The sinking of a double-deck floating roof on the 70-m diameter B962 tank containing over 62,200 m³ of crude oil was detected on July 18, 2007 in a refinery. No product had been transferred from this tank since July 2, the day of the most recent filling. Monitoring of the tank level had revealed abnormal variations beginning on July 5. The level of liquid in the container was read at 14 m when the sinking roof was first remarked, causing 3,850 m² of surface area of crude to enter into direct contact with the ambient air.The facility operator logged the tank’s electrical supply and verified the hydrocarbon vapour concentration every 2 hours at the site of the retention basin. An emergency fire-fighting vehicle from the refinery’s fleet was positioned along the edge of the basin ready to intervene. A series of hydrocarbon concentration measurement sensors installed in the town of Petit-Couronne recorded very high concentration levels (between 10,000 and 25,000 µg/m³), yielding an average background noise of 1,500 µg/m³.The Hazardous Installations Inspectorate noted these facts. The operator proceeded with a gravity transfer of tank contents into other refinery tanks until the liquid level neared that of the roof, which had stabilised at 2.8 m. The tank shell was then punctured at high pressure and water injected into the tank to enable removing the remaining hydrocarbons by means of pumping. Drainage operations continued for several weeks in order to extract via a bottom outlet all liquid contained inside the tank along with the sediments that had settled.The operator estimated the quantity of volatile organic compounds (VOC) emitted into the atmosphere during these tank drainage and safety prevention steps at more than 3,000 tonnes, including approximately 55 tonnes of benzene.While awaiting the final results of investigations performed at the time the roof was disassembled, the initial findings released by the operator tend to support the hypothesis of a loss of flotation capacity due to overload caused by both the accidental filling with crude of a leaky caisson and the accumulation of rainwater on the roof. The foot valve on the rainwater discharge trough had been closed, subsequent to the detection a few months prior of hydrocarbon leaks inside the discharge drain.Renewed use of the tank following repairs has been scheduled for the end of August 2009.

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