Pollution
Humain
Environnement
Economique

In an oil depot, 3,750 m³ of unleaded gasoline were transferred from one storage tank to another at the end of the afternoon by remote control from the site’s control room in order to prepare for a barge loading. Around 8 pm, a detector indicated the presence of hydrocarbon vapours in the “pumping station” room located 300 m from the tank. A technician dispatched to the scene smelled gasoline and informed the control room by radio, at which point the technician overseeing operations stopped the transfer process and notified the on-call supervisor. An in-house emergency response team deployed explosimeters to verify the presence of gasoline in the outer ring on the receiving tank as well as in the polluted stormwater circuit, since its floating roof was hinged to the shell’s upper edge. Gasoline was flowing into the fouled stormwater circuit via a retention basin drainage line left open by mistake, eventually emptying the basin. The on-call manager activated the internal emergency plan at 8:15 pm and alerted first responders. Fire-fighters arrived on-site at 9 pm, accompanied by the Mayor and Deputy Prefect. Together, the site’s response team and fire-fighters placed a foam blanket over the upper part of the tank and its outer ring, extending coverage to the pumping station as well. An initial attempt to recover gasoline in the stormwater network failed, only to be postponed until the next day due to the unavailability of a subcontractor’s specialised equipment. A safety perimeter was set up in conjunction with continuous measurements of explosibility throughout the night. Pumping was performed the next morning with devices designed for use in explosive atmospheres (“ATEX” designation) beginning at 10:20. The product recovered was then stored in a tank dedicated for liquid residue while awaiting subsequent processing. All 20 m³ of spilled gasoline were collected and no water pollution was reported. The emergency plan was lifted around 4:45 that afternoon. The depot operator and Prefecture issued press releases the following day. Only the tank roof had sustained damage.

The operating system had automatically tripped a transfer shutdown once the “height of high-level operations” mark had been reached in the receiving tank. This height was input by the control room technician, who verified the filling level by consulting information provided by a radar-type operational level sensor installed on the tank roof. A plunging rod-type “high- high level” safety sensor triggered an alarm and the emergency transfer shutdown step in the event of alarm activation. On the day of the accident, the rise of gasoline in the tank forced water at the bottom into the vertical measurement shaft of the operational level sensor, filling it to the 2/3 mark, since this shaft had not been fitted with orifices over its height to allow for the free circulation of gasoline in the tube. The presence of water heavier than gasoline distorted this measurement: the operational level displayed in the control room was 3.3 m below the actual tank level when the accident was detected. Despite being recently inspected, the “high high level” safety sensor failed to detect the overfilling. A test conducted on similar sensors installed on other tanks of the depot revealed a risk of the plunger mechanism seizing.

The operator proceeded by: replacing all “plunger” level sensors used on-site by more reliable and easily-controlled devices (using switches or floats); updating their testing conditions (in situ, in-product, etc.); modifying operating interfaces to provide the technician with a more accurate depiction of transfer progress; improving the procedure for handling drains on tank outer rings in the event of heavy rainfall and subsequent monitoring; and revising the subcontracting agreement to supply emergency “ATEX” equipment and qualified personnel for all circumstances. According to the profession’s best practices, access to self-diagnostic resources for malfunctions on so-called “high-high level” safety sensors placed on storage tanks is absolutely critical.

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