Straddle Carriers – Design, Operation, Advantages And Disadvantages

Transportation and logistics play a major role in the growth of the economies of the world. They determine the rate at which trade occurs and the fortunes of countries take shape.

The shipping industry contributes to nearly 90% of the total freight industry and this figure continues to grow. Simply being able to move containers or other cargo items using carriers or ships is not sufficient.

Once the vessel reaches a port, there must be sufficient equipment and technical advancements to unload and reload the container ships in as short a time as possible.

Alongside towering cranes and massive trailer bed trucks, there are a new class of vehicles aiding ports in container transport within the port area. These vehicles are known as straddle carriers and they serve a wide variety of uses in the port.

Straddle carriers are used essentially to stack containers up, but without needing to be fixed to a single spot. Spreader cranes that are used to load and unload containers from ships can generally only move along its set of rails. However, straddle carriers face no such restrictions.

straddle carrier

They are essentially transportation vehicles that are able to “straddle” and lift containers up using a spreader crane mechanism. These containers can be generally stacked three high, but there are some carriers that are able to construct four container stacks.

In this article, we will discuss the functions of these carriers, and their various uses and restrictions.

What are Straddle Carriers?

Time management plays a major role in the functioning of ports and their auxiliary facilities. Bottlenecks and high turnaround times are major reasons for certain ports gradually losing their relevance and their market to competition from neighbouring ports.

Thus, their entire fortune hinges on the amount of time that they take to handle cargo, whether it be containers or other bulk items such as oil and natural gas.

The time taken by a ship to berth, be unloaded, refuelled and reloaded is known as the turnaround time.

Long turnaround times contribute to bottlenecks in ports that can choke the influx of trade. This can have a major impact on commerce for the country. Thus, technological advancements that can aid in reducing turnaround times are important.

Generally, to lift containers off ships, spreader cranes are used. These are large cranes that lift containers by latching on to supports at the top edges.

The crane then lowers the containers onto a yard is located next to the ship. From there, these containers are transported to a variety of locations on the port premises for distributing onto trailer-bed trucks.

Port cranes and stacks of shipping containers

Image for representation purpose only

Conventional trucks require a spreader crane to load the containers on to the trailer of the truck. While this is a time tested and efficient method, a faster solution would be for the truck to load its own containers. And this is where straddle carriers come into play. They are used widely in ports to lift containers up and carry them within the premises.

Due to the way they are built, they are more apt for shorter distances compared to conventional trailer trucks. However, they are extremely useful when it comes to this type of transport, as they do not require an extra crane or forklift to help them load containers.

Design of Straddle Carriers

Straddle carriers consist of a set of four vertical frames that support the entire structure. At the top is the mechanism for the spreader crane that lifts and carries the containers.

The bottom section is attached to a reinforced frame that is connected to the driveshaft and wheels. In general, the driver who also serves as the operator, sits at the top of the crane, away from the main body of the carrier.

Their operating environment is encased in a shatterproof and extremely strong plexiglass casing. This serves to protect the operator in the event of a collision.

The reason this region is located outside the main body of the carrier is that it provides an unobstructed aerial view in all directions. This aids the operator in being able to safely navigate and stack the containers while also being able to look out for possible collisions.

It is also easier to use the spreader from a height, as compared to at the ground level. Some companies such as Mobicon that manufacture straddle carriers have a design that places the driver at the bottom of the entire structure. This is an alternative design that is often requested specifically by ports to fulfil a certain set of operating conditions.

Straddle Carrier

Straddle carriers use reinforced truss structures to ensure that their entire carrier is stable and resistant to possible damage. Since they have an open structure both at the front and rear, they can carry loads that are much longer than their average length. This was actually the primary purpose for carriers that were used in lumber yards before they were adopted for ports.

Since timber logs were often larger than the vehicle itself, they were left open at both ends to allow for longer loads to be carried. In general, modern straddle carriers are able to travel at speeds of 30 mph, for a full-load condition.

