Ryan Sharp and Arnold Williams
Mining companies around the world are continuing to feel the effects of low commodity prices, while the added pressure of China’s slowing economy is expected to lead to further market volatility. This has resulted in many companies putting even greater pressure on their plant and equipment assets in an effort to increase revenue through increased production.
Over the past few years, the mining industry has had to deal with difficult times. A reduction in demand for raw materials caused by the global economic slowdown led to coal and iron ore prices falling to their lowest level this century. As a result, mining companies are not able to justify capital expenditure on the purchase of new larger machines and much of the focus has been on devising upgrades to existing machinery to help improve production capacity through increased payloads and reduced cycle times.
However, increasing payloads and reducing cycle times often has the effect of reducing the service life of machine components and structures due to increased duty. With resistance to avoidable downtime, too often payloads are increased and cycle times reduced without the required machine upgrades being installed, based on the expectation that the increased maintenance cost and effort required would be more than justified in consideration of the increase in production.
The approach towards maintenance has often been ad-hoc and conventional, with maintenance plans for a piece of equipment often simply put together on the basis of recommendations or instructions obtained from the OEMs for the operation in the original machine configuration. As a consequence, certain preventative maintenance tasks have become standardised – remaining somewhat unchanged and unreflective of the change of duties or increased loads handled by the upgraded plant and machinery.
Towards more sophisticated maintenance
With strong emphasis on mining machinery availability and the continuing trend towards operating at increased rates of production, this conventional approach is no longer sustainable and mining companies must now look at using every available tool and technique to improve maintenance practices. Although the OEMs will provide maintenance departments with guidelines for servicing a plant based on its specification at the time of original commissioning, what many operating companies do not consider is the effect that increasing the machine’s capacity or duty cycle will have on reliability and the required maintenance.
Often, machines will be upgraded to operate significantly above their original design loading. Such upgrades create specific issues that cannot necessarily be dealt with in the traditional way, i.e. when something breaks, you simply replace it, or when it cracks, you weld it. This approach simply does not work when a machine has been pushed beyond the original design specifications as it leads to an unacceptable mean time between failures (MTBF). When increasing the load, it is important that the implications of this change are duly considered and thought is put into how to ensure the original design reliability is maintained to avoid further issues in the future. Otherwise failure rates will increase and availability will begin to fall away. A smarter approach to maintenance is certainly needed.
Advances in technology are noteworthy and have certainly impacted the way in which maintenance departments operate. The tools that are available for engineers are getting faster and more accurate. In the past, if there was a structural failure, it may have taken two to three weeks before a decision could be made as to whether to shut down production to fix the problem or continue operating the machine. With today’s structural modelling and analysis tools, such as ANSYS, Femap, IDEAS, LS-DYNA and Abaqus, these decisions can be more effectively and efficiently determined.
BMT WBM has been involved with dragline maintenance issues and improvement strategies for over 40 yr. Key areas of failure include boom, mast and roller circle. A more sophisticated approach to maintenance can, in some cases, reduce maintenance and inspection workloads and extend the fatigue life of these structures. The company has completed numerous finite element analyses, identifying high stress and fatigue prone areas of dragline structures. A map can then be created to guide maintenance inspectors to where to focus their attention, ultimately reducing the time needed for the inspections.
While maintenance planning in mining has been systematised for many years, techniques, such as reliability centred maintenance (RCM), which have been used over the last 40 yr in other industries – including aerospace – are being increasingly applied to mining machinery maintenance. RCM techniques can help identify the component failures that impact availability most significantly and thereby enable appropriate solutions to be devised.
Following RCM principles
One recent example where RCM principles were followed to significantly improve the reliability of mining machine operations is on a Marion 8200 dragline, where a substantial revolving frame floor upgrade was carried out in order to design out ongoing structural cracking issues. The cracking originated from large floor penetrations and propagated across the machine. The maintenance effort required to keep up the repairs were onerous. The problem stemmed from a pre-existing deficiency in the OEM design in this area and the accumulation of fatigue damage through the long service for the machine. Further, in an effort to increase production, the mine intended to increase the suspended load and was concerned about further exacerbating the problems in this area. BMT used a combination of field measurements to obtain the actual working stresses and analysis to propose a substantial design upgrade for the floor. The upgrade was implemented during a major maintenance shutdown for the machine. Currently, this upgrade has been in place for approximately six months.
Improving cluster joint performance
Working closely with Westmoreland Coal Co., BMT recently deployed its innovative DuraCluster modification and repair scheme, which considerably improves the fatigue performance of cluster joints on existing tubular dragline boom designs. This involved replacing a number of fatigued boom clusters with DuraCluster to demonstrate both ease of installation and operational suitability.
Once implemented, this modification for tubular boom draglines significantly reduces maintenance and inspection workloads, as well as dramatically reducing the problem of long-term fatigue cracking associated with the existing cluster design. BMT was able to offer both reduced downtime and outage costs. Once installed, DuraCluster also reduces the risks to operators and maintenance teams in having to lower the boom and carry out complicated weld repairs with limited access. Installation for Westmoreland was successfully completed in the allocated time frame and the dragline returned to duty.
The long booms of draglines comprise a number of tubular chords with interconnecting lacings welded to the chords at cluster joints. Stresses are concentrated at the cluster joint weldments and, over time, fatigue cracking becomes endemic. This methodology prevents the need to cut and replace windows in lacings by removing the problematic design detail and improving load paths. Furthermore, DuraCluster can extend the fatigue life of dragline booms by reducing the stress concentrations.
A boom replacement can cost in the region of US$20 million and would require a three month machine outage. With BMT’s modification and repair, the cluster design can be upgraded in around one week per cluster. Depending on the extent of chord repair required, multiple clusters can be modified simultaneously, with equivalent repair costs reduced to approximately US$2 million.
While DuraCluster provides a step change in life to cracking for tubular boom construction, it is equally applicable to tubular masts.The design allows lacings to be cut away from the chord, providing easy access to remove damaged or previously-repaired material. The exposed chord can then be inspected and fully weld repaired before installing the plate. Full patent rights for the DuraCluster design have been granted in Australia, South Africa, India and North America and discussions are taking place with potential clients in all of these territories.
Another area of significant technological advances made over the past 15 to 20 yr is the dragline slew bearing – also known as the roller circle. The roller circle and the supporting structures immediately above and below are vital mechanical and structural components of the dragline. Installation and maintenance activities in these areas carried out to a poor standard can lead to large amounts of cracking in the tub and the revolving frame, as well as very poor bearing life. BMT has developed supporting and repair techniques ensuring that the welding and machining of the upper and lower rail pads are done to a high standard, resulting in good bearing load distribution and long roller circle service life.
Although there is welcome optimism for the future of the mining industry as a whole, mining companies are focusing heavily on cost efficiencies and productivity gains in the short to medium term. Pushing innovation and embracing the technologies that are available can play an integral role in realising these efficiencies. But this is only one part of the jigsaw. Effective maintenance strategies that consider tools and techniques, such as RCM, and that are simultaneously aligned with the need to be production focused, will create a step change in maintenance management, helping to improve productivity and availability of critical assets and in many cases, reduce the overall cost of maintenance in the long term.
About the authors: Ryan Sharp and Arnold Williams work for BMT WBM, a subsidiary of BMT Group.
Read the article online at: https://www.worldcoal.com/mining/06112015/dragline-maintenance-developments-3125/