In a hydrocarbon dept, a patrolman noticed a domestic fuel leak from a concrete wall surrounding a 18,350 m³ tank dating back to 1972. The operator transferred half of the tank contents (7,800 m³) to another tank with out the use of pumps. Since the two tanks were at the same level, the remaining domestic fuel had to be pumped. In waiting for a sufficiently powerful pump adapted to the domestic fuel and to avoid the rejection of fuel oil, the operator maintained a level of water in the tank that crossed the supposed level of the leak. Meanwhile, the patrolman and the safety officer were instructed to especially monitor the tank where the accident had occurred. The tank was finally drained.
During the 10-yearly inspection of the tank that ended end January 2008, an inspection body had measured the thickness of the plates using ultrasound and examined the state of the welds using non destructive tests. Following these operations, the tank was partially filled from another tank to 8 m without any malfunction being detected. The tank started leaking only after it was filled to 16 m. The hydrostatic pressure exerted by 16 m of fuel oil may have caused the leak, possibly at the level of a micro crack in the side welds of the tank that went undetected by the inspection department. The operator commissioned a third party to determine if the crack could have been detected or not by the ultrasound thickness test and if it existed before the 10-yearly inspection. The operator had planned to map the bottom of the tank using magnetic flux leakage. The operator had informed the inspection authorities for classified facilities on 15 May.