Tank Farm Gas Oil Overflow

Safety Incident Topic: Tank 220x1 Overflow
Location Of Incident: Coryton Refinery, UK
Date Of Incident: 25th April, 2006

Brief Account of Incident
220x1 is a 220,000 barrel tank which is normally used to feed the distillate hydrotreater (CHD) with Atmospheric Gas Oil (AGO). In the period Feb 9th – April 23rd the CHD was shutdown for turnaround maintenance.

At around 01.00 on Monday 24th April the CHD was being commissioned with tank 220x1 designated as the rundown tank. The unit was started up successfully but the gauge on the tank did not register an increase in level. The Product Movement (PM) panelman re-set the gauge which then started to rise steadily consistent with the run-down from the CHD.

At 03.05 the gauge stuck at a reading a reading of 19.7 m. The outside operator, who is responsible for monitoring tank 220x1 failed to recognise that the product had been steadily entering the tank between 01.30 and 03.05 and interpreted from the “flat lining” of the tank gauge that no product was being delivered from the CHD. At the 06.00 shift handover the outside operator advised the on-coming shift that 220x1 was open to the CHD but nothing was coming down from the unit.

During the day shift on Monday 24th April the tank dip in 220x1 remained constant and the outside operator handed back to the on-coming shift (same operator as the previous night) with the same message he had received at the start of the shift i.e. nothing was being rundown into 220x1 from the CHD.

At 02.00 on Tuesday 25th April the PM panelman prepared to deliver a line flush of 100m3 from tank 220x1 to Jetty 5. The panelman noted that although the receiving vessel confirmed receipt of 100m3 of gas-oil from 220x1 the tank gauge had not registered this movement. The PM panelman interrogated the 220x1 gauging system between 02.09 hrs and 03.17 hrs to try and re-establish the level in this tank. After failing to establish the level he decided to wait for the on-coming day shift to take over the investigation of this problem.

At 05.09 hrs tank gauge on 220x1 changed from a reading of 0 m3 and rapidly climbs to a level 21.75m activating both the hi level alarm (set @ 21.5m) and the hi-hi alarm (set @ 21.7m) at 05.15 hrs. The outside operator was alerted and investigated the problem in the field. On seeing the tank over-filling and product entering the North Moat surface water system he closed the North Moat penstock to prevent the spillage entering the River Thames via the Dorr Oliver oil-water separator, switched the CHD run-down into another tank and gravitated tank 220x1 into a 3rd tank.

Potential Outcome(s)
The actual outcomes of the incident was a spill of 918 barrels of AGO into the North Moat system. There was potential for a more extensive environmental incident if the oil had not been isolated from the final surface water clean up system where it is then routed to the River Thames. The cost of the incident was $40K, mainly for the clean up of the spill.

What Went Well
The operator made the right decision in isolating the North Moat system before dealing with the tank spill. Clean up of the spill was started immediately and the majority of the oil recovered into the crude slop system. No oil entered the River Thames.

What Went Wrong (Possible Critical Factors)
The critical factors in this incident were the “sticking” of the gauge, the failure to recognise this, the lack of understanding of PM Operations personnel as to what the status of tank 220x1 was, the failure to manually dip when the gauge was found not to be working, and the failure to alarm at hi-level due to the “sticking” of the gauge. Additionally, after the leak the product was not held in the bund due to a broken penstock valve which allowed gas-oil to transfer into the North Moat.

Immediate Cause(s)
· Defective equipment: tank gauge and penstock valve on 220x1.
· Improper decision making / lack of judgement: improper response to the stuck gauge just prior to the spill occurring.
· Inattention / Lack of awareness: failure of the operator to accurately monitor tank 220x1.

System Cause(s)
The root cause for the “sticking” level gauge has not yet been identified and is still under investigation. The other root causes identified were:
· Inadequate monitoring of operations.
· Inadequate communication between work groups and peers.
· Failure of the hi-level alarms to annunciate due to the sticking” of the gauge.
· Inadequate identification of worksite/job hazard and subsequent poor judgement.
· Inadequate monitoring and repair of the bund penstock valve which was in need of repair.

Corrective Actions (summary only)
1· Tank gauge on 220x1 changed out to aid fault identification.
2· Condition of all penstocks reviewed and work in-progress to repair broken/leaking valves.
3· Independent hi-level alarms are already fitted on crude / gasoline tanks & all floating roof tanks.
4. Develop a plan for extending application of independent tank alarms.
5· Develop PI-data spreadsheets to aid identification of frozen gauges versus static tanks and to assist tank farm operators in monitoring operations.