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  • Writer's pictureSarah Whiteford

Container shipping - How does it work?

Shipping containers are the standardized metal box in which most of the world’s goods are transported. Today we’re going to take a look at how they are measured, where they came from, and how the vessels that transport them work. It includes a 60-year-old container design, automated transport, huge cranes, and giant ships.

Shipping containers are measured by TEU, or Twenty Foot Equivalent. This is a measure of how large a container is, and is the standard measure for all shipping. One TEU is a 20 foot long container and two TEU can be two 20-foot containers or one 40-foot container, with 40 being the most common. Standard 40-foot containers can be referred to as two TEU or one FEU, where FEU stands for “forty-foot container”. These standards have to do with space constraints, as the weight of a container can vary widely depending on the cargo.

“Two forty-foot containers stacked on top of two twenty-foot containers. These four containers represent 6 TEU.” “By Torsten Bätge, Hamburg - Self-photographed, CC BY-SA 3.0”. Image from Wikipedia.

Where did these measurements come from?

In the early 1900s, a trucking entrepreneur named Malcolm McLean thought that the container loading and unloading process was inconvenient and tedious. Barrels, crates, sacks, pallets, and anything that could hold goods were loaded piece by piece for transport. According to iContainers, McLean was at the docks one day and “thought there must be a better way to streamline and standardize the whole process”. He set to work to find a way to stop loading cargo individually, and created a standard sized cargo container that could be loaded without touching the cargo inside. He sold his trucking company and designed the containers, soon after buying an oil tanker called the Ideal X and converting it to hold 58 containers. The ship sailed from New Jersey to Houston in April 1956 with its new cargo, the first voyage for containers. McLean used 35 foot containers while Matson’s, a competitor, used 24 foot containers. The US Government wanted further standardization of container sizes, and ISO decided in 1968 that 20-foot and 40-foot containers would be the standard.

Malcolm McLean at the docks. Image from iContainers.

TEU is now used for containers, but also to measure ship capacity and port throughput. Ships are categorized according to approximate capacity, and according to iContainers, estimated sizes are as follows:

  • Small feeder: Up to approximately 1,000TEU

  • Feeder: Approx. 1,000 to 2,000TEU

  • Feedermax: Approx. 2,000 to 3,000TEU

  • Panamax vessels: Approx. 3,000 to 5,000TEU

  • Post Panamax vessels: Approx. 5,000 to 10,000TEU

  • New Panamax (or Neopanamax) vessels: Approx. 10,000 to 14,500TEU

  • Ultra Large Container Vessel (ULCV): Approx. 14,500TEU and above

TEU is a good measurement for ports, as well as a dollar value like $1 billion of goods moved or an area of 1,000 acres doesn’t mean much. TEU can effectively measure that throughput with ports like Los Angeles and Long Beach moving over 16 million TEU per year. Ports in Shanghai and Shenzhen, China move over 65 million TEUs per year!

“Goliath of global trade: The container pier at Yangshan Port, which forms part of the Port of Shanghai which last year handled nearly 740million tonnes of goods last year”. Image from Daily Mail.

There are also flat rack and open top shipping containers that can be used for Out of Gauge (OOG) cargo that wouldn’t fit inside a normal container. According to Marine Insight, OOG cargo could include wind turbine blades, industrial boilers, or a bulldozer. Open containers come in 20-foot and 40-foot lengths just like closed containers. Other less common sizes exist like 40-foot high cube, which are 1-foot taller than normal 40-foot containers, and 45-foot containers, which are slightly longer than standard. Internal dimensions of containers are slightly less than the outer measurements, with 20 feet being the overall length of a TEU, while the internal loading area is approximately 19 feet, 4 inches. They can be loaded with up to 24 metric tons, or 24,000 kilograms of weight, in addition to the weight of the container itself, about 2.24 metric tons. Unless the weight is below 1 ton, weight is used to calculate the cost of shipping.

Reefer containers are refrigerated units used to transport perishable or temperature sensitive goods. Generator sets, also called gensets, are powered by electricity and/or fuel to keep the containers at the right temperature. They are equipped with data loggers to track temperature over time to ensure consistency during long journeys. Most reefers are TEU sized, in contrast to most containers, which are FEU. Reefers are also shorter inside than a regular container, about 17 feet, 10 inches.

Row of reefer containers. Image from DHL.

Are containers inspected?

