During transfer of a hydrocarbon mix from a river barge to a refinery storage zone, a defect at the level of the pipe compensator caused a leak around 3 am.
A number of steps had been taken the day before to prepare for this transfer of 500 tonnes of n-alcanes C5-C6 (an easily flammable liquid, highly toxic for aquatic organisms) from a barge into a 1,000-m³ capacity fixed-roof tank. A hose (DN 80) had been installed on a pipe rack (DN 150) connection to hook up the tank. A checklist describing all the manual steps was followed and inspections of connections and pipes were conducted, both by the Head of Operations upon completion of this preparatory phase, and then at midnight by a night shift technician. At 12:30 am, the barge was fastened to the wharf and a sample drawn for laboratory analysis. At 1:30 am, once lab analyses were released, the pump (flow rate: 50 m³/h) was turned on and a technician noted flow heading into the tank (by acoustic verification, flow noise heard). Between 1:45 and 2, the pump malfunctioned twice for unknown reasons. Installations (pipes, valves, and measurement, control and regulation devices) were verified; no anomaly was observed. At 2:30, the pipes between the tank and the dock were inspected with no leak being detected. Around 2:45 or 3 am, the pump on the boat was reactivated at a rate of 100 m³/h. At 3:10, a control room technician noticed differences between the pump rate and the tank filling level and, once again, inspected the pipes, which enabled identifying a pool of hydrocarbon. The transfer operation was immediately stopped and both on-site fire-fighters and the municipality were notified.
Repeated pump shutdowns, combined with a pump restart at 100 m³/h instead of 50 m³/h, had caused a dilatation compensator installed on the pipe system to burst. Investigations pointed to a water hammer phenomenon being induced by:
- the 450-m long pipe unable to be positioned along a constant incline due to both geographic constraints and the site configuration;
- repeated pump malfunctions, causing its cavitation and the formation of cavitation bubbles;
- starting the pump at high speed, leading to a pressure surge exceeding the pipe design pressure (16 bar);
- the narrowing cross-section (DN 150 / DN 80), support fixtures placed inappropriately, and the dilatation compensator design that further weakened the eventual breaking point.
The fouled surface was estimated to cover 320 m². The pool was first covered with foam to avoid evaporation and pumped the next day by a specialised subcontractor. Gas measurements taken outside the site boundary indicated that the lower explosive limit (LEL) had not been exceeded. The overflow quantity of product was estimated at 80 tonnes, 20 to 30 of which were recovered during emergency measures, leaving 50 to 60 tonnes to penetrate into the unsealed ground (sandy soil). A long-term soil decontamination procedure, involving drilling wells and pumping hydrocarbons from the water table, was implemented at an estimated cost of 480,000.
A system equipped with a vacuum breaker function was installed in order to limit water hammer risks stemming from the inability to control the facilities’ technical functionalities: flow rate measurement devices around the wharf detected drops in output flow and activated a compressible gas (nitrogen) injection to ensure that moving fluid masses did not collide with static fluid masses, which would have produced pressure peaks beyond the pipe specifications. Moreover, the number of narrowing cross-sections and dilatation compensators was reduced to the bare minimum required. Following installation renovation, just one product supply line ran between the port and the tanks.
The refinery operator repaired the compensator at a cost of 10,000 and planned on expanding monitoring rounds; loading/unloading guidelines for site personnel and crew members were also verified and updated.