At around 8:55 p.m., a natural gas explosion (vapour cloud explosion – VCE) occurred on a bridge pier containing a polyethylene natural gas pipeline (DN125) passing through a stainless steel sheath, and a steel pipeline (DN 114). A fire had initially broken out at around 8 p.m. in a squat on another pier. The heat fluxes released caused the steel pipe to bend and the polyethylene pipe to melt. Natural gas was thus able to migrate through the sleeve via its couplings and form explosive pockets.

Four firefighters injured during the intervention

The fire brigade was informed about the fire upon arriving on the scene and immediately set up a 100 m security perimeter. However, they were unable to intervene due to the presence of a power line routed under the bridge. Four firefighters were injured as a result of the explosions that occurred around 9 p.m. and were hospitalised due to impaired hearing.

Significant social consequences

The gas utility services were able to disconnect the distribution network by closing the shut-off valves surrounding the bridge: 125 households were affected.  5 people were confined indoors and 35 others were evacuated. Road traffic across the bridge is prohibited until 30 November 2018. The bridge’s structure, a drinking water pipe, 20 kV electrical power line and a telecommunication cable passing under the bridge were also damaged. As a result of the explosion, 15,000 mobile telephone and Internet subscribers were also without service. The renovation and rebuilding costs were estimated at 4 million euros.

Testing of natural gas structures

Following the event, testing conducted by the gas utility services revealed no leak on the steel piping, although leaking was detected on the polyethylene pipe. Traces of the melting of the polyethylene pipe were visible along the stainless steel sleeve, however. The source of the explosion’s ignition could not be determined.

Lessons learnt

The event which occurred at the Albertin bridge demonstrates the amount of attention that must be paid to securing utility networks (telecommunications, gas, electricity, drinking water, etc.).

In addition, several lessons more specifically concerning natural gas networks call for questions about:

  • The identification, among the singular points already identified distributors or carriers (crossing of waterways, railroad tracks, etc.) of configurations similar to that of the accident (premises where there is an accumulation of an abnormal heat load, routing in a confined environment without external venting and inaccessible for inspection);
  • The relevance of an action plan to strengthen controls in the previously identified areas or the consideration of their removal.

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