Risk Assessment On Natural Gas Pipelines In An Urban Utility Tunnel

With the increasing demand on clean energy in the world,the natural gas industry has been developing rapidly.Natural gas pipeline safety management has become an important issue.Recently the government is promoting that natural gas pipelines being installed inside utility tunnels,which expected to be a more comprehensive and standardized measure to improve the safety of the cities.However,at present,there is still a lack of risk assessment on the gas pipelines entering the utility tunnels.The current sudy carried investigations on the risk and consequence of potential accidents that could happen to the pipelines inside a tunnel,including leakage and explosion.Fault Tree Analysis is employed to make comprehensive risk identification for the leakage of natural gas pipeline in the underground utility tunnel.The result is compared with that of the directly-buried pipeline.Several combination of pipe failure risks was identified by using the Fussel-Vesely method.Main factors affecting the pipeline failure are obtained through qualitative analysis: natural disasters,improper system maintenance,third party damage,and material defects,etc.It is found out that by installing the natural gas pipelines in the utility tunnel,the probability of pipeline failures is significantly decreased,and the third party damage can be effectively avoided.CFD software FLUENT is employed to simulate the diffusion of natural gas inside the tunnel if it is leaked from a pipe under a typical condition.The influence of leaking orifice size,ventilation rate on the diffusion process is investigated.Major results include:(1)with the orifice diameter of 10 mm and ventilation rate of 6 times per hour,the danger range of gas-air mixture grows with time.The natural gas concentration distribution in the tunnel tends to be steady after 5min.The explosive area is limited to 0.2 meter out of the leaking port.(2)The explosive zone increases with increasing the leaking orifice diameter.When the orifice diameter is less than 5mm,the air-gas mixture explosive range is 1m.(3)Higher ventilation rate favors the leaked natural gas to be discharged from the tunnel.Equivalent TNT method is employed to study the consequence of natural gas leakage inside a tunnel.The effect of leaking orifice diameter and ventilation rate on the consequence is studied.Under a typical operation condition,.the results show that:(1)The natural gasaccumulation and explosive radius increase with time,and then become stable after 5 mintes.(2)when the ventilation rate is greater than 6 times /h,further increasing the ventilation rate does not decrease the gas concentration significantly.f is limited.(3)The consequence of leakage and explosion increase with bigger leaking orifice diameter.