AT 3.20 am., 11.5 tonnes of flammable hydrocarbons were released (14 bar /240 °C) and ignited (probably on an unprotected hot flange nearby) in the Fluidised Catalytic Cracking Unit (FCCU) of a refinery. The FCCU was in the process of being restarted as it had been shutdown on 29/05/2000 following a power distribution failure. At 03.19 a.m. operators working on the plant in the vicinity of the Debutaniser column reported a leak of hydrocarbons. The control room operator was notified and within two minutes, despite the number of audible and visual alarms, significant isolation of inventories had taken place.
On-site emergency services set up portable fire appliances to control the spread of the fire. Further units intervened with a standby foam tanker.
Gas sampling carried out on the north bank of the River Forth found no abnormal levels of atmospheric pollutants following the incident. Towards the end of the incident, due to the large quantity of water used for fire-fighting, and the fact that the capacity of the storm water tanks was effectively reduced by recent heavy rain, the storm water tanks in the Oil Refinery effluent treatment system became filled. Contaminated effluent from the FCCU containing hydrocarbon liquids was diverted directly into the Forth Estuary.
There was substantial damage to high-level steelwork and access to parts of the site of the fire was restricted until the danger from falling steelwork had been assessed. The prevailing wind direction took the vapours into a relatively open area of the plant where the vapours were able to disperse relatively easily. The consequences may have been more serious if the wind had been towards a congested area of plant (VCE risk).
No serious injuries to on-site personnel or to off-site persons were reported. The local community expressed concern following a series of accident and on the potential contamination from asbestos insulation.
The primary immediate cause was the fracture of an unsupported 6×3″ reducing tee branch pipe to the main transfer line between the Debutaniser column and the Re-run column of the FCCU due to fatigue failure. The branch failed at welded joint to main transfer line. It had inadequate support and was vulnerable to vibration fatigue and/or shock loading. The design of the plant dates back to 1950’s, and changes were made to configuration of branch in 1987, but it was poorly controlled. The pipework were inspected only for corrosion, without proactive surveys for dead legs/redundant drains.
Serious operational problems were encountered in 1997/98 after other modifications of the FCCU, but that were inadequately dealt with despite a torch oil explosion in 1999.The operator reviewed the FCCU earlier in 2000, but the review findings were not implemented or communicated properly.
