At around 9:30 a.m., a technician working on a drinking water network, damaged a steel natural gas distribution pipe (DN 219, P 16 bar, year 1968). Firefighters evacuated five people from a garage and confined the students in a nearby school. Traffic was interrupted during the operation. The natural gas distribution network was flushed by flaring in order to undertake the necessary repairs.

Context of the work

A public works (PW) contractor had been called in to move drinking water networks. On the day of the incident, the PW contractor had wanted to remove part of a supposedly abandoned section of the natural gas network in order to add a new section to the drinking water network. To ensure that the old network was emptied of gas, the technician used a drill to pierce a hole in the line. This operation resulted in a natural gas leak. A plug was placed on the leaking pipe to stop the leak.

Causal analysis of the event

A declaration of intent to begin works (DICT) was drawn up on 10/10 by the PW contractor. The plans indicated that three gas pipes were present and, once in the trench, the workers were able to identify three pipes covered by yellow safety netting. Nevertheless, several elements contributed to the leak:

• the intervention on the DN219 steel pipe instead of on the DN120 cast iron pipe (the site foreman had little experience);
• use of a drill that was not authorised by the company’s management for such work;
• lack of a verification procedure (verification of dimensions, diameter measurement, etc.) ;
• the gas utility was not contacted to ensure that the correct network was being worked on;
• inversion of the burial depth on the drawings of the pipeline out of service (DN 120) and the one still in service (DN 219).

Measures taken

Following the event, the PW contractor developed its organisation along the following points:

• systematic participation of the site manager in the preliminary meeting to avoid loss of information;
• intervention prohibited on unidentified networks;
• verification that all site technicians in the region knew how to read the network drawings.

A feedback newsflash was also written and distributed internally within the PW company. The gas utility also assisted by providing field training to the technicians.