Coal Inspectorate | Bulletin | No.212 V 1 | 04 April 2023

Overburden drill rollover – Investigation summary

What happened?

During a night shift on 14 September 2021, two Coal Mine Workers (CMWs) (maintainers) arrived at an overburden drill rig to inspect and repair a reported oil leak. In preparation for the repair, the machine was washed and jacked to near full height with the mast raised. The two maintainers were standing on the deck of the machine and the operator was in the cab when the drill began to slowly topple, coming to rest on the drill cab side, and making contact with an adjacent light vehicle (refer Figures 1 & 2).

The operator and one maintainer exited the drill safely. The other maintainer was trapped between the deck handrail and the light vehicle parked beside the drill. An emergency was initiated, and the Emergency Response Team (ERT) assisted with the safe recovery of the trapped maintainer.

How did it happen?

Events leading up to the incident

• The operator completed drilling a hole as per design when machine sensors detected low hydraulic oil level. The mast of the drill was in a raised position at around 25 degrees off vertical.
• Two drill maintainers arrived at the overburden drill to inspect and repair a reported oil leak.
• In preparation for the repair the drill was washed, lowered off its three hydraulic levelling jacks, refuelled and the hydraulic tank refilled.
• Using the drill levelling jacks, the operator commenced to raise the drill to a height that would enable a maintainer to walk under the machine, inspect and trace the the hydraulic hosing.
• The drill operator was unable to get the left-hand cab side jack to full height as the jack stopped lifting.
• The maintainers decided to isolate the rig when it began to slowly tip to the left side of the cab, coming to rest on the ground, having made significant contact with an adjacent light vehicle (refer Figure 2).
• The operator was able to extract himself from a cab side door and one maintainer was able to jump off the walkway onto the ground away from the machine.
• The other maintainer was unable to jump off the rig and was caught between the deck handrail and the light vehicle parked beside the drill.
• An emergency was initiated and ERT assisted with the safe recovery of the trapped CMW.

Key issues

The investigation identified that the toppling of the drill while on levelling jacks was caused when the maximum lateral stable angle of the drill was exceeded. Contributing factors were:

• Unlevel ground at the drill location.
• Ground fill and degree of consolidation.
• The jacked height of the drill.
• Hydraulic cylinder creep or failure, or other form of movement.
• Lack of clear definition, knowledge and monitoring of operating limits on jacks.

The investigation identified that the cab side hydraulic levelling jack cylinder piston and rod seals were severely worn, failed to carry the load and were unlikely to support the drill. Testing of the cylinder showed the hydraulic system operating pressure and cylinder relief pressures were well below what is required according to the operating manual.

Survey data coupled with the jack extension measurements taken after the incident, indicate that the drill was outside safe working limit design parameters for roll/tilting.

Stability alarms installed at the time of the previous incident did not alert the operator when the drill was positioned outside the equipment design parameters, when supported on levelling jacks.

Drill training requirements did not include information related to the safe operation of jacks including risks associated with imbalance between jacks.

The maintainers intended to inspect and be underneath the drill whilst it was raised and only supported by the hydraulic levelling jacks.

Recommendations

All Site Senior Executives should:

1. Review and update as required, site processes and standards for the introduction of equipment to:
• ensure that third party suppliers of equipment provide up to date safety files including maintenance and operations manuals, and evidence of risk assessments covering major changes or additional items added post Original Equipment Manufacturer (OEM) build.
• require OEM / third party providers of drilling equipment to provide equipment that has alarms fitted that alert the operator of unstable conditions during travelling and drilling operations. Further, that the alarms are installed and set in accordance with designated safe operating limits and ground conditions.
• ensure operational risk assessments are conducted for the introduction and use of plant. The potential for rollover to be considered in all operating conditions. Risk assessments should include suitably qualified and competent CMWs including OEM, drilling contractors and subject matter experts.
• review operational risk assessments, including ground condition checks and grades when relocating to a new site.
2. Update site standards, related documentation and training requirements, and communicate this to all affected CMWs.
3. Ensure CMWs are aware of the risks associated with working under suspended loads unless appropriate jack stands are in place.
4. Implement and monitor systems which validate that CMWs are following the requirements of the documents held in the Safety and Health Management System to maintain an acceptable level of risk.