Characteristics of the pipeline:
- DN 800
- Pressure 63 bar
- thickness of the tubes: 9.5 mm
- tar coating of coal
- Year of construction: 1960
- operating flow: Stopping pipeline under static pressure of natural gas since January 5, 2014
Circumstances of the accident:
- Cold weather: temperature of -26 ° C at the time of the accident
- Work subjected to a single hydrostatic test since its construction
An explosion occurred at about 1:00 a.m. on a natural gas pipeline. Flames reaching a height of 300 m were visible. The emergency services evacuated 5 homes and shut down traffic on the nearby expressway. For safety purposes, adjacent pipelines were also closed. The fire was extinguished at about 3 p.m. (the gas burned for nearly 12 hours).
4,000 customers were deprived of gas for 80 hours, requiring hospitals and emergency shelters to be supplied by tanker lorries. The explosion formed a crater measuring 24 m long and 12.5 m wide. Debris was thrown within a 100 m radius.
2 ruptures over 11 m
The pipeline broke at 2 points, 11 m apart. The first failure was at a connection with another pipe and a valve. The second rupture was located at a circular joint between 2 sections of welded pipes. This 2nd failure was induced by the reaction forces of the 1st, causing the structure to lift and then bend at the circular joint.
After examining the broken sections, it appeared that:
- the pipe initially failed (failure #1) at a pre-existing crack that had remained stable for more than 50 years, and there was no indication that the defect had progressed,
- This crack dated back to the construction of the pipeline, the poor quality of the welding carried out in the workshop was determined to be the origin. The inspection of each weld by radiography was not required at the time, nor was the archiving of inspection logs,
- the strength of the steel was low and did not meet current requirements (the steel specifications were not included in the construction codes in effect at the time the structure was commissioned).
Several factors are also mentioned as being at the origin of the failure:
- ground movements creating additional stresses around the valve,
- excessively cold winter temperatures leading to deep freezing of the ground and also modifying the distribution of mechanical stresses.
Scraping was carried out on the structure in 2001 and again in 2009. Excavations to verify the integrity of the structure were also carried out in 1997,1998,1999 and 2009, in areas near the accident site. A 38 m section was changed following the discovery of traces of stress corrosion.