Once every 30 months, containers must be inspected to certify that they are transportation worthy and meet CSC 1972 (Convention for Safe Containers). They receive a CSC approval plate with container details like design, inspection, and gross weight, according to Marine Insight.

Container labels. Image from Marine Insight.

What happens when a ship arrives at port?

When the ship arrives in port, tugs help maneuver the huge ships into place next to the dock, where quay cranes (QCs) load and unload cargo. They are large cranes with trolleys that move back and forth to move containers from the ship to a transport vehicle and back. A spreader on the trolley picks up the container and moves it - one crane can unload a vessel while another loads the vessel right next to it. Transport vehicles move containers between the stacks and the QC can be completely autonomous as opposed to quay cranes, which are human-operated. Crane drivers have a lot of flexibility in how they unload containers, but loading requires every container, sometimes thousands of containers, to be in their exact spot. Destination, weight, category, size, and contents are all factors in the best location on the vessel for each container, according to Iris Vis. For example, a light container that needs to be unloaded at the next stop should be near the top, and a heavy container to be unloaded at the end of the journey should be at the bottom.

“At nearly 1,300 feet long, CMA CGM Marco Polo is the largest container ship ever to visit the East Coast. Handout photo.” Image from Chesapeake Bay Magazine.

Once on land, containers are stacked between two and eight high on the ground until they are transported by road or rail. Yard and straddle cranes can always be automated, and can make storage more efficient as they can pick up containers and drop them down anywhere within their reach, creating a solid block of containers that are still readily accessible. Automatic stacking cranes (ASCs) are used in some ports and can create rows 10 containers wide, multiple containers high, and as long as their tracks. They use laser guidance systems to move containers to within 50 millimeters, pretty impressive for a crane moving a 40-foot container at up to 5 meters per second, according to TMEIC. Check out the image below to see how gantry cranes use their sensor systems to move and deliver containers where they need to go. There are trade-offs to storing containers - high stacking takes up less real estate, but does lead to more reshuffling and moving of containers to retrieve units that are underneath others.

“Rail mounted gantry crane for automatic container stacking.” Image from TMEIC.

What about the container ships?

Container ships have grown very large over time with Panamax vessels being originally designed to be as large as possible while still navigating the Panama Canal, around 4,000-5,000 TEUs. Post-Panamax vessels were introduced in the 1990s and are too big to transit the Panama Canal. This was followed by the Suezmax, the maximum size that could go through the Suez Canal, at around 12,000 TEUs. Post-Suezmax vessels are even larger, up to 18,000 TEU and can’t transit the Suez Canal. Post-Malcamax are the largest vessels that have a draught of 21 meters or less to use the Malacca Strait - these vessels are also limited as they are too big for many ports.

In a standard container vessel, there are cell guides to help crane operators with loading and unloading containers, as well as keep them from shifting during transport. Ships have a stowage plan that is calculated by a computer, not dissimilar to solving complex Tetris problems, according to Wikipedia. Containers are put into the holds as well as on top of the cargo hatches. Containers used to be connected together, but are now stacked in independent vertical stacks that can sway like trees with the movement of the ship. Lashing rods run crosswise on the ends of the containers and connect them together in the stack, absorbing lateral movements. Lashing systems are specialized for each ship according to its specific needs, including container weights and their placement in the stack (heavier at bottom, lighter at top). If this doesn’t come together as planned, then a heavy container on the top of a stack can overload the fasteners, tip the stack and put pressure on the stack next to it, causing containers to be damaged and even go overboard. Even movement of the cargo inside the container can have this effect - if it breaks loose and moves around inside the container, then it can put extra force on the stack and cause a failure. Even the ability of the vessel to self-right in rough seas can be an issue and if a ship rights itself too slowly, then it could capsize too quickly and could put more stress on the containers. It’s estimated about 450 to 650 containers per year are lost overboard, according to the Transport Information Service.

“CC BY-SA 3.0” Image from Wikipedia.

Container ships have grown massive as some of them now have capacities close to 24,000 TEU. Bigger capacities mean less trips and greater economies of scale for moving cargo around the world. As the vessels have expanded, the ports have expanded as well with bigger cranes, deeper waters, and automated container handling systems. All of this technology was created to move the containers created in the 1930s and standardized in the 1960s.

PS - If you’d like to learn more about containerships, then check out “World's Largest Containership: Emma Maersk”.

Happy Fun Fact Friday!


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