A parcel of unleaded petrol was being delivered through the UKOP South line into
HOSL’s Tank 912 from 1850 hrs on Saturday 10 December 2005. The tank, which
had a capacity of 6 million litres, was fitted with an automatic tank gauging system
(ATG) which measured the rising level of fuel and displayed this on a screen in the
control room. At 0305 hrs on Sunday 11 December the ATG display ‘flatlined’, that
is, it stopped registering the rising level of fuel in the tank although the tank continued
to fill. Consequently the three ATG alarms, the ‘user level’, the ‘high level’ and the
‘high-high level’, could not operate as the tank reading was always below these alarm
levels. Due to the practice of working to alarms in the control room, the control room
supervisor was not alerted to the fact that the tank was at risk of overfilling. The level
of petrol in the tank continued to rise unchecked.