Open Cut » Overburden Removal
If a material or a structure is placed under stress, either mechanical, chemical or thermal, and if that stress is sufficient to alter the material's microstructure, a stress wave will be generated. These stress waves have become known as acoustic emission, and if correctly detected and analysed can provide much information relating to the structural integrity and in many cases the remnant life of the structure being monitored. In this project, acoustic emission technology has been applied to the boom of an operating dragline and the acoustic emission data has been analysed and corroborated by other monitoring results, including strain measurements and knowledge of the operating conditions.
The structural integrity of a dragline boom can be evaluated by analysing the acoustic emission data received from sensors placed on the boom structure. Variations in the nature of the detected acoustic emission will be produced by a combination of factors including the mechanical integrity of the boom; the way in which the boom is being operated (dig, swing full, empty, swing empty etc), and the nature of the material being handled, particularly in the digging mode.
To test the validity of the assumption that the condition of the boom of a dragline could be monitored using the data obtained from acoustic emission sensors, a dragline at the Howick Coal mine (Bucyris-Erie model 1570W) was instrumented initially with four acoustic emission sensors and eight strain gauges. Further data was obtained later using an extra two acoustic emission sensors and two strain gauges placed on the rear A-frame legs of the dragline. The signals were routed to a location near the operators cabin for acquisition of the data. Video monitoring of the operations of the dragline were also carried out to assist in the synchronisation of the data with the dragline operations.
The acoustic emission data was analysed using proven techniques developed by Metacoustics, and together with the results of strain gauging showed that information about boom integrity and the extent of probable boom damage as a consequence of the nature of various operations, could be determined using the available data. A large amount of information was recorded relating to the effects of different Operations on the dragline. Due to the unusual number of unscheduled events, the dragline did not achieve long term stability during the project. Thus the detection and prediction of small gas leaks at critical welds were not carried out as each monitoring program was either when the dragline was in very good condition (gas contained and no leaks) or very poor condition (no gas - all escaped). However, the available data indicates that there was sufficient sensitivity to allow detection of gas leakage within a surveillance radius of 10 to 15 metres from any transducer location. The effects of various dragline operations on the dragline structure were evaluated including variations caused by different operating techniques, different types of spoil being removed and normal operation.
The dragline was found to be a dynamic structure, subjected to both tensile and compressive fatigue as well as impact loading from both normal and abnormal operations. The over-run between operations, such as swinging with a full bucket before the bucket had been lifted well clear of the spoil, generated significantly more acoustic emission activity than completing each operation prior to commencing the next. The time difference in cycle operation was only about two seconds between the different techniques.
The project objectives did not include the estimation of the remnant life of the boom, however since there were a number of service/repairs made to the boom during the period of this monitoring program, the results of the data analysis indicated that the estimation of remnant life of the structure is feasible as part of any acoustic emission monitoring program.
It is now possible to offer a dragline monitoring technique and service to the mining industry for full, or segments of, dragline monitoring and structural integrity evaluation.