Soon after the incident BP notified the Competent Authority’s duty officer by phone of a major accident falling within COMAH Regulation 2. There was a delay in contacting the relevant HSE Inspectors and their arrival on-site. Investigation started by phone at 07:30 a.m. and HSE s were on-site investigating by 11:00 a.m. The delay did not effect the investigation as the HSE Inspectors would only attend the fire locus once it is safe to do so. In this case it was not until after 10:00 a.m. when the fire was out.
Approximately one month later after calculating the inventory that had been lost it was also reported to the European Union under COMAH Regulation 21 and schedule 7 as the losses of dangerous substances exceeded 5% of the qualifying quantity and probably also exceeded the financial loss threshold.
From the outset the Competent Authority treated the incident as a major accident and applied the major accident investigation procedure (MIRAIM) which lays down a procedural response. HSE Incident Inspectors communicated their initial findings to the HSE Board and the HSE Board directed that a Major Accident Investigation of the three incidents taken together was to be carried out by the Competent Authority.
COMAH Regulation 19 requires investigation of the technical, managerial and organisational causes by the Competent Authority, and specifies the investigation duty on the Competent Authority in detail and the outcomes to be achieved.
The Competent Authority investigation was split into the following areas:
As well as establishing the causes of the incident, it was important to ensure that the damaged site with its highly hazardous inventories was made safe, and that any actions by the Complex to rebuild the plant and reinstate its activities were appropriate with regard to the risks involved. As a result because of the risks from damaged equipment and exposed asbestos and in order to preserve evidence, access to the site of the incident was carefully controlled.
Using legal powers under Section 20 of the HSWA 1974 the Competent Authority issued a legal notice to leave the scene of the incident undisturbed and to control access to the site of the incident and the surroundings so that evidence could be obtained. Using the same powers the Competent Authority also formally requested in writing that documentation was provided by BP and impounded certain items of equipment as evidence for forensic analysis during the course of the investigations. A formal legal notice of possession document was issued.
A BP major incident investigation team was also immediately set-up to investigate the circumstances leading to the fire in accordance with the requirements of "Getting HSE Right (GHSER)" (Element 12 – Major Incidents) led by a BP senior manager from outside the Oil Refinery business unit. The BP investigation made a number of key proposals for corrective actions as a result of the incident investigation and identified a series of actions to be completed prior to the unit restart.
The leak source was identified as being upstream of the first isolation valve on the 6" transfer line at the base of the E5 column at the point where a 3" branch to a redundant pump out system connected into the transfer line. The result was that the entire contents of the column E5 plus associated vessels and pipework escaped with no possible means of preventing it from happening.
Investigations revealed a number of issues:
On examination of the failed 3" branch connection it was discovered that the connection used was a set-on tee-piece. This differed from the original project drawings and piping specifications from the 1950’s, which specified a forged weld-reducing tee-piece. The change was probably made during the initial installation in the 1950’s. Following installation the tee-piece had been covered by insulation and the plant drawings had not been updated.
At the time of the incident the failed 3" branch was supported only at the weld connection to the main transfer line. There were no other pipe supports or restraints on this section of the pipework. The branch comprised pipework suspended vertically from the horizontal transfer line with two manual valves and spool pieces fitted. Hence there was a significant load placed on the support connections when the pipework was full of liquid. Further investigation into the pipework at the base of the column revealed that there had been a modification to the Debutaniser pump out system some time before the end of October 1986. The Debutaniser pump out line had been decommissioned and disconnected from the rest of the plant pump out system by the removal of a valve at the base of the Debutaniser column below the set-on tee-piece. This removed the support provided by the rest of the pump out system pipework and only left the weld at the set-on tee-piece to support the redundant section of the pipework.
Investigation revealed several issues on the adequacy of emergency isolation of the Vapour Recovery Unit.
In addition to the above (incorrect tee-piece design and inadequate pipework support) the pipework was also subject to increased cyclic mechanical stresses as a result of the poor operating performance of the FCCU and the increased number of plant start-ups over the last two years.
Evidence was obtained from the operators that the pipework at the base of the Debutaniser column had also on occasion been subject to vibration over the previous two years and that this had occurred on two occasions previously during start-ups. This information however had largely not been passed on and no investigations had taken place. Subsequent modelling work carried out suggested that this section of the pipework was prone to flow conditions likely to promote vibration in the pipework.
The pipework which failed in the FCCU fire was examined by the Health and Safety Laboratory (HSL). The investigations were hampered by the condition of the fracture surface following the subsequent fire and the full detail of the nature of the fracture could not be resolved. It was established however that there was no evidence of progressive internal or external corrosion of the pipework that led to a condition that promoted failure, nor were there any abnormalities in terms of material composition or condition. There was no evidence of mechanical impact on the pipework.
It was established that there was an element of weld fatigue associated with the crack initiation, which originated from a number of initiation sites and grew to the point where failure occurred. What the HSL investigation failed to establish is whether the weld fatigue took place over a long period of time due to low levels of cyclic stress (as might be present during normal operation of the plant) or whether high levels of cyclic stress resulted in relatively short term fatigue failure (as might be present during abnormal operation at start up or shutdowns).
The HSL investigation concluded that the failure of the pipe was the result of multiple fatigue cracking followed by overload failure of the final ligament. The fatigue cracking probably arose as a result of vibration of the unsupported pipework.
The investigation was unable to draw any conclusions as to the potential source of vibration that led to crack initiation by weld fatigue but modelling carried out showed that the set-on tee-piece was over-stressed for the dynamic loads in the pipework system once the valve in the draindown line had been removed.
Another focus of the investigation was to identify the source of ignition following the release of hydrocarbons. The electrical equipment was found to be suitable for installation in the area concerned. The most probable source of ignition was identified as adjacent uninsulated hot slurry pipework which was used to supply heat to the reboiler for the Debutaniser column E5. This pipework was operating at a temperature of around 300oC some 80-90oC above the auto-ignition temperature of a hydrocarbon (naphtha-like) mixture.
The Competent Authority wrote a COMAH Regulation 15 letter requiring BP to inform the Competent Authority when decisions were made on the future of the FCCU and notifying BP of the Regulation 18 duty on the Competent Authority. BP confirmed that the FCCU would not be restarted without Competent Authority acceptance of the modifications to enhance safety.