In 2008, the World Shipping Council and the International Chamber of Shipping issued guidelines about the safe transport of containers by sea including a chapter on the weight of the container and the goods it carries. Since the report was published, various [major] shipping accidents have happened whereby the weight of some of the containers on-board might have been the cause. But these guidelines state that terminal operators should verify the weight of the containers before they are loaded on to the vessel. So, without any enforcement of these ‘guidelines’ are the terminal operators really weighing the containers or are they ‘happy’ with the information on the shipping documents? Bearing in mind that since these guidelines were published no actual steps have been taken to enforce these recommendations although few would deny that the weighing of all containers before they are loaded on-board a vessel would represent a major advantage in improving vessel safety. Even the European Shipping Council (ESC) supported the guidelines and takes the view that eradication of over-loaded containers would create greater efficiency and productivity in the supply chain by providing more efficient container throughput, improved vessel loading and more efficient container terminal operations. For terminal operators the main question is how to integrate technology that can detect overweight or wrongly stuffed containers without affecting the efficiency of the terminal and at what cost? Not many manufacturers of container handling equipment paid a lot of attention to this issue until French-based Lasstec came up with a detection method using the twistlocks on the spreader to identify both overweight containers and containers that have unevenly distributed weight. Beat Zwygart, Director at Lasstec explained that the company was the first to develop and patent load weighing using the spreader twistlock as a load cell. The system features the use of fibre optics which are the most compact, reliable and accurate technology available. “Development began in 2007 and since that date continuous testing and refinement has been accomplished,” says Zwygart. “Today, Lasstec’s Smart Twistlock Load Sensing System is fully tested and market ready.” A specially designed sensor is inserted into the centre line of the twistlock which measures the elongation of the twistlock shaft when under load. The load data is sent into the crane PLC and it is displayed on the monitor in the crane cabin. The system also registers the life cycles of the twistlocks so that they can be replaced when they reach ‘end of life’ based on usage and not when the date to replace them has arrived, making optimum use of the twistlocks. The load history data stays with the twistlock and can be retrieved at any time for evidence tracking. The system can be installed into new and existing spreaders and does not require any twistlock or spreader modifications. Using the spreader twistlock to measure container weights has been tried before with conventional strain gauge technologies or with hydraulic load cells. However, excessive torsional and bending forces as well as dynamic and shock loads have never produced satisfactory long term results, or accuracies were not acceptable. The only area on the twistlock where no torsional nor bending forces are present is in the neutral centre line. Fibre optic sensor technology presents itself to measure elongation of the twistlock and therefore measure container weight. The advantage of fibre optic over conventional strain gauges is that fibre optic requires very little space; it is inert to EMI and does not corrode. The Lasstec sensor has a diameter of only 3mm and the hole in the twistlock has therefore no measureable effect on strength of the twistlock. The sensor is of solid design and is totally shock proof [a patent was granted for this invention]. The sensor is bonded into the twistlock with conventional adhesive and cannot be recuperated when the twistlock is replaced. The sensor is therefore conceived as a replaceable item, again a feature possible only with fibre optic technology. The accuracy and load reading repeatability is within +/-100kg per twistlock. Temperature fluctuations of the twistlock are compensated by the sensor, which is a technology developed by Lasstec. There are three major advantages of making the sensor an integral part of the twistlock:
– Life cycles are traceable to the particular twistlock in case of an accident [the sensor always stays with the same twistlock].
– Life cycles are stored with the twistlock and twistlocks replaced when the life cycles are consumed and not when the replacement date has arrived. Its life cycle history can be reconstructed.
– Any type and brand of standard twistlock can be used.
The cost of the sensor is largely compensated with the optimised usage of the twistlock. An analysis of 4000 container moves in a RTG application has shown that 25% of all moves are below 2 tonnes per twistlock (empty containers) and 45% are below 3 tonnes. Empty containers are not considered as real load cycles. Spreader manufacturers propose to replace twistlocks after a certain number of container moves, whether these have been loaded or empty containers. On the other hand, a twistlock which has been stretched due to an accident can be spotted right away and replaced. A significant advantage of using the twistlocks for weighing containers is that in a twin-lift or even tandem mode, all container weights can be taken simultaneously. In addition to weighing containers, the system also provides a whole range of accident prevention features. These include
– Snag load detection in ship to shore applications. Measuring a snag load at the twistlock is faster than detecting it on top of the hoist trolley. It also allows detection of snags by individual twistlock in case only one corner of a container remains attached to the stack on the vessel. This minimises potentially serious problems especially in twin-lift or tandem applications.
– Prevention of lifting of the road trailer if it is accidentally still attached to the container or prevent lifting of railway wagons in case the interlocking twistlock of the wagon is jammed or not released.
– Adjustment of the spreader on a mobile harbour crane for load eccentricity when picking up containers in the ship cell. This avoids tipping of the spreader and container when exiting the ship cell. This is especially crucial in twin-lift modes.
Yet another unique feature of the system is the ability to detect if a spreader is landing on the ground or on the bottom of the vessel hull, thus avoiding the unreeling of the hoist wires. In such a situation, the landed pins of the spreader are not activated and don’t signal to stop the hoist system. This feature also allows identification of interlocking twistlocks that have been accidentally left on top of a container and prevent the spreader twistlock from entering the corner casting to pick the container up from the vessel. The Lasstec system is delivered as a “plug and play” system and can be added to the spreader by the terminal or by a Lasstec agent . Furthermore, it can be integrated into the spreader by the manufacturer during the construction of the spreader. Output signals are available in CAN-, Mod- or Profibus. Conversions into mA are optional. “The system has been on the market since early last year and the concept is now widely accepted by equipment makers and terminals worldwide,” says Zwygart.
Competition
Leading spreader manufacturer Bromma recently announced that it will launch a system based on existing load measuring technology implemented into a new format to fit with new and existing twistlock pins. This system is based on strain gauge technology implemented in a load sensor body mounted on the twistlock pin assembly. “Data from the twistlock load sensors will be collected and computed in the control system available on the spreader or on a stand-alone control unit,” said Lars Meurling, Vice-Pre
sident, Business Products Manager at Bromma. The new load sensing system for the spreader twistlocks – similar to that of Lasstec – will identify container overloads, understatement of container weight, and out-of-balance eccentric loads – all of which pose a well-known safety risk at terminals. According to Meurling the new load sensing system joins a full and expanding portfolio of other safety solutions available from Bromma including the twin-twenty detection system enabling crane operators to detect a gap between two 20′ foot containers in a 40′ cell [a real risk and a potentially dangerous lifting situation] by using a 7 photo-electric sensor. Bromma also offer their fully automatic over-height frame that is handled by the crane, which keeps ground service staff out of harm’s way and their spreaders include failsafe logic and interlocks to prevent accidental container unlocking.