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Executive summary - Direct and underlying causes

Power distribution failure

The loss of electrical power was caused by damage to a 33kV underground electricity feeder cable which eventually resulted in an earth leakage (electricity flowing to earth) from the cable. The damage had been caused to the electrical cable during excavation of a trench for the installation of a new cable, sometime before the distribution failure occurred.

The local circuit breaker on the distribution system failed to operate due to the insertion of small plastic connectors which isolated the relay. Two circuit breakers located elsewhere in the distribution system subsequently tripped to clear the fault resulting in the loss of power supply to significant parts of the Complex.

Subsequent investigations revealed a number of weaknesses in the safety management systems on-site over a period of time which contributed to the succession of events that resulted in the power distribution failure.

MP steam main rupture

The site wide power distribution failure on 29th May 2000 resulted in excess amounts of water (associated with the shutdown of utility supplies) being sent to drain, as well as the unavailability of electrical power to drainage pumps. This led to the flooding of culverts (service tunnels) beneath the A904 Bo’ness road through the site which contained medium pressure (MP) stream distribution lines. During the following investigations to determine whether the flooding had caused any damage to the pipework a steam trap located in a low point in the section of pipework beneath the road in the West Gemec culvert was closed to allow safe access for inspection. The steam trap was subsequently not re-opened and this prevented the removal of condensate (hot water produced by the condensation of steam) from this section of the system. As the liquid condensate level built up in the pipework a quantity of steam (or "steam bubble") was trapped between the hot condensate and closed isolation valves on the southern side of the culvert beneath the road. Eventually collapse of the steam bubble resulted in a phenomenon called "condensation induced water hammer" which led to a gross overpressure and the subsequent catastrophic failure of the pipeline.

Subsequent investigations revealed a number of weaknesses in the safety management systems on-site over a period of time which contributed to the succession of events that resulted in the MP steam main rupture.

Fluidised Catalytic Cracker Unit (FCCU) fire

The Fluidised Catalytic Cracker Unit had been shutdown as a direct consequence of the power distribution failure. During start-up of the unit on 10th June there was a leak of hydrocarbons which were subsequently ignited and resulted in a fire on the plant.

Investigations revealed that the leak was as a result of failure of a tee-piece connection at the base of the Debutaniser column which then found a source of ignition nearby (probably an uninsulated hot flange).

During the investigations the tee-piece connection which had originally been installed in the 1950s was found to be correctly specified but incorrectly fitted and then covered in lagging. (A set-on tee-piece had been installed whereas a seamless forged weld reducing tee-piece had been specified.) There had been no subsequent amendment to the plant layout drawings to identify the change. 

Prior to the mid 1980’s modifications had been made to the pipework at the base of the column and a valve removed which resulted in there being inadequate support for the remaining pipework and the tee-piece connection.

Further modifications to the FCCU in 1996/1998 had resulted in the FCCU being increasingly difficult to operate reliably. This had resulted in an increase in the number of start-up/shutdown cycles for the plant and pipework.

Failure of the tee-piece connection pipework was probably caused by a combination of the incorrectly fitted tee-piece connection, the inadequately supported pipework and the cyclic stresses/vibration caused by the increased start-up/shutdown activity on the plant. Eventually this led to "fatigue" failure of the pipework in the vicinity of the welded connection.

Subsequent investigations revealed a number of weaknesses in the safety management systems on-site over a period of time which contributed to the succession of events that resulted in the FCCU fire.

2010-10-25