By Ryan Sharp and Arnold Williams of BMT WBM
Mining companies continue 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 greater pressure on plant and equipment in an effort to increase production and boost revenue.
|BMT WBM has been involved with dragline maintenance issues and improvement strategies for over 40 years.|
Ryan Sharp and Arnold Williams of BMT WBM, a subsidiary of BMT Group, believe a sustained increase in production can only be truly realised when robust maintenance procedures are in place. Taking a closer look at draglines, they consider current maintenance challenges and highlight how technology can help optimise and in some cases, reduce maintenance and inspection workloads.
THE mining industry has been dealing with difficult times. Reduction in demand for raw materials caused by the 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 purchasing new machines and have been focusing on devising upgrades to existing machinery to help improve production capacity through increasing payloads and reducing cycle times.
However, this 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 Original Equipment Manufacturers for operating 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 upgraded plant and machinery.
With strong emphasis on machinery availability and the continuing trend towards operating at increased rates of production, this ‘conventional’ approach is no longer sustainable and mining companies must look at utilising every available tool and technique to improve maintenance practices. Although the OEMs will provide maintenance departments with guidelines for servicing plant based on the specification on which it left the factory, 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 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’s important that the implications of this change are duly considered and thought is put into how you ensure 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 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 but with today’s structural modelling and analysis tools such as ANSYS, Femap, IDEAS, LS-DYNA and Abaqus, these decisions can be determined more effectively and efficiently.
Draglines are large items typically used to remove overburden. BMT WBM has been involved with dragline maintenance issues and improvement strategies for over 40 years. 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.
BMT WBM 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 on where to focus their attention, ultimately reducing the time needed for 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 years in other industries including aerospace, are being increasingly applied to mining machinery maintenance. RCM techniques can help identify component failures that impact availability most significantly and thereby, enabling appropriate solutions to be devised.
One recent example where RCM principles have been followed to significantly improve reliability of mining machine operation on a Marion 8200 Dragline is where a substantial revolving frame floor upgrade was carried out in order to design 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 of the OEM design in this area and the accumulation of fatigue damage through a long service for the machine. Further, in an effort to increase production, the mine was intending to increase the suspended load and was concerned about further exacerbating the problems in this area.
BMT WBM used a combination of field measurements to obtain the actual working stresses and analysis to propose a substantial floor design upgrade. The upgrade was implemented during a major maintenance shutdown for the machine and this upgrade has been in place for approximately six months.
Working closely with Westmoreland Coal Company, BMT recently deployed its innovative DuraCluster modification and repair scheme which dramatically 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 ease of installation and operational suitability.
Once implemented, this modification for tubular boom draglines significantly reduces maintenance and inspection workloads and dramatically reduces the problem of long-term fatigue cracking associated with the existing cluster design. BMT was able to offer 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 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 dramatically extend the fatigue life of dragline booms by reducing stress concentrations.
A boom replacement can cost around $20 million and 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, while multiple clusters can be modified simultaneously. With equivalent repair costs reduced to approximately $2 million, this is an attractive incentive for mining companies. While DuraCluster provides a step change in life to cracking for tubular boom construction, it is equally applicable to tubular masts.
|The BMT DuraCluster modification and repair scheme improves the fatigue performance of cluster joints.|
The innovative 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.
Another area of significant technological advance made over the past 20 years is dragline slew bearing, also known as the roller circle. The roller circle and the supporting structures 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 revolving frame, and very poor bearing life.
BMT WBM has developed supporting and repair techniques ensuring that the welding and machining of the upper and lower rail pads are done to a very high standard resulting in good bearing load distribution and long roller circle service life.
Although there is optimism for the future of mining 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 available technologies can play an integral role in realising these efficiencies but this is only one part of the jigsaw. Effective maintenance strategies which consider tools and techniques such as RCM and simultaneously align with the need to be production focused will create a step change in maintenance management, helping improve productivity and availability of critical assets and in many cases, reduce the overall cost of maintenance in the long-term.