According to a Mines Inspectorate significant incident report issued by chief inspector of coal mines Gavin Taylor, the employees took evasive action to avoid being hit by the fast moving cable drums.
The incident happened after an electrical fault caused the workers to start recovering a borehole pump using a 30-tonne mobile crane to lift it to the surface.
The pump was suspended from a drill rod string some 450m down the 650m cased borehole with the report saying a power cable and control cable were attached to the pump and secured by cable ties to the rod string.
Both the control and power cables were manually wound to the cable drums, with the latter also going over a sheave wheel above the borehole collar.
As the pump was being lifted the cables started to twist around the rod string.
To counter the problem, the report said as each rod was raised out of the borehole the rod string was rotated using Stillson wrenches to untwist the cables.
But once 130m of rod was removed the cables whipped downhole pulling the cable drums to the borehole collar.
With both drums placed some 10m away, the larger power cable drum managed to ricochet off the crane outrigger as it moved to the hole and struck the sheave above where the operators had been standing, according to the report.
After hitting the sheave the drum then hit a nearby truck before stopping some 15m away from its original position.
In identifying the cause of the incident the Mines Inspectorate said as each rod was rotated to untwist the cables the rod string was uncoupling at a cored joint in the borehole.
The Mines Inspectorate noted the cable drum frames were not secured to the ground and there was no mechanism to control the rotation of the drums.
Heavy corrosion of the threads of the drill rods where the rod string parted was also observed.
The inspectorate said no procedure to remove the pump from the borehole had been developed and the life cycle of the pump installation had not been risk assessed.
The application of a formal risk assessment that covers the design, operation and maintenance aspects of borehole pump installation has been recommended by the inspectorate.
“Designate lowering and raising cables installed in boreholes to be high risk activities and implement risk assessed procedures,” the inspectorate said in its report.
Other recommendations included making sure employees involved in these activities were familiar with relevant procedures and applied a risk assessment process before starting and that cable drum assemblies were anchored to handle a maximum anticipated load.
Powered cable drums to control drum rotation and breaking have been recommended along with the use of delineated no-go zones in cable paths between the hole and the drums.