Most accident models view accidents as resulting from a chain or sequence of events. At Buncefield, the last event in the sequence that resulted in a massive explosion at the oil storage depot may well have been activation of the fire system itself.
A report issued by the Buncefield Major Incident Investigation Board states there is evidence of an internal explosion in a fire pump house, located on the Hertfordshire Oil Storage Limited West site on the northside of the Buncefield depot.
"It is believed that the pumps should have started when the emergency fire alarm was activated" just before the blast occurred, the report states.
At around 6 a.m. on Sunday, December 11, 2005, a number of explosions occurred at the Buncefield Oil Storage Depot north of London. At least one of the initial explosions was of massive proportions, followed by a large fire that engulfed more than 20 large fuel storage tanks. Forty-three people were injured with significant damage to commercial and residential property nearby.
The following is the official timeline of events published by the investigation board.
December 10, 2005
Around 7 p.m., Tank 912 in bund A at the HOSL West site started receiving unleaded motor fuel from the T/K South pipeline, pumping at about 500 m3/hour (flow rates are variable within limits).
December 11, 2005
At approximately midnight, the terminal was closed to tankers and a stock check of products was carried out. When this was completed at around 1:30 a.m., no abnormalities were reported.
From approximately 3 a.m., the level gauge for Tank 912 recorded an unchanged reading. However, filling of Tank 912 continued at a rate of around 550 m3/hour.
Calculations show that at around 5:20 a.m., Tank 912 would have been completely full and starting to overflow. Evidence suggests that the protection system which should have automatically closed valves to prevent any more filling did not operate.
From 5:20 a.m. onwards, continued pumping caused fuel to cascade down the side of the tank and through the air, leading to the rapid formation of a rich fuel/air mixture that collected in bund A.
At 5:38 a.m., closed circuit television footage shows vapor from escaped fuel start to flow out of the northwest corner of bund A toward the west. The vapor cloud was about 1 m deep.
At 5:46 a.m., the vapor cloud had thickened to about 2 m deep and was flowing out of bund A in all directions.
Between 5:50 a.m. and 6 a.m., the pumping rate down the T/K South pipeline to Tank 912 gradually rose to around 890 m3/hour.
By 5:50 a.m., the vapor cloud had started flowing off site, following the ground topography. It spread west into neighboring commercial property.
At 6:01 a.m., the first explosion occurred, followed by further explosions and a large fire that engulfed more than 20 large storage tanks. The main explosion event was centered on the car parks between HOSL West and commericial properties to the west. The exact ignition points are not certain, but are likely to have been a generator house in the Northgate car park and the pump house on the HOSL West site.