Consequences of a 12-mm diameter high pressure gas release on a buried pipeline. Experimental setup and results

When considering the transportation of gas through high pressure pipeline and associated permitting, the 12-mm diameter breach is one of those commonly considered as accidental scenarios and the associated consequences must to be calculated for determining the required safety measures. Up to now this “12-mm” scenario was modeled at the convenience of each risk analyst with no certitude on the real behaviour of the gas in the soil for these types of releases. To obtain concrete information on the “12-mm” scenario considered as the sizing event for safety distances associated to a little breach due to corrosion for instance, AIR LIQUIDE, ENGIE, NATIONAL GRID, PETROBRAS and TIGF decided to launch in 2013 a JIP (Joint Industrial Program) named “CRATER”. The aim of this project was to improve knowledge on the consequences of leakages occurring on buried high pressure pipes, and to determine what were their behaviour and their impact on the soil – i.e. crater formation, or not, according to release parameters – in order to use the appropriate methodology for risk and consequences assessment. Thus by changing several parameters – like nature of gas, initial gas pressure, type of soil … – the threshold between crater formation and gas dispersion in the soil following such leakages was investigated (specifically for methane and hydrogen, flammable and light gases). INERIS was chosen as subcontractor to perform nearly-full scale tests on its experimental site in order to collect reference data, understand phenomena and correctly assess the gas behaviour for accurate risk evaluation. •Nearly-full scale experiments were performed simulating light gas accidental releases on a 1-m depth buried pipeline.•The 12-mm breach scenario was studied for several pressures and leak orientations in a sandy and clayed soils.•Depending on the release and the soil, crater formation or crown uplift of the soil were observed at the ground surface.•When a crater is formed, releasing light gas evacuates as a high pressure free jet.•When an uplift is observed, releasing light gas evacuates through the crown at very low velocity and is quickly dispersed.