Open Cut » Overburden Removal
In this project CMTE developed a rigorous procedure to assist the dragline owner to identify the optimum operating point for a given dragline in terms of its bucket size. This optimum point is based upon the trade-off between two opposing factors: the increased instantaneous productivity that may arise from a larger bucket; and the loss of reliability, increased downtime, and lost production that may arise from increased rate of wear.
The operation of the optimisation procedure is demonstrated on DRE23 of Peak Downs, which is a BE 1370W machine. The determination of the optimum point is based on the following assumptions:
- The direct costs associated with maintenance and production functions are not affected by varying the payload. This is not an unreasonable assumption especially if the maintenance work is carried out by the company staff, rather than contractors called in on demand. In any case, it is not difficult to modify the method by incorporating the variation of production and maintenance costs with payload.
- CMTE are assuming 100% utilisation. In other words, if the machine is available for production, it can be used in production.
- The payload is uniform through the year, i.e. we use the same bucket through the year.
- CMTE also assume that they are able to sell all they can produce at the same unit price.
- CMTE assumes that there is a baseline recommended maintenance program corresponding to the payload size recommended by the manufacturer and a preventive maintenance program associated with this recommended payload.
They then calculated the effect on availability of varying payload by reference to this baseline. This is probably the most critical assumption of all and would require a detailed analysis of the machine strength unless such is already available from the manufacturer.
With these assumptions, the payload optimisation problem is reduced to maximising the number of tonnes produced over a certain period. Based on a number of maintenance parameters and using functions that describe the relations between payload and reliability, it is possible to solve this optimisation problem.
As the suspended load is increased, the increased productivity benefits are first large enough to compensate for the decreased machine availability and the tonnes shifted per year increases with increasing the bucket size. However, the wear and tear on the machine is a nonlinear function of the suspended load and if the suspended load is increased above a certain point (about 130% of the recommended suspended load for the dragline DRE23), the increased productivity benefits are no longer sufficient to compensate for the increased downtime.
While reading this Report, it should be noted that the project moved through two stages:
- The first stage established the safe working envelope for the DRE23 and the relation between the individual component duties and the suspended load. This required construction of electro-mechanical and finite-element models and validation of these models through field testing. The results of this first stage have been released as an Interim Report in September 1999, which is included in the final report.
- After carrying out this work, they were in a position to formulate the reliability relationships for individual structural, mechanical and electrical components. The resultant objective function for optimisation and its solution are described in this Report.