secured to the bridge using lashing rods and turnbuckles, whilst the whole stow is secured throughout with twistlocks. The lashing bridge allows the anchoring points for each stack to be moved higher up the stack, which allows the lashings to be more effective in reducing the tipping moments acting on a stack when a vessel is rolling heavily. However, the practice of fitting the bridges between forty foot bays means that the twenty foot containers can only take advantage of the lashing bridges at one end. So, in effect, the twenty foot stacks have to revert to the limits of a conventional lashing system.. This is the case, because the practice of estimating the forces acting on a stack divides the container weight equally between each end of the container. So the weight in each twenty foot container is limited by the capacity of the lashing system at the container end, which does not have the advantage of being secured by a lashing bridge. On smaller vessels, the whole stow is also secured throughout with twistlocks, and the lowest three tiers are secured to the hatch cover or support post using the lashing bar/turnbuckle combination. However, since the mid 1980s, naval architects have produced computer programs to calculate the dynamic loads acting on container stacks. Such programs have been designed for use by ships' officers and container planners. On modern vessels, 5-high and 6-high stowage on deck is common; the use of onboard computers to check the dynamics of the stow in all weather conditions is vitally important for the safe carriage of the cargo.
CONTAINER POSITION NUMBERING
| |
You can see these numbers painted along the deck. |
To enable the position of a container on a ship to be specified, a standard numbering system is used. Container slot positions aboard ship are expressed by three co-ordinates indicating :
Bay -- Row -- Tier
Bays: Are numbered lengthwise from bow to stern with odd numbers for 20' containers and even numbers for 40' containers. The even number between two 20' containers is used to define 40' bays.
Rows: Are numbered from centreline to portside with even numbers and from centreline to starboard with odd numbers. The container row stowed on the centreline is marked 00.
Tiers: In underdeck stows, containers are numbered vertically downwards with even numbers from top to bottom. The bottom row will be 02, except where as a result of the hull contour, the bottom of an adjacent row is at a higher level. In case of two half heights the bottom ones are to be numbered by an odd number. On deck stowage is indicated by code key 8 followed by an even number sequence.
