Risk Analysis And Study On Calculation Method Of Safety Distances For Natural Gas Pipelines

Pipeline transportation is the preferred alternative for the transport of natural gas,and occupies an important position in national economy and social development.In recent years,the rapid development of natural gas pipelines provides great conveniences to national industrial production and people's life,but it also presents new challenges to the reasonable set of safe distances between the pipelines and the public.Too large safe distances will waste the land resources,but too small safe distances will increase the risk of fire and explosion accidents.Determining reasonable safety distances is of great importance for guaranteeing the safe operation of natural gas pipelines,people's life and property safety.Presently,the standards for safety distances are insufficient,and are based on the analysis of the foreign standards and the summarization of the domestic experiences.In this thesis,the calculation method of safety distances based on risk analysis for natural gas pipelines is proposed.The approach that considers both the accident occurrence probability and the accident consequences severity is an effective means for guiding the reasonable set of safety distances for natural gas pipelines.The present approach for estimating safety distances of natural gas pipelines can be briefly described as follows.Firstly,based on the statistics of the failure data of natural gas pipelines in China and abroad,the risk factors and the failure probability data of natural gas pipelines are analysed.The basic failure probability is determined by Bayesian theory,and the failure probability of a specific pipeline is calculated using the empirical correction equation.The calculation procedure of accident occurrence probability for natural gas pipelines is established combining with the ignition probability analysis.Secondly,through the analysis of the consequence calculation procedure,the accident consequence types and the accident consequence calculation model,the accident consequences of jet fire and vapour cloud explosion are numerically simulated.The influences of the factors on the accident consequences are analyzed,including the pressure,the pipe diameter,the leakage hole diameter,the leakage direction and the wind speed.Then,the individual risk and the social risk are selected as the risk calculation indexs,and the quantitative risk calculation model for natural gas pipelines is established.The individual risk acceptable criteria and the social risk acceptable criteria for natural gas pipelines in our country are suggested according to the ALARP principle.On this basis,the calculation method of safety distances for natural gas pipelines based on the risk analysis is proposed.In addition,on the basis of the worst credible accident scenario analysis,the calculation method of safety distances based on the consequences is proposed to examine the superiority of the present approach.The primary contents of this thesis are as follows.(1)Research on risk factors and failure possibility levels of natural gas pipelines.According to the analysis of accident cases of natural gas pipelines in China and abroad,the construction operations,the corrosion,the material defects,the improper maintenance,the ground movement and the wrong operation are the important causes for pipeline failures.Based on the accident statistics of natural gas pipelines of EGIG,PHMSA,UPOKA,AER and China,the accident causes and the proportions are ascertained.The third party damage,the inner corrosion,the outer corrosion,the construction defect/material failure and the soil movement,etc,are determined as the most primary causes for natural gas pipeline failures.On this basis,the primary risk factors are summarized,including third party damage,corrosion,faulty operation,construction defect/material failure and soil movement,etc.The uncertainty measurement theory is put forward for evaluating the failure possibility levels based on the risk factors analysis,and provides theoretical basis for mastering the dangerous levels and identifying the high-risk pipe sections of natural gas pipelines.(2)Study on calculation method of accident occurrence probability for natural gas pipelines.Through statistical analysis the failure data of natural gas pipelines in Europe,the United States and other countries,we found that the failure probability recommended by EGIG is 0.329×10-3/(km?a),PHMSA is 0.131×10-3/(km?a),and UPOKA is 0.255×10-3/(km?a).In Europe,the failure probability of pinhole/crack is the highest,the next is the hole,and the last is the rupture.Third party damage mainly gives rise to the hole failures,the next is the pinhole/crack and the rupture.Corrosion mainly causes the pinhole/crack failures.Construction defect/material failure mainly causes the pinhole/crack failures,the next is hole failures.Soil movement mainly causes the rupture failures.Rupture is mainly caused by the third party damage,and the next is the soil movement.On the basis of EGIG failure data in Europe,the basic failure probability is analyzed using Bayesian theory,the results show that the failure probability declines steadily on the whole in recent years,and stabilizes gradually with the increase of failure database.Presently,the basic failure probability of natural gas pipelines is 0.39×10-3/(km?a).Based on EGIG failure database,the primary failure probability and the secondary failure probability of each failure cause are analyzed,the correction parameters and the correction formula of each failure cause are determined.According to the ratio of the secondary failure probability and the primary secondary failure probability of each failure cause,the correction coefficients of each correction parameter are determined.The basic failure probability of each failure cause is corrected using the correction formula,and the pipeline failure probability for per unit length is calculated using the sum function.The basic failure probability is corrected for a specific natural gas pipeline using the empirical correction formula.Combining with the ignition probability analysis of natural gas pipeline failures,the calculation model for accident occurrence probability is established.