Whilst conducting a daily inspection at 2.27pm
a shift deputy noted that at the point where the counterweight and man riding cage pass
the counterweight appeared misaligned.
The deputy contacted the control room operator to advise them,
then continued with his inspection until he reached pit bottom
412m below the surface.
Engineers were informed of the misalignment of the counterweight.
A mechanical engineer attended the shaft winder to investigate.
While the mechanical engineer was investigating,
a work crew and the deputy travelled to the surface
using the winder without encountering any problems.
The deputy then discussed his observations
in relation to the misaligned counterweight
with the mechanical engineer.
At 3.20 pm,
about 60 afternoon-shift workers
travelled to the pit bottom.
They felt vibrations and heard noises.
Immediately after the crew arrived at shaft bottom
a shift deputy contacted the control room
to report the unusual vibration and noises.
At 3.26pm,
two workers walked past the deputy on the phone
and entered the No.3 shaft conveyance
and began to travel to the surface.
During the trip
the counterweight collided with the number 4 water ring
which tripped the winding system seam profile switch
stranding the workers for 8 hours
160m below the surface.
The shaft winder uses four head ropes.
when all four ropes are of equal tension,
the net torque on the counterweight is zero.
When there is uneven head rope tension
a rotational torque is applied to the counterweight.
The investigation identified that the rotation of the counterweight
was generated by out of balance head rope tensions.
As the tension variation in the head ropes increases,
the wear rate on the drum groove liners accelerates due to rope slip.
This further aggravates the variation in head rope tension.
At the time of the incident there was a variance of up to 4.5 millimeters
between the four head rope drum grooves
- which over a depth of 400 metres
would result in a 1.456 metre
difference in rope travel between the ropes.
The rotation of the counterweight meant that is was possible for the
counterweight to contact the conveyance or the shaft wall whilst it was in motion.
Other causal factors include:
The maintenance and inspection scheme
did not recognise the criticality of the balanced head rope tensions
Early indicators were not recognised
and properly communicated to senior personnel to action.
Mine operators should review their safety management systems
relating to shaft winding systems, and:
- consult with appropriately skilled people to identify the control measures
that are critical to maintaining designed clearances and safe operating limits.
- developing or reviewing defect reporting systems
to include subjective and verbal comments by the workforce
- maintaining accurate records for all baseline data.
- challenging the effectiveness of maintenance tasks
to provide consistent outcomes
to maintain a safe state
and clearly identify a deviation requiring escalation
and/or isolation of equipment
- identifying the critical tasks in the maintenance management system
and a response plan if those tasks are not completed
- considering a change of duty
that should trigger a review of the maintenance schedule.
For more information and to view the full report:
resourcesregulator.nsw.gov.au