Inside a refinery, the floating roof tank used in the production of 98-octane gasoline had since 2:30 am been filled with the basic ingredients (butane, naphtha base, alkylate, ETBE) entering into the mix at an average flow rate of 630 m³/h. At 2:23 pm, a high level alarm (alarm button plate by mechanical contact with the floating roof relayed to the control room) was activated while the gauge was indicating a level of 10.04 m for a high level mark set at 14.6 m. Nonetheless, the tank continued to be filled and at 4:45 pm, the gauge posted a level stabilised at 11.135 m. An error message notifying the disagreement between tank status (filling) and gauge stability was sent to the control room. A technician was deployed to the site and observed a gasoline spill within the retention basin. The instrument supervisor then stopped the filling process and alerted emergency services.

The tank’s foam boxes, blocked by the secondary joint of the floating roof that had derailed from its track, were no longer in working order. The operator estimated at 10 m³ the quantity of hydrocarbons spilled into the basin, which also contained rainwater. Total property damage to the tank roof was appraised at €100,000, while clean-up costs amounted to €12,000. An inspection of the tank’s structural integrity along with expanded monitoring of groundwater was undertaken after detection of a thick layer of hydrocarbons floating at the level of the piezometers installed around the retention basin. The tank overflow was caused by both a defect in the level gauge (due to a 4-m offset, a malfunction known since 10th January yet went uncorrected) and failure to comply with the procedure imposing pump shutdown upon activation of the high level alarm.