Description of the accidents

We have been made aware of several accidents relating to drill rigs toppling over or moving whilst set-up and drilling. Three of these instances are summarised below:

1. A significant amount of rain resulted in the ground becoming saturated causing a rear outrigger to sink into the soft ground. This resulted in the drill rig toppling over.

2. The piston on a hydraulic outrigger leaked which caused the outrigger to move resulting in the drill rig toppling over.

3. A diamond core drilling machine, involved in drilling shallow angle boreholes (inclination close to 45 degrees) suffered a snapped quill rod (kelly rod) when the drill rig moved off of the line of the borehole.

Apparent causes

In each of the incidents, rig stability and mis-alignment were caused by different factors; soft ground conditions, a faulty hydraulic outrigger cylinder and reactive forces while drilling and tripping drill rods.

In a vertical borehole all reactive forces generated through pulling the drillstring or placing weight on bit act perpendicular to the earth and so will not result in the rig moving out of alignment with the borehole. When drilling angled boreholes however, part of the reactive force acts vertically and part acts horizontally. As borehole inclination flattens, the horizontal component increases in magnitude and this can lead to the drill being pushed out of alignment. Even slight movement of a diamond core drill can lead to drill rod failure.

Additional factors

In all cases, the drill rigs were levelled using a spirit level - no integrated levelling system was built into the rigs and so detecting small amounts of movement was not possible.

Recommendations

Maintenance of drill rig stability and alignment are critically important particularly in diamond core drilling operations where rotational speeds are very high.

Additional controls such as increasing the diameter of outrigger footings, and regular checks to ensure that drills are level should be considered particularly in wet weather conditions or in circumstances where drill pads have been back-filled.

Rigs drilling flat angle boreholes must be very securely anchored and regular alignment checks must be conducted.