A man was killed last year after being buried alive when a metal crane rope snapped and more than a tonne of concrete landed on him. The 48-year-old labourer was working on a building site in a suburb of Melbourne, Australia on 6 September 2018 when the crane’s cable, which was lifting a bucket full of concrete, broke. For several frantic minutes, the victim’s co-workers tried desperately to free him, but he died before they could pull him out of the rapidly drying concrete. A second man was rescued after suffering critical injuries, and a third escaped with minor injuries.
This horrific incident highlights the vital need to test wire rope to prevent catastrophic failure. But there’s a snag: a visual inspection will not necessarily show problems, because any deterioration that might have occurred in service will probably be in the core of the rope.
Thankfully, there is an answer: metal rope testing (MRT). Denis Hogan, performance and special projects manager at the Lifting Equipment Engineers Association (LEEA), explains: “There is only one MRT method, which involves passing the rope through a permanent magnet. This sets up an electromotive force, which is picked up with electronic sensors that can detect any breaks in the rope or any corrosion that occurs throughout the section of the rope, known as ‘loss of metallic area’. There are different MRT equipment makers, but the method is exactly the same with each one.”
According to LEEA, the MRT process inspects even the centre of the rope, giving a clear indication of any broken wires, voids and corrosion. Rope manufacturers perform an MRT on some products during the final stage of manufacture when they close the strands round the core. This produces a trace showing the state of the rope when it is created, but before it’s installed.
It adds that by checking on a regular basis (typically six months, although the period will depend upon how often the crane is used), one trace can be compared with the next to observe developing defects. When rope meets the rejection criteria, change can be planned in without fear of rope failure, maximising uptime.
Hogan says that examining the core of the rope without MRT requires a special tool to open the rope’s strands. But, he points out, this still only gives visibility of a small percentage of the rope’s length. He adds: “On multi-strand crane ropes, you can never see the core because the multiple layers can’t be opened up, due to the underlying layers being laid in the opposite direction to the outer strands.”
This makes MRT a vital tool for safety and maintenance systems. LEEA points out that the process was developed in the mid-1950s by the National Coal Board in conjunction with British Ropes (now Bridon-Bekaert The Ropes Group) for deep-shaft mine ropes hauling cradles.
Barrie Mordue (inset), director of wire rope specialist Tensology, points out that the bulky equipment of 40 years ago has been replaced by lighter, more powerful magnets. He adds: “But the biggest change [to MRT technology] is computerisation – digital information and signal analysis is much better on computers. You can generate automatic reports that tell you how close to discard the rope is; previously, it was all done on old analogue X-Y recorders.”
For lifting applications, he says, there is a legal requirement under the Lifting Operations and Lifting Equipment Regulations (LOLER) that every 12 months – or sooner if deemed necessary by a responsible person – all lifting equipment including the wire rope should be thoroughly examined. If the equipment is lifting people, the inspection period should be once every six months.
Mordue adds: “The main standard for carrying out the thorough examination of a wire rope is ISO4309: 2017 Cranes – Wire ropes – Care and maintenance, inspection and discard. This tells you how to inspect, what to look for, and contains tables telling you when a rope needs to be taken out of service. Ropes are consumable items – they deteriorate and have to be changed; ISO4309 sets out the criteria for discard.”
According to that standard, non-destructive testing of steel wire rope for cranes is important to ensure that the condition of the rope is monitored from cradle to grave. This allows operators to predict the point where the wire rope actually needs to be changed, allowing planned preventive maintenance and maximising uptime.
MARINE APPLICATIONS
But construction is not the only industry that uses wire rope. MRT has also become crucial for routine monitoring of ropes in offshore applications, where minimising costly downtime is particularly critical. The technology is discussed in a guide on the condition monitoring of wire rope in offshore installations published by the International Marine Contractors Association (IMCA), which is starting to drive greater awareness for using the technology in ports.
MRT technology has progressed greatly, and IMCA has revised its document – Guidance on Examination of Steel Wire Rope through Magnetic Rope Testing (originally published in August 2009) – to reflect this. The latest document (https://is.gd/awuquz) provides guidance on the use of non-destructive examination by means of MRT for wire ropes, to assist with inspection and integrity management. It contains new sections including rejection criteria and MRT equipment operator levels of competence and training. The latter outlines three levels of competence:
● Level 1: Can set up the MRT instrument, perform tests, record the trace, conduct a visual inspection and understand modes of deterioration
● Level 2: Can perform and supervise MRT testing according to recognised procedures. Has carried out a minimum of 10 MRTs as level 1. Qualified to choose the test method technique, set up and calibrate the instrument, interpret and evaluate the results and prepare written MRT instructions
● Level 3: Carried out at least 20 MRT inspections as level 2. Qualified to supervise level 1 and 2 personnel, organize and report the results of MRT inspections, recommend the use of other non-destructive tests and be aware of materials, fabrication and product technology to select MRT methods.
Mark Ford, technical manager at IMCA, outlines a particular characteristic of wire ropes used in marine applications: “At depths of 1,500m to 2,000m, water pressure can force the manufacturer-applied lubricant out of the rope. Wire rope is effectively a machine; it relies on lubrication, otherwise there can be severe wear where the wires cross over and where they bend over the sheaves.
“MRT gives a good insight into what is going on inside the rope. In the offshore industry, we work with rope diameters of more than 180mm. You need a very strong magnet to test rope of that diameter.”
He adds that although older MRT equipment could only really cope with smaller-diameter ropes, the new MRT equipment enables testing of larger ropes; the standard was revised to reflect this.
LEEA’s Hogan has the last word: “Many operations still rely only on visual inspection of wire ropes. Plant engineers need to be made aware of the existence of MRT and the crucial role it can play in maintenance – particularly on high-integrity cranes that are being used intensively. MRT gives the ability to predict the life expectancy of the rope, allowing them to plan replacement in a just-in-time manner, without the need for bulky wire rope to be consuming space in stock.”
BOX OUT: Grease extends steel wire rope life
Lubrication used in multi-strand metal ropes performs several useful functions, reducing friction and boosting performance.
However, steel wire ropes used for marine applications – on ships and offshore rigs, for example – are usually exposed to extreme weather conditions and will, of course, come into contact with the harsh effects of seawater. In this environment, a lubricant has to fulfill a demanding task: it has to offer not only good adhesiveness and water resistance, but also meet requirements regarding eco-compatibility as well.
Klüber Lubrication has developed Klüberbio AM 92-142 grease for precisely this kind of application. This environmentally-acceptable lubricant provides corrosion protection to steel wire ropes exposed to salty seawater. Due to its adhesion, water resistance and thermal stability, the grease – also compatible with galvanised ropes – is said to be highly effective when used in small quantities, a characteristic that also reduces costs.