Container Stacking on Large Vessels

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(Image Courtesy: Hofstra People)

Container shipping has evolved as an indispensable vertical of maritime trade. Until the 1970s, virtually all goods were shipped around the world loose, crammed into the holds of cargo ships. The impact of containerisation has been enormous and today, the shipping industry boasts of over 55000 cargo ships, active in international trade and about 20 million containers are traveling across the oceans every day.

If we lined up all the world’s containers, the line of containers would stretch halfway around the planet.

(Image Courtesy: Crescent Transportation)

Locking Containers

On most ships, which are specially designed for container traffic, the containers are carried lengthwise. This stowage method is sensible with regard to the interplay of stresses in rough seas and the loading capacity of containers. Stresses in rough seas are greater athwartships (being across the ship from side to side), than fore and aft and the loading capacity of container side walls is designed to be higher than that of the end walls. Besides, the first ‘tier’ of containers that are carried above deck are secured to deck sockets using ‘twistlocks’.

Twistlock

The twistlock was developed in Spokane, Washington, in the 1950s by transport engineer Keith Tantlinger. This lock made handling and stacking standard containers much easier. A major advantage of this approach is that containers, which may be stored or transported without being inspected for months at a time, do not require any maintenance in order to function effectively.

Even with long term exposure to the weather, the container remains as simple to move as ever. Only when corrosion is very extensive does the twistlock become dangerous to move the crate. The system is adaptable with all standard containers.

(Image Courtesy: gCaptain)

Auto Twist Locks

For higher tiers, normal twist-locks become difficult to use.  For these containers, automatic twist-locks are used.  These are generally attached right after the container is raised from the pier. When placed on top of another container, the automatic twist-lock pops into the locked position. While unloading that container, it can be switched to the unlocked position using a long pole with a hook.

For containers stowed inside the holds, things are easier. Most ships have cells (vertical slots made by long metal rails) in the cargo holds that do most of the work of keeping containers from sliding from side-to-side. Additionally, the bottom tier will be twist-locked to the deck, and stacking cones do the rest of the work.

Interlocking Crate and Shipping Container

A shipping container system comprises a bottom panel, side panels and a top panel, each with interlocking features. The side panels interlock with the bottom panel to assemble a self-supporting shipping container.

The horizontally oriented slots are formed in an opposing pair of the side panels, and arranged at different heights between the bottom panel and a top edge of the side panels. A cross member is positioned at a selected height by insertion into an opposing pair of the slots, in order to restrain one or more shipping units against vertical motion inside the shipping container. The top panel interlocks with the side panels to cover the shipping container.

(Image Courtesy: www.sensiblehouse org)

Athwartships Stowage

On many vessels, the containers are stowed in athwartships (lying across the ship from side to side). This must be taken into consideration when packing containers and securing cargo. This stowage method is not sensible with regard to the stresses in rough seas and the loading capacity of containers. As stated earlier, the stresses in rough seas are greater athwartships than fore and aft but the loading capacity of container end walls is lower than that of the side walls.

Preventing Slippage & Toppling

Even unusual stowage methods like this, where some of the containers are stowed athwartships and others fore and aft, are used, but they require greater effort during packing and securing operations. When securing containers on board, the stresses resulting from the ship’s movements and wind pressure must be prioritised.

Forces resulting from breaking-wave impact can only be taken into account to a certain degree. All the containers on board must be secured against slippage and toppling, with care being taken to ensure that the load-carrying parts of the containers are not loaded beyond admissible levels. Except in the case of individually carried containers, securing is affected by stacking the containers in vertical guide rails or by stowing them in stacks or blocks, the containers being connected together and fixed to parts of the vessel.

(Image Courtesy: _ PT.Gema Nawabintang Surabaya)

The Containers’ Journey

A container is loaded and then brought to the port by a trucking company. Bringing containers to the port or from the port is called “drayage”. The most common containers are 20 foot dry and 40 foot dry containers. There are also refer (refrigerated), high cube, 45 foot, flat racks, open top, open side, liquid and many other specialty containers available for every conceivable cargo load. The container is kept at the port in the container stacks until the designated ship arrives. Once the designated ship has arrived, the container is brought to the ships side by a special chassis and cab called a bomb cart.  At most ports, container movement are computer controlled.  It is necessary to place each container in its correct position on the ship based on important factors such as container weight and the port designated for off loading, which will ensure that the boxes reach their intended destinations ‘unhurt’.

(Sources: www.containerhandbuch.de, TJ Willis, Merchant Officer (Quora), www.freepatentsonline.com, www.pacificmarine.net)

Sea News Feature, February 13