At 6pm on 21st March 2019
a supervisor working at Metropolitan Colliery's longwall face
detects increasing carbon dioxide levels coming from floor cracks.
Production in longwall 303 panel stops due to the elevated
carbon dioxide levels at the tailgate end of the face.
When the longwall deputy goes to investigate, the carbon dioxide level
rises above the measuring range of his hand held gas detector.
The longwall crew hear hissing sounds behind the face supports
and notice that all supports along the face are yielding.
At 6:45pm the Undermanager arrives at the longwall.
He and the Deputy begin to inspect the longwall face
and notice bubbling from the floor.
At 7pm the face power is automatically tripped at 1.25% methane.
The level measured on the tailgate chock monitor reads 2.31%.
The longwall crew and Undermanager withdraw to the crib room.
After confirming with control that the gas levels are increasing,
the mine follows their withdrawal trigger action response plan,
withdrawing workers to the surface and removing power.
(Vehicle starting up, and driving)
Gas levels continue to be monitored as workers make their way to the surface.
By 8:45pm, the 70 underground workers are out of the mine
and accounted for on the surface.
Underground power is confirmed isolated at 8:50 pm.
All mine entrances are barricaded to prevent access.
The quantity of gas is significant, and the speed of the release
has overcome the mine's ventilation system
which was unable to dilute the quantity of gases released to maintain safe limits.
Methane is a flammable gas with explosive limits in air between 5 and 15.4 percent.
The legislation deems it unsafe for persons to be exposed to greater than 2%,
or electric power and diesel engines to operate at greater than 1.25%.
Carbon dioxide, at high concentrations, depresses the
central nervous system resulting in impaired consciousness.
The legislation deems it unsafe for persons to be exposed to greater
than 1.25% on a time weighted average, or 3% in short term exposure.
As levels increase above 10% this may cause collapse, coma and death.
Both methane and carbon dioxide displace oxygen.
Normal air has 20.9% oxygen content.
Current legislation deems it unsafe for oxygen levels to be less than 19.5%.
In the longwall 303 return airway the gas levels peaked at 16.1% methane,
and 15.9% carbon dioxide, with the oxygen reduced to 13.5%.
Further outbye in the North West Mains return,
the gas levels reached 4.3% methane and 4.6% carbon dioxide.
At the base of the upcast shaft
the gas levels reached 3% methane and 4.16% carbon dioxide.
Workers remain out of the mine for 7 days
until carbon dioxide and methane levels returned to non-hazardous levels.
A combination of circumstances led to the release of gases stored in the strata.
The most likely source of the gas release is from undrained interburden and coal measures
that contain carbon dioxide and methane.
The contributing causal factors had not been considered holistically.
Had a systematic approach been taken,
it may have resulted in the consideration and implementation
of additional control measures.
Contributing causal factors have been identified as:
Incomplete knowledge of the strata, gas contents
and geology in the current mining area.
Elevated stress loading on the longwall face supports,
influenced by increasing depth of cover.
The fracture profile into the strata around the extraction area
exacerbated by regional syncline.
Gas migration due to a reduction of confinement
resulting from geological conditions.
The mine continues to conduct further analysis
of the causal factors following the investigation.
For more information and to see the full report, visit our website