A major leak of hydrocarbons occurred at 9:45 am upon restarting a furnace following maintenance (decoking) on the steam cracker line of a chemical plant; 6 t of a gas mixture containing 5% benzene and 3% butadiene were released into the atmosphere for 30 min. The site IOP was activated.

Odours were noticeable several kilometres from the platform. A few dozen residents and employees were affected and/or suffered from eye irritation; the staff of a neighbouring business (200 pers.) confined themselves inside shelters. Some 50 employees from the platform had to undergo urine testing in a search for biological indices demonstrating benzene exposure.

Oily deposits mixed with rainwater were also observed in a nearby municipality, as well as iridescence effects on vehicles due to oil traces. The operator absorbed the costs for facility cleanup (170 vehicles and 30 homes) at a cost of between €100,000 and €500,000. Operating losses, on the other hand, amounted to between €500,000 and €2,000,000 (with a 9-day shutdown). The operator issued a press release.

The transition phase between decoking and restarting the furnace required the movement of two valves: one of them had to be closed to release decoking gas into the atmosphere after passing through a cyclone, while the other had to be opened to direct the cracked gas to the oil quenching tower located downstream. During the accident, a mechanical problem prevented closure of the first valve without the automatic controller or staff detecting this malfunction. Under these conditions, the valve between the furnace and quench bath was opened, causing 6 t of cracked gas to be released into the atmosphere from the quench bath sector. Gas emissions via the 41-m high chimney stack lasted 25 minutes until manual closure of the valve (as automatic closure proved impossible) between the furnace and quench bath.

An analysis of defects performed by the operator identified a break in the coupling system between the electric motor and the decoking valve stem. Furthermore, design of the limit switch (of an electrical type), based on the detection of a number of motor rotations and not on the physical position of the valve stem, prevented detection of this valve non-closure.

Several improvements were introduced before restarting the furnace: fitting of a new valve (modified design), fitting of an additional physical position detector on the valve stems, integration of these limit switches into the existing safety sequence, report writing, preparation of feedback experience, and a reminder issued to staff regarding the operating procedure for reversing these valves and the specific points to be monitored. Lastly, the valve between the furnace and quench bath that could not be operated electrically during the accident was to be investigated. Authorities and elected officials were notified in both France and Germany.