| This dissertation is supported by “Safety Risk design and control technology” of the independent innovation engineering on the seventh generation deepwater semi-submersible platform from the Ministry of Industry and Information Technology of China,and “Research on Evolution of Chain Risk,Disaster Mechanism,and Response Mechanism of Major Accident for Oil and Gas production in Offshore Industry” from National Key R&D Plan of China.According to the site actual operational requirement of deepwater drilling in South China Sea,taking the 7th generation deepwater semi-submersible platform that is an original design as the main research object,a systematic research on blowout accident evolution and safety barrier is carried out.The research has made significant progress in blowout vulnerability analysis of deepwater drilling system,blowout accident evolution,safety analysis of well control system,blowout gas dispersion,and quantitative assessment of safety barrier system.Furthermore,the risk assessment and control method for deepwater drilling blowout accident are formed,providing effective technical references for the smooth and safety operation of the deepwater semi-submersible platform in the South China Sea.The main progress of the paper is summarized as follows:1.Vulnerability analysis of the complex deepwater drilling systemConsidering the structure composition of the complex drilling system for the deepwater semi-submersible platform,the complex system characteristics and vulnerability characteristics are analyzed.Aiming at evaluating the the vulnerability of the drilling system,the risk indicators and entropy indicators are put forward and the two indicators are integrated into risk entropy indicators.In view of the topological structure and non-topological factors of complex systems,the Complex Network method and the utility evaluation method are used to measure the vulnerability characteristics of the drilling system,respectively.According to the development process for the blowout accident of the deepwater drilling system in combination with the hierarchical structure of the system,an index system is established in terms of the facility,process,human,management,and environment,to provide the underlying basis for systematically and quantitatively evaluating the vulnerability and risk of the deepwater drilling system.2.Accident evolution for deepwater drilling blowoutBased on the index system of vulnerability and risk evaluation for deepwater drilling system,considering the logical relationship and hierarchical categorization of the vulnerability factors,a chain risk evolution model for deepwater drilling blowout accident is constructed,which takes the risk factors as nodes and the risk transitive relationship as the connect edges.The evolution model integrates the risk entropy theory and Complex Network theory to the deepwater drilling procedure and the development process of the blowout accident.In the model,the Clustering Coefficient is used to measure the cluster degree of the nodes in the network,and the risk entropy that covers both the stochastic uncertainty and fuzzy uncertainty is used to evaluate the weight of the transmission path.To recognize the most potential failure modes composed by the risk nodes,the shortest path is applied to measure the blowout accident accident with the Dijkstra algorithm.Furthermore,to evaluate the dynamics of the risk,the maintenance factors are introduced into the model,and the values of entropy and probability in different time intervals can be calculated dynamically.3.Safety analysis of deepwater well control based on STAMP/STPAIn view of the characteristic of complexity and dynamism,the safety analysis of deepwater well control during the drilling process is regarded as system control and feedback problem in this paper.Different form the traditional methods based on failure probability,we use the STAMP(system-theoretic accident model and process)accident model based on system theory and STPA(system-theoretic process analysis)method to construct the control interaction model and feedback control loop of well control system.In the model,the systematic risks and restrictions are identified.Then the control structure of safety-related is defined,and the key factors which contribute to unsafe control behaviors are analyzed.Using the dynamic multiphase simulation software OLGA,we construct the process of well control to simulate the unsafe control behaviors during the deepwater drilling.The simulation takes the no providing of control after well kill and the delay of control from shut in to well kill for examples.It turns out that the STAMP/STPA method is an effective solution to evaluate the safety of deepwater well control from the perspective of control and feedback.It quantitatively indicates that the rational control can prevent accidents occurring and escalating and ensure the system safety in certain time horizon,by modeling the process of well kick,shut in,and well killing.In the meanwhile,the results also demonstrate that the method is available and useful for the safety analysis of deepwater well control.4.Research on blowout gas dispersion law during the deepwater drillingAiming at the problem of gas dispersion after blowout out of control in deepwater drilling,this section takes the seventh-generation semi-submersible platform that is under construction as research object to analyze the blowout and diffusion of natural gas.A simulation test system,based on geometric and dynamic similarity theory,is designed,which could monitor the concentration of flammable gas at different sampling points with time.A CFD(computational fluid dynamics)-based numerical simulation model using the FLACS software system is established to predict and evaluate the consequences of gas diffusion.The simulation results are consistent with the results from the experiment,the simulation results,while the simulation results obtained from FLACS are more intuitive and precise.The contrast between the two methods illustrates the validity of the numerical model in handling with flammable gas blowout and dispersion law.On the basis of simulation model,wind direction and wind velocity of the wind field in which the platform locates,whose influence on the gas diffusion behavior and motion direction was analyzed in detail.Taking the shale shaker house as an example,the 3D simulation model is constructed to analyze the diffusion process that the flammable gas and toxic gas enter into the confined space,and determine the fire and explosion zone and the poison zone.Finally some recommendations of prevention,control and emergence response for engineering application are proposed,according to the structural distribution in combination with the gas diffusion behavior.5.Quantitative analysis of safety barriers for preventiong blowout accident escalationConsidering the system vulnerability and the chain of events caused by the deepwater blowout,an integrated method for risk assessment based on DEMATEL-BN(Decision Making Trial and Evaluation Laboratory-Bayesian Net)is proposed to evaluate the vulnerability of the accident escalation dynamically.First,a three-level index for risk assessment of risk escalation is established,which includes factors,events and subsystems.Second,the interactions among the factors that influence the development of accident are analyzed based on DEMATEL method.Last,to realize dynamic analysis the cascading risk of the platform,a sequential model of blowout accident is built to evaluate the safety barrier failure probability and consequence likelihood of abnormal events using accident precursor data based on the BN method.It turns out that the proposed approach of dynamic risk analysis for deepwater drilling systems overcomes the disadvantages of traditional risk assessment methods that the operating data cannot be used effectively.It can dynamically assess accident probability and dynamically reflect risk change of the deepwater drilling system to provide references for risk analysis and control of the semi-submersible platforms. |
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