Theory of Constraints in Strip Mining

This research project was commissioned to investigate the feasibility of using Theory of Constraints to assist in planning the stripping process of a coal mine. The explicit aims were:

· Investigate how Theory of Constraints might be used in the stripping process;

· Model the stripping process;

· Find the constraints/bottlenecks in the system;

· Track the Work In Progress Queues through time;

· Model the effects of machine delays; and

· Investigate possible proactive application of TOC using Drum-Buffer-Rope.

To enable the strip mining process to be modelled effectively in terms of Theory of Constraints, a consistent "manufactured unit" which could be tracked through the mining process had to be defined. The chosen unit of manufacture is the "block", a unit readily used in mine planning.

In modelling the process there were a number of complications:

· Different blocks of material require different numbers and types of activities;

· There are a number of resources (machines or machine types) which perform multiple activities on the same block;

· The progress of the blocks is not explicitly tracked by the site; and

· There are machines which fit the definition of a bottleneck in all except the processing rates.

In response to this the following innovations were made:

· Software written to convert current (machine) progress recording to block progress recording;

· Defining the bottleneck by activity as well as machine group;

· Defining the actual and effective processing rates of an activity or machine group; and

· Identifying multiple bottlenecks in the system.

The modelling was done primarily on an 11 months plan provided by a site which had a combination of dragline and truck shovel stripping operations. Custom software was written to find the bottlenecks, display the work in progress queues and simulate delays to the plan. All of these applications could potentially provide benefit to sites. Analysis of the plan was provided in most sections to provide an example of how such information may benefit sites.

The software receives machine scheduling data from long term planning software such as XPAC and changes the data into information which tracks the progress of the blocks through the mining process, an essential part of assessing theory of constraints performance. The software display shows bottlenecks and queues either overall or divided by pits. The software also provides delay generation to test the plan's sensitivity to previously unplanned-for delays.

Of particular note was the delay simulation which provided analysis of a "stick to the plan" approach when delays occurred, rather than immediate relocation of resources whenever delays or conflicts occurred, which is often standard practice.

Drum Buffer Rope could not be adequately modelled with the resources available, however there is a significant body of work in this report showing how it might be achieved using the software tools developed for this study.

The research showed that Theory of Constraints can be useful to sites. Furthermore, it does not

require wholesale replacement of the current planning and management process. Using some of the tools made available in this project, the site can make incremental improvements in managing resources. These improvements should lead to:

· More consistent (or improved) production;

· Better production risk planning;

· Decrease focus on 100% machine utilisation (utilisation of 100% only matters for bottlenecks);

· Reducing the need for constant replanning when unexpected delays hit.