Optimising Production Scheduling for Maximum Plant Utilisation and Minimum Downtime
The Reliability Revolution
A Conference Paper presented to the Dollar Driven Mining Conference
Perth, Western Australia 
  By Sandy Dunn   July 1997
 

 

Introduction
     Traditionally, Mining organizations have focused on the key measures of Plant Availability and Utilization to measure equipment performance. This paper sets out to demonstrate that these measures alone are insufficient to make informed decisions about equipment strategies. It also sets out to demonstrate that, in practice, there is one factor, often overlooked, that has a significant impact on Equipment performance and that is Equipment Reliability. A focus on Reliability is revolutionizing the way that mining companies look at Improving Short-Interval Scheduling, and improving equipment performance. 
Production Scheduling Timeframes
     The following diagram indicates the different timeframes with which Operations and Maintenance are Planned and Scheduled. 
Timeframes of Maintenance and Operations Planning Diagram
     In the longer term, Life of Mine Plans help to determine the quantity and type of equipment required for achievement of that plan (and vice versa), thereby maximizing equipment availability and utilization.
     In the medium term, Operations plans interface with the Maintenance plan in order to maximize equipment availability and utilization by
  • adjusting planned maintenance start times due to changes in production schedules or shipping schedules.
  • taking advantage of maintenance windows as they become available.
  • ensuring preventive maintenance on critical equipment is carried out.
  • ensuring equipment is available for maintenance when planned.
     These concepts are well understood, and are generally well implemented in most mining organizations today, but this is not the focus of this paper. In this paper, we will focus on short interval planning and scheduling - the planning and scheduling that occurs on a week to week, shift by shift basis. It is in this area that the reliability revolution is occurring.
Availability, Utilization, and Reliability
     Before continuing, it is important to make sure that we all have the same understanding of the terms Availability, Utilization and Reliability, and introduce the concept of Overall Equipment Effectiveness.
    Availability: the proportion of time the equipment is able to be used for its intended purpose.
    Utilization: the proportion of the time that the equipment is available that it is used for its intended purpose. 
     It is important to realize the difference between availability and reliability. While availability measures the proportion of the total time that the equipment is available, reliability measures the frequency with which it breaks down.
    Reliability: how often the equipment does not fulfill its intended purpose - usually measured by Mean Time Between Failures (MTBF). 
     Clearly Reliability and Availability are related, but not necessarily directly - it is possible to have a piece of equipment that breaks down frequently, but for short periods, which as a result has a reasonable level of availability. Similarly, it is possible to have a piece of equipment that is highly reliable, but has a low level of availability because it is out of service for maintenance for long periods at a time.
     The traditional view of Availability and Utilization maintains that achieving high equipment Availability is a Maintenance responsibility, while achieving high utilization is a Production responsibility. By maintaining both high equipment utilization and high equipment availability, maximum output will be achieved from the equipment.
     Consider, however, the situation where a haul truck is operating, but, because of a problem with the engine, it can only haul at 80% of its normal speed. The truck is available, and being utilized, according to our definitions, but clearly maximum output is not being achieved.
     Consider also, for example, the situation where a shovel trips, causing a 15 minute delay while it is reset. During this time, the Haul trucks queue at the shovel. Once again, those trucks are available, and being utilized, but maximum output is not being utilized.
     Clearly, we need a better measure of overall equipment performance.
     We achieve this by including an additional measure - which I will call Production Efficiency.
    Production Efficiency: the ratio of actual output from a machine (which meets the required quality standards) to its rated output, during the time that it is operating.
     Poor reliability, while having some impact on equipment availability, is likely to have a bigger impact on Production Efficiency, due to the inefficiencies associated with starting up and shutting down equipment, and the time and effort that it takes to get the production operation back to a steady state situation. It is fair to say that the costs of poor reliability, generally show up in lower Production Efficiency. This is a measure that is often not given the same emphasis as Availability or Utilization measures, and in any case is generally considered to be a Production responsibility, with the impact of Maintenance on this figure generally being ignored.