(3)Study on calculation and numerical simulation of accident consequences for natural gas pipelines.Based on determining the representative material,choosing the leakage hole diameter,estimating the total leakage substances,ascertaining the gas leakage rate and the leakage type,etc.,the calculation procedure of accident consequences for natural gas pipelines is established.The accident consequence types,the accident consequence damage forms,the accident consequences model,the damage models for fire thermal radiation and shock wave overpressure of explosion are analyzed.Based on selecting calculation procedure and determining simulation conditions,the typical accident consequences,such as the jet fire and the vapor cloud explosion,are numerically simulated.The influences of factors on the accident consequences are analyzed,including the pressure,the pipe diameter,the leakage hole diameter,the leakage direction and the wind speed.The influence distances of accident consequences in different conditions are determined as well.The results show that the heat radiation influence distances of jet fire increase with the increasing of the pressure,the pipe diameter and the leakage hole diameter,and increase with the decreasing of the angle of leakage direction.The thermal radiation intensity of jet fire increases with the increasing of wind speed when the downwind distance is smaller,and is less influenced by wind speed when the downwind distance is larger.The influence distances of shock wave overpressure of vapor cloud explosion increase with the increasing of the pressure,the pipe diameter,the leakage hole diameter and the level of atmosphere stability,and increase with the decreasing of the angle of leakage direction.The overpressure is less infulenced by wind speed when the downwind distance is smaller,and increases with the decreasing of wind speed when the downwind distance is larger.The accident consequences of the jet fire are more serious than the vapor cloud explosion under the same conditions on the whole.(4)Research on quantitative risk calculation and risk acceptable criteria of natural gas pipelines.By analyzing the individual risk,social risk and environment risk existing in natural gas pipeline system,and comparing the public concern degree on the risk types,the individual risk and the social risk are selected as risk indexes for quantitative risk calculation and safety distances calculation of natural gas pipelines.Based on the study of the accident occurrence probability calculation model and the accident consequences calculation model,the quantitative risk calculation procedure,the individual risk calculation model and the social risk calculation model are established.Through the analysis of the principle and method for determining the risk acceptable criteria,and the research results of risk acceptable criteria for hazards(including pipelines)in China and abroad,the individual risk acceptable criteria for new pipelines and pipelines in service are proposed in our country,and the social risk acceptable criteria for the length of one kilometer pipeline are suggested as well.(5)Study on calculation method of safety distances for natural gas pipelines.Based on the relative explanations for safety distances of natural gas pipelines,the safety distances focus on the influence of thermal radiation and overpressure,and two methods for estimating safety distances are determined,including the risk based approach and the consequences based approach.Considering the differences of the sensitive degree and the intensive degree of population around the pipelines,four types of protective objects for safety distances are defined.Based on the quantitative risk calculation model and the research results of individual risk acceptable criteria for different protection objects,the risk based calculation procedure and calculation model for safety distances is established.According to the secondary failure probability analysis of each failure cause,the risk reduction measures are put forward,which provide a direction for reducing safety distances requirements.The risk based safety distances are applied to divide risk zones,and the land use planning suggestions for each risk zone around the pipelines are proposed according to the acceptable criteria for the individual risk and the social risk.Based on the literature analysis,the simulation and comparison of accident consequences,the worst credible accident scenario is determined.Combining with the safety distance setting standard,the consequences based calculation procedure and calculation model for safety distances is proposed.The consequences based safety distances are applied to divide the emergency response areas,and provide theory basis for formulating accident emergency preplans for pipeline companies or local governments.(6)Based on the research results of this thesis,the risk based approach for estimating safety distances of natural gas pipelines is applied to engineering cases.The changes of safety distances before and after taking the risk reduction measures are analyzed,and the risk based safety distances are compared with the consequences based safety distances.The results show that the consequences based approach only considers the accident consequences severity,the safety distances determined are often too large,and the feasibility of this approach is poor.However,the risk based approach considers both the accident occurrence probability and the accident consequences severity,and the safety distances for different protective object types are determined according to the individual risk acceptable criteria.The accident risk of different objects types are effectively controlled within permissible ranges.In addition,the risk reduction measures decrease the safety distances requirements effectively.Therefore,the risk based method for estimating safety distances of natural gas pipelines is scientific,reasonable and feasible.It has important theoretical significances and reference values for guiding the reasonable set of safety distances and the land use planning around the pipelines.The main originalities of this thesis lie in:(1)Based on the Bayesian theory,the present basic failure probability of natural gas pipelines is determined as 0.39×10-3/(km?a).According to the analysis of EGIG failure data,the correction parameters,the correction formula and the correction coefficients of each failure cause are determined.It relealizes the correction of the basic failure probability of a specific natural gas pipeline.(2)The individual risk acceptable criteria for four types of protection objects and the social risk acceptable criteria for one kilometers pipeline are studied and determined.(3)The risk based approach for estimating safety distances of natural gas pipelines is proposed.Moreover,the proposed risk reduction measures provides a direction for effectively reducing the safety distances requirements.