Pollution
Humain
Environnement
Economique

In a petroleum storage facility at around 7 a.m., during heavy rainfall (more than four times the monthly average), the floating roof on a gas tank began to gradually sink. Having a capacity of 30,000 m³, the level in the tank was 3,900 m³. Residents were complaining of the strong hydrocarbon smell, and some individuals felt ill. The operator, who also detected a 300-litre leak in the tank’s containment area, initiated its internal emergency plan, alerted the emergency services, the municipal council, and the various governmental departments. The Classified Facilities Inspection Authorities went to the site.

After consulting the body specialised in managing emergency situations and the BARPI, the Classified Facilities Inspection Authorities validated the operator’s proposal to empty the tank slowly without deploying a foam mat. This decision was based on the following elements:

  • the petrol that had collected in the retaining basin was pumped out and absorbent material was put in place to recover any residual product,
  • the results of the measurements conducted using portable explosimeters, installed around the tank’s retaining basin, were negative and allowed the situation to be monitored,
  • the calculation of the potential overpressure associated with the explosion of the vapour cloud in the open air, considering the amount of hydrocarbon and the configuration of the tank, showed that it would remain confined to the tank’s walls,
  • the deployment of a truly efficient foam mat in this configuration would prove to be very difficult,
  • the application of foam using a foam cannon is not recommended owing to the risk of fire due to the static electricity generated by this operation.

The petrol present above the floating roof was pumped out the next evening. Two measurement campaigns were carried out by the emergency services in different locations of the neighbouring community. No hydrocarbons were detected in the ambient air.

The prefecture issued an emergency measure order to keep the tank shut down.

The operator analysed the incident retroactively. Under the weight of the rainwater that had accumulated on the floating roof, it began to sink and flex in the centre. As the roof was in the low position in the tank, its flexing led it to touch the bottom of the tank and the base of the valve, causing the valve to open. The open valve allowed the petrol (having a lower density than water) to flow toward the top of the tank, thus loading it down even further. The effectiveness of the water discharge drain on the roof was questioned: on the one hand, its discharge capacity was insufficient and, on the other hand, its automatic closing system in the event of the presence of hydrocarbons had not functioned correctly, which explains the small amount of fuel in the retaining basin into which the drain discharges.

The following are among the organisational failures that led to this event:

  • insufficient assessment of the impact of changes in the plant carried out during the 1990s:
    • the strut of the 2 additional valves, installed in the 1990s in the centre of the floating roof, was longer than the roof’s support legs
    • the drain’s discharge capacity was not reassessed after the installation of the automatic closure systems in 2001
  • poor design of the modifications:
    • the deflection of the floating roof’s membrane resulting from stresses caused by heavy rainfall was not taken into account
    • the drain’s automatic closing system did not prevent the release of hydrocarbons
  • incomplete maintenance plans: lack of a preventive maintenance plan concerning the cleaning of the roof and unclogging of the drain line, partial inspection of the inside of the drain (internal corrosion was observed after the incident).

Based on its analysis, the operator undertook several corrective actions: verification of the length of the legs on all of the depot’s breather valves with reconditioning as required, review of the inspection plans of the floating roof tanks, roof deflection study and optimisation of the drainage system.