Paper published by Port Equipment Manufacturers Association (PEMA)
The Port Equipment Manufacturers Association (PEMA) has published an information paper on container weighing technologies – a timely resource for the ports industry following the IMO’s recent decision on a mandatory worldwide system for verifying container weights.
“Container weighing is an increasingly hot topic in the global shipping industry and recent legislation passed by the IMO indicates that ports worldwide will have an increasingly critical role to play in checking and verifying container weights,” notes Ottonel Popesco, PEMA President.
Following much international debate on the best approach to reduce the incidence of mis-declared and overweight containers, on September 20 the International Maritime Organization (IMO) finally approved a compromise proposal for verifying the weight of containers before they are loaded onto ships. The new regulations will make container weighing compulsory, or to have their weight otherwise calculated.
PEMA’s new report, entitled Weighing Containers in Ports and Terminals, provides ports, terminal operators and other interested parties with information about the port-based container weighing technologies and systems that are currently available and their relative capabilities to accurately verify container weights. The paper covers both weighbridges and weighing systems for use on various container handling equipment, including ship-to-shore container cranes, mobile harbour cranes, RTGs, RMGs, straddle carriers, reach stackers and container handling forklift trucks. The document concludes a table summarising the various technologies and their accuracy.
The goal is to provide ports, terminals and otherinterested parties with a clear understanding of thevarious technologies available today and their relativecapabilities.
Image Source: PEMA
Certification, Calibration & Accuracy
Container weighing systems must be certified if theinformation generated from the system (i.e., weightdata) will be used as the basis for commercialtransactions. Weighing system certification isperformed through national regulatory authorities orcompanies accredited by those authorities.
Container weighing systems must be certified if theinformation generated from the system (i.e., weightdata) will be used as the basis for commercialtransactions. Weighing system certification isperformed through national regulatory authorities orcompanies accredited by those authorities.
When discussing the precision of weighing systems,the practice in this document is to refer to thesystem’s inaccuracy, rather than its accuracy.The weighing measurement inaccuracy as referencedin this paper always refers to a percentage of fullscale. This means that if the maximum weightrange of the weighing device is 40 tonnes and itsinaccuracy is 1% as a percentage of full scale, then the inaccuracy expressed as an absolute weight willbe +/- 400kg. It should be noted that the absoluteinaccuracy will also depend upon the position of themeasuring device.
For instance, if the measuring device is positionedin a quay crane trolley, its measuring range must bespecified to cover not only the maximum weight ofthe container, but also the weight of the spreader andheadblock, as well as the crane cables and ropes.The weight of these additional items can easily bededucted from the total weight. However, the potentialdeviation resulting from the higher measuring rangewill affect the absolute inaccuracy of the device whenused as a container weighing system.
As this aspect is highly dependent both on the typeand manufacturer of the equipment in question, andon the specification of its component parts, it is notincluded in the indicated inaccuracies given in thenext chapter for the various weighing alternatives.
However, ports and terminals must be aware of thisrequirement when planning to implement handlingequipment-based container weighing systems.
Weighing System Alternatives
1. Weighbridges
Weighbridges offer a vehicle weighing solution, oftenpositioned at site entrances/exits. The incoming truckpasses over the weighbridge and the total weight isregistered. In order to determine the container weight,the tare weight of the truck is deducted from the totalweight. The tare weight of the truck can be measuredif the truck leaves the terminal empty or the weight ofthe truck can be declared when entering the terminal.Weighbridges should be recalibrated periodically byan accredited technician to ensure accurate weighing.
2. Load cells on STS cranes
Load cells on ship-to-shore (STS) cranes are installedat the rope ends on the crane trolleys or booms, inthe sheave pins, or elsewhere in the rope system.They are often intended to perform principally as anoverload protection, rather than a weighing, system.Load cells have a weighing inaccuracy of about 3-5%of the maximum STS crane lift weight and need to beregularly re-calibrated.
Nowadays, all new STS cranes have such an overloadprotection system installed as standard. There arealso several suppliers that provide such systems forretrofit installation on existing STS cranes.
3. Load cells on RTGs
Load cell systems are also available for use on rubbertyredgantry cranes (RTGs). In this application, theload cells are installed in the shafts of rope sheaveson the trolley or in the rope anchors. These systemshave an inaccuracy of about 3-5% and need to bere-calibrated regularly. Again, installation on existingcranes can be complicated if there are no drawingsavailable of the sheaves and shafts. If the cranes areequipped with twinlift spreaders, the load cells cannotmeasure each container individually.
4. Weighing systems onMHC
A mobile harbour crane (MHC) needs a weighing system to control the stability of the crane, as theload weight is strictly limited depending on the boomoutreach. These systems are therefore normallyintegrated by the crane manufacturers during theproduction process. MHC weighing systems oftenmeasure the hydraulic pressure in the boom liftcylinders or may be built into the rope system.
5. Load cells on straddle carriers
Load cell weighing systems have also been installedon straddle carrier hoist systems. However, thedegree of inaccuracy is quite large. Similar tocontainer crane load cell systems, these systemscannot weigh containers individually if the straddlecarrier is fitted with a twinlift spreader, nor can theydetermine container loading eccentricities. Accidentsare common with straddle carriers during twinliftoperations, when one container is loaded and one isempty or very lightly loaded.
6. Weighing systems on reach strackers
Most reach stacker manufacturers provide builtinsystems which use the hydraulic oil pressure inthe boom lift cylinders to measure load weight. Thesystems are rather limited in accuracy and performancemay vary depending on whether the reading is takenafter a hoist movement or after the lowering of a load.
7. Weighing systems on container handling FLTs
Weighing systems for container handling fork lifttrucks (FLTs) usually work by measuring the hydraulicoil pressures in the lift cylinders. Alternatively, loadcells may be installed under the chain anchors. Theinaccuracy of these systems is usually due to frictionin the hydraulic cylinders. Currently, it is not verycommon for weighing systems to be installated oncontainer handling FLTs.
8. Weighing systems using spreader twistlocks
More recently, weighing systems have been developedwhich measure the load weight and eccentricity onthe crane spreader twistlocks. These systems aremore accurate than any of the above-mentionedtechnologies, with the exception of weighbridges.They can also weigh each container individually intwinlift mode, determine container weight eccentricity,and have a variety of safety features to help preventaccidents during handling operations. Twistlockbasedsystems require no infrastructure changesto the terminal and can be installed on all type of spreaders at point of manufacture or as a retrofit.Use of this weighing technology does however meanthat the terminal must install the system on all of thespreaders used together with the crane(s) in question.
9. Where in the terminal to install a weighing system
There is no universal place to install a weighingsystem in a terminal. The choice depends heavilyon the logistics flows of an individual facility, not leastits mix of export/import versus transhipment traffic. Italso depends on the size of the terminal and the typeof container handling equipment in use. In general,though, weighing a container only when it beingloaded onto a vessel is too late in the process, as thiswill not allow the ship stowage plan to be updated incases where container weight has been mis-declared.
The earliest a container can be weighed whenbeing exported out of the country is with aweighbridge as it enters the terminal. However,transhipped container weights are more efficientlyverified when they are in the stacking yard.
An important factor for the terminal to consider ishow the strategy to verify container weight affects thelogistics flow in the terminal. Many of the alternativesdescribed in this document measure the weight aspart of the regular lift cycle, while the weighbridgealternative requires that a transport vehicle takesthe container to a weighing station. If weightverification is needed, for instance in a transhipmentterminal, this extra activity within the terminal willmean a significant change in terminal logistics.
For more information read the PEMA Paperon Container Weighing Technologies
