Explosion – hot work in a flammable atmosphere (Transport Malta)

A large tanker was in transit to her scheduled dry-docking. Additional technicians were on board to assist with preparations for the dry dock.  As part of this process, walkway ramps on the main deck were being dismantled to check on their condition and that of the piping passing underneath. Two of the bolts on the last ramp were particularly hard to undo and the assigned technician opted to use an angle grinder to facilitate the work. Sparks emitted from the grinding disk flew towards the vent head of a fuel oil tank (see photograph). Subsequently, an explosion rocked the vessel, causing damage to her steel structure, and port side lifeboat and life-rafts. There were no injuries.

The full investigation report is detailed and instructive; however, IMCA will draw out only a few conclusions:

• Acceptance of risk/perceived pressure to finish the job:
  • The technician had failed to loosen the bolts, having tried with an impact wrench, smaller sized spanners, and a chisel, he was unable to remove the last two bolts. Eventually, their hexagonal shape was rounded, making it even more difficult to undo them. Then he reached for the angle grinder.
  • The vessel would dry dock the following morning and things would become very busy. The technician continued his work into the evening.  Daylight was ebbing and this might have also prompted the technician to find a quick solution to the issue at hand prior to sunset.
  • The technician was motivated to quickly complete the task expected of him and the solution seemed to be a simple one. The cutting of corroded nuts and bolts using an angle grinder would have speeded up the process.
  • The safety investigation believes that, although the technician was aware that sparks would be generated by cutting the bolts with the angle grinder, he was unaware of the flammable gases accumulating around the vent head of the bunker tank in the vicinity.