Open Cut » Maintenance & Equipment
The chronic failures and repairs of field plant structurals have been investigated with a particular emphasis on BE1370 and Marion dragline structurals. The investigation steps included:
- Review of history of structural failures, their classification and mapping (global and localised) of the location and type of failure
- Review impact of the failures and repair on the basis of maintenance cost, downtime and safety
- Review of present repair practices and techniques and their effectiveness
- Review and identification of best practices (including those successfully applied across other industries)
- Definition of factors contributing to selected critical chronic failures (e.g. boom clusters)
- Detailed investigations to validate the main contributing factors
- Development of guidelines for improved repairs for the chronic failures
The maintenance cost analysis, covering a period of ten years for eleven BE1370 and five Marion 8050 draglines, revealed that BE 1370 booms are the highest cost item in structural maintenance, followed by tubs. The BE 1370 boom repair cost is more than three times that of Marion 8050 boom repairs. In the case of Marion 8050 machines, the highest structural repair cost item is the tub repairs. The cost of unscheduled down time was several fold higher than the direct maintenance cost. In addition to the maintenance and downtime cost, the catastrophic failure risk emanating from chronic failures is highest for the boom. Based on this information, and with the agreement of the industry monitors for the project, the investigation focussed on the chronic cluster repairs of BE dragline boom clusters as the most critical area. The tub repairs, the second highest item identified through the cost analysis, were also investigated, albeit to a lesser degree.
The major factors influencing the BE main chord cluster cracking were identified at the early stages of the study and can be summarised as follows:
- High stress ranges in clusters (compressive for most clusters)
- High tensile residual stresses (as high as yield stress) causing the compressive stress ranges to become effective tensile stress ranges
- Quality of initial fabrications and repairs, particularly at the root pass of the lacing to main chord welds.
Improving any one of the above factors can contribute to an improvement in fatigue life of the cluster, but optimal results may be obtained by improving all three of these together.
The repair history showed that the most common remedial measure taken is to reduce the stress ranges in the main chords by increasing the chord wall thickness. This approach provided an improvement, but was of limited value if the other factors were not addressed. Therefore, minimising the residual stress effects and improving repair quality should be given higher emphasis in order to improve fatigue life of the clusters.
The measured and simulated stress ranges in the main chords of BE 1370 and Marion 8050 machines have been carefully examined. For similar RSL's the main chord stress levels in BE 1370 main chords are slightly higher than that in Marion main chords. However, the cluster failures in BE 1370's are several fold higher than that for Marion 8050 machines. The major reasons for the difference are the quality of initial fabrication and the residual stresses. By nature of the overlapping BE cluster geometry, it is difficult to achieve the same level of quality that is possible in Marion clusters.
The clusters more prone to cracking have been identified using history information and the results of FE analysis. The main areas in a cluster where cracking is common have also been identified for focussed attention during fabrication and repair. The investigation also shown that the welding sequence of lacing members did not have a significant influence on the stresses observed in these clusters.
The detailed investigations into repair quality indicated that the specifications in the BE Tubular Manual for fabrication and repair are reasonable, but the quality achieved in practice, particularly in cluster repairs, had not been satisfactory. The main reasons for this appear to be: lack of emphasis on quality; conditions under which the repairs are conducted; and the need to complete the repairs within a very short period of time to get the machine back to work. The desirable level of quality for specific application should be determined by individual operators on the basis of commercial and technical considerations.
As residual stress was identified as a major factor affecting fatigue life of the clusters, a detailed residual stress measurement program was undertaken to quantify the levels of residual stresses. These measurements showed very high levels of residual stresses in the clusters that can significantly affect the fatigue life of the clusters.
The key recommendations from the project can be summarised as follows:
- Identified critical locations in clusters for improved fabrication quality
- Improved weld details for critical areas in BE 1370 clusters
- Improved weld procedure for repairs - temper-bead welding
- Improved guidelines for window repairs
The recommendations are detailed in Section 11 of this report.
The project has significantly improved the knowledge and understanding on dragline structural failures and repairs, through a series of systematic and detailed investigations. The recommendations given were formulated based on this new knowledge and understanding. The implementation of these recommendations should significantly reduce the structural maintenance and downtime cost of draglines.