The work by the Maritime Safety Committee (MSC), within the auspices of the International Maritime Organization (IMO), on the verification of container weights prior to loading on to a ship is progressing. Currently, expectations are that legislation will come into force in 2017 at the latest, and possibly in 2016.
Many terminal operators are concerned about how to comply with the upcoming legislation, and how it will impact on logistic flows in terminals.
One of the challenges facing terminals is how to weigh containers with little or no impact on operations. Transferring containers to separate weighing stations will affect productivity. Terminals are likely to need additional space and transportation capacity to cope effectively.
Solutions that weigh containers as part of existing logistic flows and operations will therefore deliver significant advantages for terminal operators.
Weighing alternatives
This article outlines technologies that are available for managing weighing or verifying weight. It should be noted that requirements for weight accuracy is not included in the current draft text from the IMO and will likely put further constraints on available options. The discussion here indicates what level of accuracy can be expected from the various options available.
Three different types of weighing or load measurement devices will be discussed: commonly available weigh bridges; load sensing devices in cranes and other lifting equipment and load sensing devices fitted to, or integrated into spreader twistlocks.
Weigh bridges
The first system that comes to mind when looking at weighing a container is the weigh bridge. Weigh bridges are a longestablished and recognised technology to measure the weight of a vehicle. When the weight of the container being carried by a vehicle is of interest, the tare weight of the vehicle must be deducted. The measuring accuracy of the weigh bridge is very high but the tare weight deduction process either introduces additional inaccuracies or becomes complicated and time consuming.
If a standard vehicle tare weight is used, the inaccuracy comes from such things as variations in fuel level, driver weight and the weight of miscellaneous materials also loaded in the vehicle. These may seem like minor aspects when considering a truck carrying a 40ft container, but it easily adds up to a few hundred kilos, thereby significantly affecting the accuracy of the container weighing process. The alternative to using a standard tare weight is to include weighing of the unloaded vehicle in the process. This will give an accurate vehicle tare weight and ultimately, an accurate container weight, but it adds steps to the process which takes time and uses terminal resources.
Using weigh bridges to weigh containers is likely to result in changes to the internal logistics of most existing terminals. All containers entering terminals by road would have to pass through the weighing station. The most critical factor in this scenario would be to have sufficient weigh bridges to avoid the bottlenecks and resulting congestion.
Containers arriving by train or sea (for transhipment) would have to be sent to a weighing station, a step which is uncommon in terminal logistics today. This additional step would tie transporting vehicles to specific containers for longer periods of time ultimately resulting in additional resources being needed to handle the same container volumes. Sufficient resources in terms of weigh bridges and transportation space therefore need to be allocated to avoid congestion
and delays.
Another situation which would require a specific process to be in place is where vehicles arrive at the terminal gate with two twenty-foot containers loaded. Weigh bridges can only determine combined weight. Because containers have to be weighed separately, this would imply a relatively complicated process involving not only the truck carrying the equipment but also terminal resources to facilitate the loading and unloading of the containers.
Load sensing devices in cranes or other lifting equipment
The second type of load measuring device is, in effect, several devices with common features. This group includes load sensors and devices on ship-to-shore (STS) cranes; rubber-tyred gantry (RTG) cranes; railmounted gantry (RMG) cranes; mobile harbour cranes; reach stackers; straddle carriers and so on. Most of the load sensing devices in this group are used for safety and/or stability systems, but the information is available to provide weight information with some limitations as outlined below.
The biggest question mark related to these systems is accuracy. Will the accuracy of these systems meet the requirements to come? The answer is most probably no, but until the requirements are defined, this option should be mentioned. Sensors in these devices are typically fitted to rope and chain anchors, in trollies or on booms. Distance from the container, and the dynamic effect this introduces adds to inaccuracy. These systems will typically have a measuring accuracy of plus or minus five percent.
