Study On Risk Analysis And Control Of Offshore Natural Gas Hydrate Trial Production

Offshore natural gas hydrate trial production is faced with high accident risk,which is different from conventional offshore oil and gas production methods and characteristics,and there are still unclear fault types and risk evolution mode.Therefore,it is of great significance to carry out the study on risk analysis and control of offshore natural gas hydrate trial production.This study is aimed at the exploitation demand of natural gas hydrate and based on the national key research and development plan project“Safety monitoring and risk assessment technology of offshore natural gas hydrate trial production”（2016YFC0304005）and major science and technology projects of China National Petroleum Corporation Ltd“Gas Hydrate Test wellhead,sand control equipment and safety key technology”（ZD2019-184-004）,systematically develops the study on risk identification,risk assessment of borehole wall collapse,consequence assessment of fire and explosion of wellhead leakage,and risk control decision in offshore natural gas hydrate trial production.It provides reference for safe natural gas hydrate trial production.The main research is as follows:1.Risk identification of offshore natural gas hydrate trial productionIn offshore natural gas hydrate exploration technology and equipment as the study object,analysis natural gas hydrate characteristics,investigate characteristics of natural gas hydrate reservoir and production technology,identify key risk factors in the trial production using HAZID approach,analyze failure mode and consequences of each component of key equipment in the trial production using FMECA approach.This provides a basis for risk assessment and control of high-risk accidents in the trial production.2.Risk assessment of offshore natural gas hydrate borehole wall collapseAnalyze the risk factors of the borehole wall collapse accident in the trial production,constructe the T-S fuzzy faultt tree and fuzzy Bayesian network model of borehole wall collapse;determine the prior probability of each basic event by fuzzy evidence theory,according to the probability calculation,the occurrence probability of borehole wall collapse is 1.04×10^-5,which is in the risk reduction area,determine key influencing factors according to the importance analysis and sensitivity analysis;analyze the consequence of borehole wall collapse,constructe the Bayesian network model of borehole wall collapse consequence,evaluate the losses combining with the consequence loss classification table,and the risk value of borehole wall collapse consequence is 2.28×10^-4.3.Risk assessment of offshore natural gas hydrate gas overflowEstablish the simulation model of deepwater semi-submersible drilling platform with FLACS software,build wellhead gas leakage and diffusion accident scenario model to study the law of gas leakage and diffusion;simulate fire and explosion accident scenarios to study the influence degree of different factors on the consequences of fire and explosion accident;designe the orthogonal test for the influencing factors of fire and explosion to analyze the primary and secondary degrees of the influencing factors on the consequences in the accident scenario,in the condition with the greatest consequence,the average daily gas production of the second trial production was selected as the leakage rate for simulation,the results show that the risk of fire and explosion caused by gas overflow in the trial production process is small.4.Risk control decision of offshore natural gas hydrate trial productionTo study control decision-making of risk in the natural gas hydrate trial production,analysis of the optimal security configuration method based on risk and Bayesian influence diagram model to put forward a new risk control decision-making method,take the borehole wall collapse as an example;make fault diagnosis by equipment fault diagnosis method based on fuzzy Petri net,take the drilling fluid cooling system as an example,the results show that the maximum confidence of the drilling fluid cooling system fault is 0.2768 and the first order of fault detection is compressor;make maintenance decisions by equipment maintenance decision-making method based on fuzzy comprehensive evaluation,take the compressor in the drilling fluid cooling system as an example,the results show that regular maintenance should be adopted.