During rounds while changing shifts inside a refinery, an employee witnessed a fuel oil storage tank overflowing at around 5 am. The filling operation was immediately stopped and the tank isolated; moreover, 1,000 m³ of hydrocarbons recovered in the retention basin were transferred to other tanks, as well as to the grease trap system for recovery in recycling tanks.

The tank was already full when at 1:30 am it was mistakenly scheduled for filling. The control room operator had requested that the field operator close the manual pouring valve leading to this tank, but the field operator instead closed the tank’s recirculation valve. By 4 am, the tank was overflowing its retention basin.

Detected by the centralised control alarm system, these unwanted product transfers had not been recorded in time by the control room operator. The radar-type level sensors equipping the northern site’s tanks were being replaced due to obsolescence. The newly-installed sensors triggered many false alarms in the control room on the account of the cold spell, causing higher than normal consumption in the tanks at night. This problem of adjusting the detection threshold was being resolved when the accident occurred: the control room operator was unable to identify the alarm indicating an overflow on the affected tank amidst all the very high level alarms on adjacent tanks continually relaying false readings to the displays.