These carriers are also able to lift up to 60 metric tons, that is equivalent to two containers. They can lift their load up to a height that allows for stacks between three and four containers. Any higher than this can compromise the structural integrity of the design.

Functions of Straddle Carriers

First, let us have a look at the various functions that these carriers perform.

Straddle carriers serve three main functions in the port area:

  • they shift containers within the port,
  • load and unload trailer-bed trucks, and
  • stack containers.

Once the primary spreader cranes unload containers off ships on to the port yard, they often must be transported to the individual trailer loading areas. For this purpose, straddle carriers approach individual stacks, and latch on to the topmost container.

Once it is lifted, they proceed to the allotted drop off location. If provisions are made, two containers can be interlocked, and then simultaneously lifted. This tends to slow down the carrier, but reduces the number of trips and increases efficiency. Once the carrier reaches the required zone, containers are stacked in the reverse order in which they must be arranged on trucks. With this, the first function of the straddle carrier is over.

Next, separate carriers are assigned to load trucks with their respective containers. The individual containers are lifted and correctly aligned with the trailer bed on which they must rest.

Once this is complete, the straddle carriers move on to the next trailer for loading. Several ports use large spreader cranes for this purpose, as compared to straddle carriers. However, in places with space restraints and a need for quick transport, these carriers are more efficient and favourable when compared to spreader cranes.

The last function that these carriers serve to perform is the stacking of containers. Often, containers are required to be stacked in a specific order for a variety of purposes. This does not have to be on port premises. In warehouses and container depots, there is a need for maximum space utilization.

Such locations do not have the infrastructure or facilities for a spreader crane, and so are dependent on straddle carriers for this function.

These carriers are used to arrange containers on specific rails that align and lock in the stacks. Individual containers of a stack are also locked in position. This allows for large towers of containers to be safely stacked.

Advantages and Disadvantages of Straddle Carriers

Although straddle containers seem to be the solution for nearly every logistical issue in ports, it is important to carefully analyze their pros and cons.

The main advantage of using a straddle carrier over a spreader crane is that it is able to easily fit in tight locations that would otherwise not have been possible. Since spreader cranes are in general confined to rails, they cannot often reach all parts of a port. This is another reason to have straddle carriers.

Along with this, they allow for fast transport of containers due to their average speeds of 30 mph. However, the main advantage in the overall usage of straddle carriers, is that they do not require forklifts or other cranes to help them load containers. All that is required is for them to simply straddle and lift the container unit.

Now, if we analyze the disadvantages of using straddle carriers, it is their cost that acts as a primary deterrent. Since they are a relatively new technology that will take some more time to catch up with the market, they are still slightly expensive.

However, given sufficient time, they can turn out to be a very useful investment. Another problem with these carriers is that they can only stack containers up to a height of three to four units.

The true stacking potential of containers is much higher, especially with improved interlocking mechanisms. Spreader cranes are able to stack much larger units and hence can drastically conserve space.

Another issue, which is more of a restriction as opposed to a problem, is that these carriers can only carry up to two containers on every trip. With higher technological standards, we can hopefully see innovations in this area that allow for a larger cargo load.

Lastly, the structural integrity of straddle carriers continues to be a bit of a concern for some critics. Weights nearing 60 tons can pose a serious threat to the operating life of several components on the carrier.

Having to frequently replace or repair components can be costly and place a strain on the finances of any organization. Similarly, the entire design has minimal transverse support. Although the design and material choices ensure the integrity of the carrier, it can still pose a problem during the application of major transverse loads.

In Conclusion

Straddle carriers appear to be the latest successful innovation in the field of transportation logistics and have immense scope in applications for the maritime industry.

Given sufficient time and advancements, we hope to see innovations in the design of these carriers, that allow them to safely and effectively reach new heights, and bear larger loads. For e.g. some ports have implemented automated straddle carriers provided by companies such as Kalmar for better efficiency and safety.

Do you have any ideas regarding how these carriers can be improved? Let us know in the comments section below.

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