On the evening of the 9th June the FCCU was in the process of being restarted. The "front end" i.e. the cracking section was started up successfully by about midnight. The intention was to then start the "back end" Vapour Recovery Unit (VRU or "light ends" section) of the plant by progressively introducing hydrocarbons into each of the columns in turn.
The main fractionator and absorber columns were successfully brought on line and then material was introduced into the Debutaniser column (E5) sometime after 01.00 a.m.
At this stage the control room operator had difficulty in achieving the correct temperature and pressure at the base of the column E5 and this resulted in the relief valves lifting.
Problems were also experienced in maintaining the correct levels in the next column downstream (the Re-run column E6) due to gas being present in the pump which controlled the liquid levels in the column. This was a common problem experienced during start up conditions.
During start-up and before transferring liquids from E5 to E6 standard operating practice for plant operators is to dewater the transfer line using a number of drain points on the system. Dewatering is essential because if the water in E5 (which is operated at a high temperature and pressure relative to the conditions in the column E6) was transferred into E6 it would result in the water immediately "flashing off" to produce steam. As discussed previously (for the MP steam main rupture incident) the transition between liquid and vapour phases is accompanied by significant volumetric changes and this could potentially damage the Re-run column E6.
In order to dewater the column a drain point at the base of the column E5 on the outlet pipework is opened and water is allowed to escape to the local ground drains until hydrocarbons are observed coming out. This operation is carried out several times prior to transfer commencing in order to ensure that all the water has been removed. Note: The hydrocarbons discharged to the drain system are subsequently removed in the effluent treatment systems for the Complex and are not discharged off-site.
Following the dewatering procedure the transfer of liquid from the base of column E5 to column E6 was initiated by the control room operator at about 03.15 a.m. This was achieved by opening a flow control valve in a 6" transfer line between the two columns.
Shortly afterwards (approximately 03.19 a.m.) operators working on the plant in the vicinity of the Debutaniser column reported a leak of hydrocarbons (described as a white vapour smelling of "spirit") coming from the base of E5 and drifting northwards towards the hot oil pumphouse. The control room operator was notified of the leak and immediately stopped the feed to the unit.
At approximately 03.23 a.m. whilst the plant operators were investigating the source of the leak and beginning to isolate valves, the release ignited and the operators ran quickly to a place of safety. A serious fire developed at the base of E5 and this then had an impact on the adjacent columns and equipment. There were no injuries as a result of the incident. The fire was automatically detected by a fire detector located in the Hot Oil pump house facing the debutaniser column E5, which alerted the Control Room
The control room operator isolated the feeds to the "light ends" system using the DCS and within two minutes despite the number of audible and visual alarms significant isolation of inventories had taken place. The damage to instrumentation during the fire also meant that the control room operator was unable to view the condition of parts of the process through the DCS system. For a summary of the key findings of the investigation into the alarm handling issues of the FCCU fire and the other incidents see the alarm handling investigation.
The prevailing wind direction at the time of the incident took the vapours into a relatively open area of the plant where the vapours were able to disperse relatively easily. If however the wind had been in a different direction towards an area where there was a congestion of plant and equipment and ignition had occurred the consequences may have been more serious. Such a scenario had previously been identified in the BP CIMAH/COMAH safety reports for the FCCU whereby the possibility of a vapour cloud explosion (VCE) had been considered.
In response to the original identification of the leak, the on-site emergency services were called to attend (03:21 a.m.) and by the time they arrived at the scene, ignition had occurred. They quickly set up portable fire appliances to control the spread of the fire.
The Central Scotland Fire Brigade was notified by a 999 call at 03:29 a.m. following the ignition. They arrived at the scene at 03.39 a.m. and assisted in controlling the incident. The fire was initially attended by five mobile units along with a decontamination and control unit. Subsequently a further five mobile units attended with a standby foam tanker
At 03.33 a.m. the Oil Refinery muster point alarm was sounded.
Plant operators helped the emergency services by closing manual valves (for example at the fractionator column) and by shutting down pumps to minimise the quantity of material released. The control room operator was limited in the extent to which he could assist to isolate inventories remotely because many isolations could only be done manually.
The BP Grangemouth Incident Management Team was called out and within 30 minutes the Grangemouth Petrochemical Complex Major Incident Control Committee had been activated at Grangemouth police station. Some difficulties were experienced with the reception of emergency responders and with MICC communications. Control of the access to the incident site was initially difficult to achieve.
The fire reached its maximum intensity within 20 minutes but within 40 minutes started to decline as a result of a reduction in available fuel and the use of aggressive fire-fighting techniques (water and foam). Within 90 minutes the fire size had reduced considerably to the point where the remaining inventory could be allowed to burn off.
The incident was brought under control and several hours later (around 10:00 a.m.) the incident was declared to be over. Damage was mainly limited to columns and associated equipment on the "light ends" section in the vicinity of column E5. Passive structural fire protection prevented significant damage to surrounding structures.
During the initial incident response, consideration was not given to the potential asbestos hazard to fire-fighting teams. It was only later that it was suspected that asbestos from vessel/piping insulation may have become damaged either directly by the fire or by the fire-fighting attempts and tests were carried out to determine the extent (if any) of the asbestos contamination. Whilst most of the asbestos was retained on the plant, some asbestos material was subsequently found in the plant drains, most probably as a result of having been deposited by firewater. There was no evidence of air-borne asbestos having been deposited outside the plant area. For a summary of the key findings of the investigation into the response to asbestos concerns following the FCCU fire see the asbestos investigation.
Gas sampling carried out on the north bank of the River Forth found no abnormal levels of atmospheric pollutants following the incident. For a summary of the key findings of the investigation into the environmental impact of the FCCU fire and the other incidents see the environmental impact investigation.
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.
BP made the plant safe by isolating sections of the plant using existing valves and then by determining how to safely dispose of inventories of flammable material. 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.
For a summary of the key findings of the investigation into the emergency response to the FCCU fire see the emergency response investigation.