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Oil tank exploded after rinsing, injuring three workers

Date of incident: March 2013
Notice of incident number: 2013170090056
Employer: Heavy-equipment firm servicing the oil and gas industry; firm providing services including rinsing oil tanks

Incident summary
Workers were rinsing out 400-barrel storage tanks at a worksite. One of the tanks still contained flowback fluid, which is a waste product from the drilling of a natural gas well and contains combustible material. After a tank truck removed the flowback fluid, two workers from a heavy-equipment firm removed the door from the tank. Then another employer's crew rinsed the inside of the tank with warm water.

After the tank had been rinsed, the first two workers approached the tank to remove ice buildup at the doorway. The workers lit a propane torch, called a tiger torch. When the torch was lit, it caused the combustible material inside the storage tank to ignite, causing an explosion in the tank. The tank top blew off. Three workers were injured.

Investigation conclusions

Cause
  • Torch ignited gases and vapours in tank, causing an explosion: The combustible material (gases and vapours), from liquids that were previously stored in the tank, was ignited when a propane torch was lit. This caused an explosion, resulting in serious burn injuries to one worker. Two other workers received injuries as a result of the concussion forces from the explosion.
Underlying factors
  • Failure to conduct a hazard assessment: The firm doing the rinsing failed to conduct a hazard assessment by a qualified person before allowing a worker to enter a confined space. The prime contractor of the well site failed to conduct a hazard assessment before the heavy-equipment workers used a tiger torch on a storage tank that had recently contained flowback fluid.
  • Failure to adequately vent tank and control concentration of combustible material: The failure to ventilate the tank and to maintain the concentration of combustible material below 20% of the lower explosive limit was an underlying factor in this workplace incident.
  • Failure to test for the presence of combustible material: Although the tank had just been drained of flowback fluid, no one tested the tank for the presence of combustible material before workers entered the confined space to rinse the tank. As a result, the hazardous atmosphere was not identified. Workers from the heavy-equipment firm relied on a signal from the other employer to proceed with their work and did not test the atmosphere to determine the percentage of combustible material before lighting their torch.
  • Inadequate coordination of work crews: The prime contractor failed to adequately coordinate the activities of the workers involved with the removal of the combustible material from the storage tank and the rinsing of the storage tank. This resulted in combustible material being present in the tank within the flammable range.
  • Ineffective system for ensuring compliance: The prime contractor’s system or process for ensuring compliance with the Workers Compensation Act and the Occupational Health and Safety Regulation and for following its own policies was ineffective at preventing violations related to controlling the ignition source or confined space entry requirements.

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Publication Date: Mar 2013 Asset type: Incident Investigation Report Summary NI number: 2013170090056