Analysis On Risk Due To Corrosion And Leakage Of High Sulfide Natrual Gas Gasthering Pipeline

Quantitative risk research on corrosion and leakage of high sulfide natural gas gathering pipeline was systemically conducted in this dissertation, which is a part of the sub-topics'gathering process and safety control techniques in high sulfide gas field'of national science and technology special project"Large-scale oil & gas fields and coal bed gas Exploitation"(No. 2008ZX05017).Through theoretical research, numerical calculation and computer simulation, progress were made in these aspects including the corrosion factors of high sulfide natural gas gathering pipeline, quantitative risk prediction on corrosion and leakage, pre-warning technique on corrosion and leakage, risk evaluation index system and measures of corrosion and leakage, inspection method optimization on corrosion and leakage and so on. The main works are summarized as follows:1 Analysis on Corrosion Factors of High Sulfide Natural Gas Gathering PipelineThere are three types of corrosion factors in high sulfide natural gas pipeline including internal corrosion, buried metal corrosion and atmospheric corrosion. The internal corrosion mainly includes products corrosion and inside corrosion protection of the pipe; the buried metal corrosion mainly includes underground environment factors, cathodic protection and coating; atmospheric corrosion mainly includes atmospheric exposure, atmospheric type and wrapped layer and so on. The influence of pipeline operation parameters such as flow rate, pressure and temperature to the internal corrosion was analyzed. Hydrogen sulfide concertration, the main influence factor leading to internal corrosion was identified based on grey relevance method. Detailed analysis on corrosion factors of the high sulfide gas pipeline was conducted by means of Fault Tree Analysis, the structure importance degree of each factor and the minimal cut sets causing corrosion failure were figured out.2 Quantitative Risk Prediction on Corrosion and Leakage of High Sulfide Natural Gas Gathering PipelineQuantitative risk prediction for the consequence caused by leakage of high sulfide gathering pipeline was systematically conducted using Computational Fluid Dynamics(CFD) method. The CFD model of high sulfide natural gas leakage in gas station was established. Also the influence of grid dependency, horizontal homogeneity of Atmospheric Boundary Layer, and validity of model were take into consideration. The prediction of the consequences as result of leakage of high sulfide natural gas under a variety of working conditions was achieved. The prediction results show that the risk of individual poisoning was obviously higher than the risk of explosion of inflammable gas.The technic of computation grid generation over complex mountainous terrain was presented. First, GIS data of pre-modelling terrain was dealed to get the coordinate of the longitude, the latitude and the altitude. Then the reference point was fixed meanwhile the coordinates were transformed to planimetric rectangular coordinates with a XYZ format. Finally the CFD mesh of real terrain was established to make realistic prediction of dispersion consequence in result from the high sulfide natural gas leakage over complex mountainous terrain. A case study under some working condition has been carried out to making prediction for the dispersion rules of high sulfide natural gas accidental leakage over complex mountainous terrain in 30 min.A CFD numerical simulation and dose–response model combined approach has been proposed for quantitative analysis of acute toxic gas exposure threats. First the toxic gas concentration distribution in time and space was got by CFD simulation, then toxic dose according to gas concentration and exposure time was calculated. Evaluating exposure personnel death percentage according to dose reaction model is the last step. Quantitative risk analysis of individual poisoning in high sulfide natural gas station was conducted. An example of leakage under a certain working condition was studied to evaluate the personnel poisoning death percentage assuming workers stay at original exposure location without moving or make emergency evacuation.3 Pre-Warning Research on Corrosion and Leakage of High Sulfide Natural Gas Gathering PipelineThe technical flowchart of pre-warning system on corrosion and perforation of high sulfide natural gas gathering pipeline has been presented. The pre-warning contains three stages including gathering pipeline corrosion risks identification, corrosion risks prediction and pre-warning, corrosion and perforation emergency response and control. Detailed technical analysis of every stage has been conducted. This work could provide dependable references for the decision making of pre-warning and emergency responses in related areas.The prediction mathematical models under multiplicate corrosion rates for pre-warning system were set up including the grey model, BP neural network model, Monte-Carlo model and grey neural network model, which could provide dependable references for the decision making of pre-warning on corrosion and leakage.A remaining strength assessment based pre-warning method of corroded pipelines transmission capacity decrease was proposed. The classification standards and the corresponding thresholds of pre-warning incidents were established.4 Risk Assessment Index System on Corrosion and Leakage of High Sulfide Natural Gas Gathering PipelineThe relative risk of pipeline corrosion was assessed by corrosion index which includes atmospheric corrosion index, internal corrosion index and buried metal corrosion index. Each index contains some 2nd indicators and 3rd indicators. Detailed index scoring standards were given. Cumulative score of each index was the relative risk of pipeline corrosion, range from 0 to 100. The higher the score, the lower the risk of corrosion.To avoid the subjective uncertain factors in the scoring process, Analytic Hierarchy Process was introduced according to the factors such as the expert's knowledge background, work experience and expertise to identify each expert's scoring weight of internal corrosion index, atmospheric corrosion index and buried metal corrosion index. The weight multiplied the score and then was accumulated to get the final score of corrosion index in a more objective way.The relationship of pipeline relative and absolute risk was investigated. The correspondence of corrosion index relative risk, corrosion speed and the possible failure time of corrosion perforation was set up to increase the intuition on the risk evaluation result of corrosion index.5 Detection Optimization Research on Corrosion and Leakage of High Sulfide Natural Gas Gathering PipelineThe method of detection optimization on leakage of high sulfide natural gas was presented based on CFD. The layout of the gas detection and alarm instruments in a high sulfide natural gas station has been optimized by means of setting up a series of monitor points in CFD model. The methane and hydrogen sulfide volume fractions of monitor points were recorded by the simulation of natural gas release and dispersion process. The sensitivity to combustible gas detectors high and low level placement, validity and response time of both combustible and toxic gas detectors in large and small release rates were investigated. Then some advices about optimization of gas detection and alarm networks on high sulfide gas gathering station were provided.