In an oil depot, an explosion was followed by fire inside an empty tank allocated to storing premium unleaded gasoline and equipped with a stationary roof and a 5,000-m³ diaphragm float. The accident occurred while 2 subcontractors were scraping the floor inside the tank to remove residual deposits. The facility’s internal emergency plan was activated. Fire-fighters brought the situation under control in 2 hours using a fixed fire hose and 2 foam nozzles. The 2 workers were seriously burned and required hospitalisation. The tank was heavily damaged. Activity at the depot was suspended for 2 months. Damages amounted to €1 million on the property side, €200,000 for safety measures and dismantling, and €600,000 for operating losses. The gendarmerie proceeded with an investigation.
Works were underway before reaching a gas concentration 10% less than the lower explosive limit (LEL). Moreover, the tank had only been equipped with a single manhole; its vents had not all been opened and the ventilation system installed to discharge gasoline vapours was turned off during the scraping intervention. The explosive atmosphere inside the tank was likely ignited by a spark caused by an accessory (rivet on a boot sole, metal scraper, steel snap, etc.) carried by one of the subcontractors. The diaphragm float had been placed at just 1.2 m off the ground: the repair crew was operating under difficult conditions, which were capable of producing friction with equipment on the floor or sides of the tank, with the confined space only exacerbating effects of this explosion.
As proposed by the Inspection authorities for classified Facilities, whose inspectors visited the site the same day, the Prefect issued an emergency order requesting the following prior to facility restart: completion of a study on the accident causes and circumstances; determination of measures to be adopted to limit the chance of incident recurrence; safety verification of both the given installation and neighbouring ones. The operator recalled applicable intervention guidelines for all subcontractors working on tank maintenance and moreover modified the protocol inside hydrocarbon tanks to include: adaptation of procedures to the various types of tanks; focus on cleaning/degassing operations only after validation by a depot manager or assistant manager; verification of vapour concentrations (as specified in existing procedures) before any intervention inside the tanks; enhanced ventilation by opening taps; and removal of all manhole valves and continued forced-air ventilation throughout the maintenance intervention. The group owning the depot decided to implement the following measures: dissemination of accident feedback, strengthening of subcontractor controls, stricter control over equipment potentially present inside the tanks, systematic installation of 2 manholes during 10-year inspections for the largest tanks.
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