Simulation Of Operational Vulnerability For Urban Metro System And Its Application

As the essential and indispensable public transport means of urban daily traveling,metro plays a role of crucial importance in the modern people's daily life.Considering its advantages in land use,energy consumption,air quality,landscape quality,and passenger service quality,metro is favored by urban planners and city dwellers,and is increasingly being the backbone of public transportation system in many big cities.Nevertheless,metro is a typical Socio-Technical System(STS)characterizing the feature of susceptibility to interferences and disruptive events during operation,which is apt to have metro operation accidents.Once metro operational accident occurs,it will bring out negative effects on metro system,ranging from operational delay or outage to inefficient passengers' evacuation.At worst,it would give rise to major equipment destruction or even the paralysis of the entire line network.Such metro operation accidents not only caused huge economic losses,including loss of life and property,but ignited public debate about the safety of metro system.More than providing fast and convenient services,metro system is preferred to be a safe and reliable transportation mode.There is a growing consensus among metro professionals and researchers that safety must always come first in metro operation.For such reason,it is necessary to further excavate what has caused the insecurity of metro operation.Researchs from the perspective of vulnerability provide a new field of vision to explore this phenomenon.After making the meaning of the Metro Operation Vulnerability(MOV)and its formation mechanism clear,metro operation disruptive events and metro operation vulnerability factors are believed to be the two important aspects of addressing the vulnerability of metro operation.Based on these analysis,countermeasures and strategies controlling the MOV are put forward to build a more safety operation environment.Such corresponding improvement susggestions would provide theoretical and technical support for the decision-making and management department,which has great theoretical significance and application value to improve the metro operation safety.(1)After systematically combing the extant researches,literature review is conducted from four different summarized fields of the accident model,general safety operation of metro system,the status of vulnerability research in various area,and simulation technologies to keep abreast of current development in metro operation.The existed deficiencies in current research are pointed out.Furthermore,the research objectives and contents are clarified.Following that the research framework and the structure of this paper are proposed in the end of the chapter.(2)Konwing the operating charactristics of metro system is crucial to explore Metro Operation Vulnerability(MOV).Metro system is introduced and the features of metro operation are summarized to better understand the inherent property of metro operation,Referring to the definitions of vulnerability in natural sciences,social sciences and engineering sciences,the connotation of MOV is formed.Furthermore,the difference and connection between vulnerability and its relevant concepts including reliability,stability,robustness and resilience are illustrated which has a positive effect on expanding the intension of vulnerability.The formation mechanism of MOV is explained later,in which metro operation accidents are triggered by the complex disruptive events.On the basis of the formation mechanism of MOV,the process of capturing the vulnerability of metro system is given.Finally,a simple statistical analysis of metro operation accidents is carried out from the angle of MOV.(3)On the basis of the metro operation risk checklists,the types of metro operation disruptive events are accessed and summarized from the collected metro operation accident cases.Meanwhile,the interrelations among these disruptive events are obtained.Regarding disruptive events as nodes and interrelations as edges,Metro Operation Disruptive Event Network(MODEN)is generated.With the assist of complex network theory(CNT),four statistical indicators including network density,degree and degree distribution,average path length,and clustering coefficient are selected to explore whether MODEN occupys small-world and scale-free properties.The dynamic feature of MODEN is simulated by observing the changes of the overall property of MODEN after losing specified nodes,in which three node centralization indexes including degree centrality,closeness centraility and betweenness centrality are employed to evaluate the importance of each node in MODEN and the network efficiency is adpted to reflect the change of network property.The results indicate that MODEN is a scale-free network for the cumulative degree distribution obeys power-law distribution.Moreover,MODEN also possesses the small-world property for having a relatively high clustering coefficient and small shortest path length.As for reducing the network efficiency of MODEN,selective removal strategy is superior than random removal strategy.With regard to the selective removal strategy,BC-based removal strategy works better in the early stage,while DC-based removal stategy has a more ideal effect latterly.In terms of removal scale,11.54%of the central nodes control 79.76%of the initial efficiency of the network.(4)Instead of concentrating on a portion of metro system,metro operation vulnerability factors are collected from following aspects of human,equipment and facility,environment,management,structure,and emergency based on a thorough retrieved literature.These vulnerability factors are further streamlined by interviewing five experts in the field of metro operation safety.In addition,the interrelations between these factors are obtained from the expert interview.Utilizing the integrated Interpretative Structural Modeling(ISM)and Cross Impact Matrix Multiplication Applied to Classification(MICMAC)methods,a hierarchical structure and a driving dependence graph which are used to interpret how these factors impact the metro operation are generated.The analysis suggests that these vulnerability factors can be divided into three layers and four categories including autonomous,dependent,linkage and independent factors.(5)For the purpose of quantifying the importance of each vulnerability factor to the metro operation,a questionnaire is designed and employed to obtaining the related fundamental data in this paper.Based on the ISM model of vulnerability factors,six hypotheses about the influencing paths among the vulnerability factors are put forward and allow for the production of first-order model.While from the Hierarchical Holographic Modeling(HHM)model,second-order model is developed.The Structural Equation Modeling(SEM)is utilized to analyze the data as well as verify the first-order and the second-order model.The results indicate that the first-order model and the second-order model could give an ideal expression of the contribution of each vulnerability factor to metro system and represent the interrelations among the six dimensions of vulnerability factors.What's more,it is obvious that human,management,and emergency dimensions occupy higher importance.Management factors have significant impacts on human and emergency factors,while human factors contribute marked effects on equipment and facility,and emergency factors.(6)With the purpose of revealing the nonlinear relationship between vulnerability and metro system,a system dynamic(SD)model of metro operation vulnerability is constructed based on modeling principles and procedures.After a careful fliter of metro operation vulnerability factors,the casual loop diagram of vulnerability factors is developed from human,equpipment and facility,and management subsystem.From this casual loop diagram,all positive feedback loops and negative feedback loops can be identified,which is beneficial to clarify how these vulnerability factors affect the operation safety of metro system.The system dynamic simulation platform Vensim PLE is employed to build the operation vulnerability flow diagram of metro system.In the end,an empirical research on metro operation vulnerability of Nanjing Metro is simulated for 40 years,which allows multiple comparison.The sensitivity analysis from the aspects of management superivision and safety investment is conducted to formulate recommendations,which could improve the safety of metro operation.