 
 
     Furthermore, analysis of reliability figures at many mining operations indicates that the Mean Time between Failures (i.e. process interruptions), can be as low as a few hours. Not unsurprisingly, Production output in these operations falls well short of the theoretical rated capacity. In these operations, the impact of poor reliability far outweighs the costs associated with equipment availability and utilization (which are generally quite high).
     A relatively new measure being used for Equipment performance is Overall Equipment Effectiveness. This gives an overall measure of how effectively an asset is being used and is given by the following formula.
    Overall Equipment Effectiveness: 
     Overall Equipment Effectiveness is closely linked to the accounting measure, Return on Assets, and provides us with an indication of how well we are using our investment in Plant and Equipment. 
     If Availability, Utilization and Production Efficiency were all equal to 90%, we might be tempted to think that we are doing a pretty good job, but in fact, the Overall Equipment Effectiveness for this example only equals 73%. This means we are only getting 73% of the total potential output out of this equipment. Increasing this figure will mean that we can produce more with the same equipment, or potentially, could produce the same amount with less equipment - with an investment of in excess of $2m required for a large haul truck, the savings could be considerable.
The Scheduling Process
     The scheduling process (indeed any planning or management process) is made up of four key activities, as shown below:
     In short Term Mine Scheduling, forecasting is the activity that most determines the success, or otherwise, of the schedule. It is only when we can forecast with some degree of certainty, that we will be able to achieve the schedules that we have established. This, in turn, has a direct impact on equipment availability, utilization, and operating and maintenance costs
     For example, when scheduling routine haul truck services (typically done a week in advance), a forecast is made of the date and time at which the service will become due. In most Maintenance Management packages, this forecast is based on the Service Meter Unit (SMU) reading at the time the forecast is made, and a projection of the average running hours per day for the truck, based on past usage patterns. However, a breakdown, or some other process interruption, can significantly affect the running hours for the truck - to the extent that the service is now performed (unnecessarily) early - with increased maintenance costs, and increased downtime. Alternatively, a change in the forecast running hours of the equipment could lead to the service being performed late, with the implication of potentially increased engine wear, and future reliability and maintenance problems.
     Once again, it is reliability that has the greatest impact on the degree of certainty that we can have regarding forecasting - poor reliability means a high level of uncertainty regarding forecast operating hours for individual machines. This leads either to equipment being serviced, and components repaired, earlier or later than optimal, or equipment being given maintenance at short notice, and therefore in an unplanned manner, which increases the level of downtime associated with the service, overhaul or repair. 
The Reliability Revolution
     So, to summarize so far, better mining operations are realizing that: 
  • Poor reliability has a far greater impact on operating efficiency, and therefore unit operating costs, than it does on the traditional measures of availability and utilization.
  • The cost of poor reliability is, for the most part, hidden to most mining operations today.
  • When measured, the true cost of poor reliability in most mining operations is very significant.
  • Poor reliability also has an adverse impact on our ability to provide accurate short term forecasts for equipment operating hours. This, in turn, leads to either:
  • Equipment services being performed unnecessarily early, with resulting increases in maintenance costs and downtime, or
  • Equipment services being performed late, leading to the risk of in-service failures and reduced equipment life, or
  • Equipment services being performed at short notice, in an unplanned manner, increasing the downtime associated with these services.
     These better mining organizations are beginning to focus less on the traditional measures of equipment performance - availability and utilization, and are starting structured programs to address reliability issues. A new shift in paradigm is occurring, which I term the Reliability Revolution.
Factors Impacting on Reliability
     As defined previously, reliability is the frequency with which equipment does not fulfill its desired purpose. Anything, therefore, which causes an interruption to the normal operation of the equipment can be defined as a failure. While it is common to consider reliability as being primarily a Maintenance concern, in fact, there are generally as many Production issues that cause interruptions to normal operations. The factors that could interrupt normal operations include:
  • Geology - Variability in digging conditions can lead to the need for shovels or trucks to stop, even momentarily. Similarly, ore grade that is different from what is expected also can cause the need for an interruption to production.
