A 1,000 m³ tank containing 500 m³ of a mixture of methyl ethyl ketone, toluene and a little water ruptured at around 2:30 am in the oil unit of a refinery. The bottom of the tank that had detached from its liner collapsed onto a neighbouring tank and the product within it spilled extensively into the concrete retention system, shared with 2 other tanks (one empty and the other 20% full). A technician who was carrying out a visual inspection of the tank was sprayed by the wave. He was hospitalised and as a precaution remained in observation for 24 hours.

The preliminary internal emergency plan unit was activated. The municipality’s mayor was notified, along with the CHSCT (Corporate committee for hygiene, safety and working conditions) which was associated with the emergency response operations. Hot work on the site was suspended. In the morning, the effluent treatment monitoring revealed a slight increase in the COD content (indicating that perhaps a limited product leak may have escaped from the retention system?). The regulation therefore diverted the load into the storm tank. As a precaution, from the start of the event a layer of foam was spread over the retention system affected and a safety perimeter was defined on the site, of which the various operating sectors were notified. The operator, who did not requested the intervention of external emergency services, attempted to make the system as leaktight as possible. In the evening, the level in the system had stabilised, and only a small leak persisted in one of its corners in line with a pipe feedthrough. It was contained by a sand dam, pumped out and then recovered in a drainage tank. The retention system was monitored by the site’s safety department which reapplied the foam covering. The product contained in the system was pumped out and initially directed to available tanks pending its transfer to a unit, if it was reusable, or to a disposal centre. Given the quantities involved, the emptying operations were planned to last for 2 days and one night.

The inspection authorities for classified facilities recommended that the Prefect issue a formal notification order instructing the operator to review the leaktightness of the site’s retention system. In addition, it requested information regarding the stability of the tanks in question and the general maintenance of this type of equipment. According to the operator, there was no risk of environmental consequences and studies were carried out concerning the stability of the neighbouring tank, onto which the tank in question had collapsed. The hypothesis of an internal pressure surge related to the accidental presence of steam in the tank has been proposed.