Pollution
Humain
Environnement
Economique

Pipe characteristics:

  • Steel pipe with cathodic protection
  • DN 250 (273 mm)
  • Medium pressure (MPB) grade (P<4 bar)
  • Year laid: 1967
  • Burial depth: 2 to 3.80 m
  • Coal-tar pitch coating

At around 4:00 p.m., the firefighters informed the gas company that a pungent odour of natural gas was coming from a street (Montée des Esses) facing a detached house. A gas technician arrived and found high concentrations of natural gas. The house’s occupants were subsequently evacuated.

Hard-to-find leak

Excavation work to locate the leak began at 7:30 p.m., but by 1:00 a.m. the leak still had not been found. The firefighters and the city decided to cut the gas supply to 906 private customers and 26 businesses and facilities (including a retirement home). The gas leak on the network serving the Montée des Esses was pronounced over at 2:43 am. The gas lines serving the Part-Dieu/Tête d’Or districts were used to supply the Croix-Rousse district. The leak was located on a pipe conduit running under the road at around 3:00 p.m.

Investigation findings

On 23 January, the inspection authorities for classified facilities went to the site to speak with the gas company about the technical causes of the leak and how the emergency was managed. Excavation work was still in progress as the exact location of the leak had not yet been found. A company was brought in to look for the leak using an endoscope camera. However, it did not find any corrosion or breaks and cautioned that it would be difficult to detect small cracks or holes. The inspection authorities asked the gas company for information on the monitoring of the pipe’s cathodic protection.

After the portion of the pipe running under the road was excavated, a 15 cm-long crack along the top of a weld bead was found. This section of the pipe was removed for metallurgical analysis.

Hypothesis for the cause of the crack

Mechanical strain that accumulated since the pipe’s installation and resulting from shifts in the ground is believed to have caused the crack. In addition, the ground is located on a steep incline and the road had been widened in recent years. Elastic kickback of the pipe was observed during the excavation work, probably reflecting the release of the accumulated mechanical stresses. The metallurgical analysis laboratory corroborated this hypothesis and found a penetration defect on the weld bead. However, no weld inspection recommendations were yet in force at the time the pipe was laid.

Following this event, the gas company called on a working group to catalogue all the pipes subjected to similar stresses. This work was carried out in early 2018.