H2S Alarm at the Shale Shaker: Evacuation and Rescue
At 22:15 the fixed H2S detector at the shale shaker reads 18 ppm and rising, and the shaker hand's personal monitor alarms. The well is at 3,350 m and offset wells produced up to 40 ppm H2S. The wind is from the north at 3 m/s. The shaker house is on the south side of the rig and the muster point is 60 m north of the substructure. The night toolpusher is in the office; two floorhands are on the drill floor and one has left his mask in the doghouse. The shaker house ventilation fan is switched off for maintenance, and the driller is at the controls with the top drive turning at 120 rpm. The mud logger has not called the driller, and the H2S alarm is not repeated on the driller's console.
1. State the alarm level this reading represents and set out your immediate actions in the first five minutes, including who does what.
Read the worked answer
At 18 ppm the reading is above the 15 ppm high alarm but below the 30 ppm critical level. Treat it as a high alarm and act immediately; do not wait for the toolpusher or the mud logger to confirm.
Driller:
- Stop the top drive, stop drilling and stop the mud pumps if that can be done without losing well control; keep the well shut in and monitored.
- Make a clear PA announcement: H2S release at the shaker, all personnel don breathing apparatus and muster upwind at the north muster point.
- Send the floorhands to the doghouse for their masks by the upwind route, not through the shaker area.
Shaker hand:
- Leave the shaker house immediately, moving upwind and north, and report to muster with the personal monitor reading.
Toolpusher or senior person at muster:
- Take a headcount against the board, account for visitors and contractors, and call the emergency response team and onshore support.
- Confirm wind direction and keep everyone north and upwind; do not start the shaker house fan, because it could spread gas and it is not rated for the atmosphere.
Do not send anyone back to the shaker without SCBA and a backup.
2. A floorhand collapses near the shaker. Outline the rescue and first-aid response, including what must not be done.
Read the worked answer
Do not run in and drag the casualty out. H2S at this level can render a rescuer unconscious within seconds, and the shaker area is still contaminated. The rescue must be by trained personnel wearing SCBA with a backup person also on SCBA, a lifeline if practical, and a third person controlling the entry from a safe point.
Once the casualty is out and in fresh air upwind:
- Check responsiveness, breathing and pulse.
- Give oxygen by trained first aiders if breathing; use a bag-valve mask if breathing is inadequate.
- Start CPR and apply an AED if the casualty is unresponsive and not breathing normally.
- Keep the casualty warm and still, and monitor continuously until medical help arrives.
- Arrange urgent medical evacuation and tell the receiving hospital that this is suspected H2S exposure, with the time and duration of exposure.
What must not be done: entering without SCBA, using a dust mask or escape hood as a substitute for SCBA, giving mouth-to-mouth without a barrier device, or moving the casualty through the gas cloud to reach the muster point. The rescue team must approach from upwind and decontaminate before removing SCBA.
3. What conditions and actions are required before normal operations resume, and what records should be kept?
Read the worked answer
Before resuming:
- The area must be tested and below the action level (normally 10 ppm or the rig's low alarm) at the shaker, flowline and drill floor, with the detector calibrated and functioning.
- The source must be understood: check the mud/gas separator, degasser, shaker ventilation, mud weight and whether the gas came from the formation or from the mud system.
- The shaker house ventilation must be repaired and running, with the fan rated for the area.
- A permit to work and a restart briefing should be issued, with SCBA and gas monitors available at the shaker.
- Personnel who were exposed should be assessed medically, and the emergency response team should debrief the rescue.
Records:
- Alarm times, readings at each detector and personal monitor, and the wind direction.
- Headcount and muster time, names of rescue team members and their SCBA checks.
- First-aid treatment, hospital transfer and exposure times.
- The investigation findings, corrective actions and the restart authorisation. The alarm was not repeated on the driller's console; that defect must be recorded and fixed before the next tour.