Description of the accident

Whilst towing a trailer-mounted core drilling rig up a ramp of an open cast mine, the “A” frame of the trailer snapped, resulting in the drill rig rolling backwards, down the ramp where it came to rest against a retaining wall.

Fortunately, there were no other vehicles using the ramp at the time and so no injuries or other damage were sustained.

Apparent causes

The drill rig was approximately eight years old and had been used on a continuous basis for most of its life. The drill rig and carrier had not undergone any structural integrity tests prior to its deployment to the mine. The mine inspection procedures did not require integrity tests to be conducted on the drill rig mast or carrier.

Recommendations

Exploration drill rigs are frequently towed or transported over very rough terrain that can result in accelerated fatigue in critical elements of the structure. Visual inspection of the drill rig, carrier and mast will not identify fatigue in any of the equipment and so all drills should be inspected by a certified NDT inspector at regular intervals.

There is much debate about what tests should be done and how frequently they should be conducted. The following recommendations are made:

Masts and drill rig carriers must be inspected at least every three years by a certified inspector, the tests should include:

1. Wall thickness tests on all tubular sections of the mast and the mast support structure.

2. Crack detection tests on all critical welds on the mast and mast support structure.

3. Crack detection tests on all outrigger mountings.

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