| As a continental country as well as a coastal country, China has a coastline of over 18,000 kilometers and territorial waters of almost 3 million square kilometers. The exploitation and utilization of submarine resources is of great strategic significance for the sustainable development of China. The exploitation of marine oil and gas is started relatively late for China where, however, the marine oil and gas industry has been initially formed with a medium scale after 30 years of practice. The transportation of hydrocarbons by subsea pipelines is highly efficient and convenient, whilerequiring minimal cost. As a result, offshore pipeline is gradually becoming the primary mode of transportation of oil and gas at sea. China currently has 6,000 kilometers of installed offshore pipelines, and is planning to increase that length threefold in the next decade. Some hazards associated with operating offshore pipelinesinclude leakage, rupture and even bursts that result ininterruption to transportation and production of hydrocarbons, require clean-up operations, and can cause catastrophic health, environment and safety accidents. Consequently, maintaining the integrity of offshore pipeline transportation networks is of vital importance to a nation’s economy and peoples’ lives.To strengthen pipeline safety not only requires technical innovation, but also demands the improvement of safety management philosophy and fundamental revolution of management mode. Proposed in this context, the concept of “Pipeline Integrity Management(PIM)” has been widely adopted by most international pipeline operation companies, and it plays a more and more important role in keeping the safety of pipeline transportation. Risk assessment is a critical step in PIM, because the results from it serve as the main basis to judiciously divide resources for inspection and maintenance among different pipelines, segments, or assets. What risk assessment does could improve the efficiency of the next steps of PIM, such as integrity assessment, inspection and maintenance, so as to reduce protection cost.This dissertation mainly research about the risk assessment and decision-making method foroil and gas pipeline. With the purpose of combining the concepts of pipeline integrity management and dynamical risk assessment, an improved pipeline integrity management system is proposed. Then this dissertation puts forward a rule based fuzzy synthetic evaluation(RB-FSE) method used in pipeline risk assessment. RB-FSE method is useful to deal with the complex mechanism of pipeline accident and the scarcity as well as the uncertainty of the data related to pipeline safety condition, and the relative risk evaluation results from this method is of service to support risk ranking and specific risk reduction measures. With the increasing requirements of pipeline safety, more and more pipeline operation companies demand 1) probabilistic risk assessment results which could support economic evaluation and decision-making optimization, and 2) dynamical risk assessment model that can be continually updated during the life cycle of pipeline so as to offer more precise results. Therefore a comprehensive risk assessment and decision-making model is built with the integration of Bayesian network(Bayesian Network) and cost-benefit analysis(CBA) method. And two important functions of Bayesian Network – probability inference and probability updating- were used to analyze damage probability and update model parameters, respectively. Based on the above-mentioned purposes, this research mainly consists of the following work.(1) On the basis of dynamic risk assessment, an integrity management system of oil and gas pipelines is built, and IDEF0 method is applied to functional modeling. According to the improved integrity management system, it could realize 1) thetimely updating of risk related data in order to respond with emergency, 2) the continuous improvement of risk management ability, and 3) the prefect connection of risk assessment and integrity assessment. The feasibility of the system is clarified by applying a case containing the convergence between the small cycles of dynamic risk assessment.And the introduction of the improved integrity management systemestablishes the foundation of the rest research of pipeline risk assessment methods.(2) A fuzzy logic model for calculating the generalized structural reliability of oil and gas pipeline is proposed in this dissertation. Fuzzy set is used to express the vague variables – resistance and stress in reliability analysis. And fuzzy rules are employed to encode the failure mechanism. To fully utilize the expert knowledge on the empirical data and the ambiguous cause(s) – effect(s) failure mechanism and then keep a good balance between precision and practicality, this dissertation then puts forward a RB-FSE method which combines fuzzy synthetic evaluation with fuzzy logic. It is applied to the pipeline risk assessment for third-party damage. The proposed method is compared with scoring-type method in a case study. Results indicate that the relative assessment values from RB-FSE model could better support risk-ranking and decision-making in the pipeline integrity management.(3) Bayesian Network models are proposed in this dissertation to determine the probability of anchor damage and trawling damage to subsea pipelines. The Bayesian Network models are developed by integrating directed acyclic graphs, and four computational methods(Boolean operation, standard and historical statistical analysis, fuzzy set theory and data learning) to elicit marginal probability tables and conditional probability tables. A case study illustrates the utilization of two Bayesian Network-related functions – probability prediction and probability updating – to determine final probabilities of damage to a subsea pipeline. The results of the analysis support risk ranking and risk reducing decisions associated with maritime operations in the area of offshore pipelines.This dissertation further builds risk assessment and decision-making model based on the combination of Bayesian Network and CBA. The concept and computational method of cost-benefit ratio(CBR) are presented. By using this CBA, the economic gain/loss and the probability analysis results from the Bayesian Network modelare considered together to evaluate the economic effects of any risk reduction measures. Two case studies about anchor impacting accident and objection impacting accidentillustrate the utilization of the Bayesian Network and the CBA. The results of CBA can support decision-making for economical measure protecting offshore pipeline.Compared with past researches at home and abroad, the original works in this dissertation may be represented as follows.(1) Past researches have discussed the utilization of fuzzy synthetic evaluation method or fuzzy inference method in risk assessment. In this dissertation, fuzzy synthetic evaluation and fuzzy inference are combined to build RB-FSE method, which can not only solve multi-target and multi-level evaluation problem, but also simulate the cause and effect mechanism of accident by “IF-THEN” rules. RB-FSE method is applicable to deal with the risk assessment problem which involves various influence factors and complex emerging and developing mechanism.(2) This dissertation put forwards the Bayesian Network risk assessment model, which is firstly used in offshore pipeline engineering. The modeling procedure and fourparameter computational methods(Boolean operation, standard and historical statistical analysis, fuzzy set theory and data learning) proposed in this dissertation effectively solve the Bayesian Network building problem in the risk assessment of offshore pipeline.(3) On the basis of Bayesian Network and CBA, a risk assessment and decision-making model for offshore pipeline is created in this dissertation. Pipeline managers could determine the unsafe pipelines depending on whether the damage probability from Bayesian Network model exceeds the risk acceptance criteria, and then choose the optimal risk reduction measure according the calculation results of cost-benefit ratio(CBR). The merits and the novelties of this model are manifested in the combination of Bayesian Network and CBA, the benefit valuation method of risk reduction measures, and the proposition of CBR.The research results of this dissertationnot only can be applied to the offshore pipeline integrity management and risk management, but also the RB-FSE method and the risk assessment and decision-making model based on Bayesian Network and CBA are both of significance to the risk analysis of the complicated safety accident scenario. |
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