  • The Mine Plan - The mine plan generally calls for equipment to be moved periodically as different areas are to be mined. This causes an interruption to a steady-state production process.
  • Accident Damage - A production issue, which causes an interruption to the production process if the equipment must be taken out of service for inspection or repairs.
  • Equipment Failure - Clearly a Maintenance issue, which causes an interruption to the production process.
  • Routine Maintenance - Routine Servicing, Component Replacements and Overhauls cause interruptions while the equipment is taken out of service.
  • Weather - Rain or fog can interrupt the production process.
  • Downstream Processes - If in a direct tipping situation, if the downstream process stops, this can cause an interruption to the mining operation. If during this stoppage, ore is tipped onto a stockpile, a further interruption is experienced when the downstream operation starts up again.
  • Shift Changes and Crib Breaks - Every shift change and crib break causes an interruption to the steady-state nature of the operation.
  • Spillage and Housekeeping - The need to stop to clean up spillage in the vicinity of the shovel, or in the dump area, also causes an interruption the process.
  • Minor Production Stoppages - "Comfort stops", mirror adjustments and other minor stoppages interrupt the production process
  • The Blast - Often there is a need to stop the equipment during a blast. 
  • Ineffective Blasting - If the blast is ineffective, this can lead to problems with diggability in certain areas. This also causes equipment not to operate reliably.
  • Refueling and Lubrication - Stopping equipment to refuel and lubricate them also interrupts the production process.
Improving Reliability
     So where to from here?
     The first step is to measure the number of times your equipment is stopped, together with the reasons for those stoppages. It is a truism that "What cannot be measured cannot be managed". Clearly those organizations with automated pit control systems will find it easier to record the number of stoppages than those without these systems, but in either case, it is vital to be able to accurately record the reason for the stoppage - and this is a human issue, not a technology issue.
     The second step is to perform a Pareto Analysis on the data collected in the first step - that is, identify the few reasons that account for the majority of the stoppages. Focus your attention on these stoppages, because a small improvement in reliability in these areas will have the greatest impact on overall equipment performance.
     The third step is to analyze these high frequency stoppages, and identify the opportunities for improvement. It is worth noting, at this point, that the possible causes of stoppages listed above can be categorized as being either planned or unplanned stoppages - that is, the stoppage is generally known about in advance, or it is not. These are tabulated below.
Planned Unplanned
The Mine Plan Geology
Routine Maintenance Accident Damage
Shift Changes and Crib Breaks Equipment Failure
The Blast Weather
Refuelling and Lubrication Downstream Processes
  Spillage and Housekeeping
  Minor Production Stoppages
  Ineffective Blasting
 
     Clearly, for those stoppages that are planned, the strategy should be to combine the stoppages, and reduce their duration - for example, conduct blasts during crib breaks, or refuel at shift change. These concepts are generally well understood and applied in all mining operations. However, some lateral thinking may suggest some innovative approaches - for example, performing "pit stop" services during crib breaks, with the aim being to turn the equipment around within the time allowed for crib. 
     For unplanned stoppages, the aim should be to eliminate them or reduce their frequency, or to convert the stoppages into planned stoppages that can be combined with other planned stoppages. There are a number of analytical techniques available to assist in achieving this objective, such as Ishikawa or "Fish Bone" diagrams. For analyzing Equipment Failure issues, Reliability Centered Maintenance (RCM) techniques have had significant success with both fixed plant and mobile plant in a mining environment - doubling reliability in some instances.
     It is not the intention of this paper to go into detail on these analytical techniques, rather, if, after this conference, you all return to your minesites with the determination to formally measure, analyze, and improve the reliability of your equipment and production process, then this paper will have achieved its aim.
This article is contributed by Sandy Dunn, who maintains excellent plant maintenance reference material at:
www.plant-maintenance.com
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