Environmental Risk Assessment Of Natural Gas Pipelines By Using Modified Catastrophe Progression Method

With the advantages of safety,economy,energy saving and convenient,pipeline transportation has become the main mode of transportation of natural gas.However,in view of the flammable and explosive nature of gas,and the characteristics of outburst,clusters and complexity of gas pipeline,and natural gas environmental risk assessment guide isn't temporarily issued,so it is particularly important that comprehensive and typical index system of environmental risk assessment are constructed and the suitable assessment method are applied.(1)Risk source of natural gas pipeline is divided into material risk and production equipment risk.Natural gas are flammable and explosive gas,belonging to the fire hazard Class A,which its main ingredient is asphyxiation methane.Production equipment risk mainly included gas transmission station,block valve chamber and the equipment of pipeline.Based on risk identification,the multi-level index system environmental risk assessment for gas pipeline was constructed,including the goal layer,system layer,criterion layer and index layer.Among them,with environmental risk of natural gas pipeline as the goal layer,the system layer consists of the risk source,risk receptor,and environmental risk prevention measures.The risk source index,ecological risk index,natural environmental risk index,social economic risk index,and environmental risk prevention measures index formed the criterion layer,which were divided into 16 indicators,such as the main raw material ratio of the maximum storage capacity and critical mass,flammable and explosive accident risk,biological abundance index,the comprehensive air pollution index and accident emergency plan.(2)Modified catastrophe progression method was that the entropy weight method was modified traditional catastrophe progression method,considering the subjectivity of the index sort.By calculating the entropy of index,the indicators of the same level were sorted from big to small according to entropy weight,and different catastrophe model were selected to calculate the membership degree according to the number of control variables.The models established above were employed for environmental risk assessment of natural gas pipelines in the Anhui Jiangbei Industry Zone.The results indicated environmental risk value of construction period and operation period were0.9361 and 0.9221,respectively,belong to ? level.The risk of flammable and explosive accident,forest coverage rate,population density,and the environmental monitoring and early warning system were the main limiting factors.At the same time,the results were consistent with that derived from fuzzy comprehensive assessment method.Theoretical practice proves that this method is feasible and the effect is realistic.Thus modified catastrophe progression method can be used as a quantitative approach which reflect system mutation,and avoid the subjectivity of traditional mutation progression method on index sort.(3)Monte carlo method was applied to three parameters(leakage aperture,wind speed and temperature)to simulate pollutants diffusion of natural gas pipeline leakage.The radius of the maximum allowable concentration was calculated on condition that95% cumulative probability levels for methane,to determine the protection distance of natural gas pipeline accident risk,with coupling of MATLAB.It not only simplify calculation process,also can effectively solve the problem of uncertainty and randomness in the risk assessment.The natural gas pipelines in the Anhui Jiangbei Industry Zone as an example,the results indicated that the methane hazard distance of methane suffocation threshold concentration(176825 mg·m-3)and the maximum allowable exposure concentration(6700 mg·m-3)under 95% cumulative probability levels were123.8 m and 698.5 m,respectively,so the crowd should be outside the scope of this to reduce the harm of gas pipeline accident.Setting its protection distance can be reference to leak source radius,that is,698.5 m.Practice has proven that the evaluation model established above has a certain reference value of environmental management and accident emergency response.