CREEPING CHANGES CAUSES AN INCIDENT

The Fluidised Catalytic Cracker Unit (FCCU) was shut down on the 29 th May 2000 following the power distribution failure and was being restarted after an 11-day shutdown. On 10 th June 2000 during start-up a significant leak of hydrocarbons was discovered, creating a vapour cloud which ignited resulting in a serious fire. Workers escaped before the blast, nobody got injured in the incident.
Key learning points
The leak was as a result of failure of a tee-piece connection at the base of the debutaniser column which found a source of ignition nearby. The tee-piece connection which had originally been installed in the 1950’s was correctly specified but incorrectly fitted, and then hidden by lagging. There was no subsequent amendment to the plant layout drawings to identify that change.
Since the 1950’s, sections of the FCCU had been significantly modified. Prior to the modifications in 1986, changes had been made to the pipework at the base of the column and a valve had been removed. This resulted in there being inadequate support for the remaining pipework and the tee-piece connection. Between 1996 and 1998 the FCCU had been experiencing considerable difficulties and did not operate consistently. This resulted in an increase in the number of start-up/shutdown cycles for the plant and pipework. 

An incident occurred in 1999 during a prolonged start-up on the FCCU. It resulted in an ignition
of a torch oil vapour cloud. Contrary to plant operating instructions in the master operating manual, the torch oil had been admitted to the regenerator when the unit was at too low a temperature. As a result, ignition of the torch oil did not occur in the regenerator. Although ignition had not been verified, a considerable further quantity of torch oil was injected, and it is believed that hot spots in the slumped catalyst bed vapourised the torch oil. The provision of a temperature interlock had previously been considered and discounted, as it was decided that operating procedures alone provided enough control.
In the 11 weeks preceding the incident in 2000, 19 start-up attempts had been made and only 7 were
successful. 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 number of start-up/shutdown activities on the plant. Eventually these led to fatigue failure of the pipework in the vicinity of the welded connection. The company reviewed the FCCU to find out why it did not operate properly but the findings were never implemented or communicated properly. The safety report failed to reflect the reality of the condition of the FCCU. The 1997/98 revision concluded that “hardware and software controls in place on the FCCU are adequate to prevent the occurrence of a major accident”. 

Incidents with vibration of the transfer line had occurred over the two years prior to the
explosion. These events were not reported or investigated. There were two incidents preceded the blast on 10 th June, a power distribution failure on 29th May 2000 and the medium pressure steam main rupture on 7th June 2000. Construction of a new facility had started in early 2000. The company hired a sub-contractor for the underground works and the sub-contractor sub-contracted the actual excavation work to an excavation contractor. The company also engaged a main electrical sub-contractor for the electrical and instrumentation work to be carried out. The electrical subcontractor further contracted the laying of the cable in the excavated trench to a cable-laying contractor. The schedule for the excavation and cable laying was very complicated and supervision of the excavation work was limited. On the 25th May a cable-laying operative from the cable-laying contractor observed a damaged tile and cable in preparation for laying a cable but he did not report the damaged cable in the belief that it was dead and it had already been reported. Before that, on 20th April an excavation contractor had been found using a clayspade to the trench at a depth greater than the instructions from the toolbox talks. The earth fault was caused by physical damage to the cable from a clayspade. This case is not a standalone event related to creeping changes. For example, the 2006 Royal Air Force Nimrod crash, Texas City refinery explosion, Buncefield, Shell Moerdijk, the Columbia space shuttle disaster, Bhopal or the Herald of Free Enterprise are cases similar in nature.

Source:IChemE