| This dissertation is supported by the National National Key R&D Program of China“Risk assessment and prevention technology for coastal oil and gas pipelines and onshore terminals”.Due to a seires of factors,e.g.corrosion,fatigue,third-party damage,et al.,subsea oil and gas pipelines face the high leak risk.Furthermore,the leak evoluiton mechanism has not been revealed clearly,and the available methodologies for risk analysis and assessment of subsea pipelines are not enough.A systematic research on risk evoluation mechanism,prevention and control methodology for subsea oil and gas pipelines leak risk is conducted,to obtain a certain progress in the aspects including comprehensive risk analysis and evaluation methodology,dynamic risk assessment methodology under uncertain conditions,dynamic leak characteristic of subea pipelines considering the intervention of control measures,underwater gas plume behavior,safety assessment of gas dispersion above sea,maintanence planning methodology and leak emergency,prevention and control framework.Through the research,the mechanism of subsea oil and gas pipelines leak is revealed,and risk assessment,prevention and control methodology of subsea pipelines leak is proposed,which could provide a scientific basis for operational safety and risk management of subsea oil and gas pipelines.The detailed progresses are summarized as follows:1.An integrated framework for safety analysis and evaluation of subsea pipelinesEnsuring the operational safety of subsea pipelines is a growing challenge.It requires monitoring of failure causation factors and the detailed safety analysis.This paper proposes an integrated framework for a detailed safety analysis of subsea pipelines.This framework is implemented based on a risk evaluation index system,which incorporates interdependency analysis of failure causal factors,hazard coupling analysis and risk grade evaluation.The Decision-Making Trial and Evaluation Laboratory(DEMATEL)method is utilized to analyze the interdependencies among risk factors.The fuzzy reasoning algorithm is employed to calculate the risk grades of these factors in the evaluation index system.This framework has also considered the effect of hazard coupling(combination of hazards)on subsea pipeline failure.The framework is tested on a case study.The case study demonstrates the practicality and usability of the proposed framework.This framework will serve an important tool for swiftly manage risk of subsea pipelines.2.Dynamic risk assessment of subsea pipelines leak under uncertain conditionsAccidental leak in subsea pipelines poses a severe threat to human life,environment,assets and corporate reputation.Quantitative risk assessment of such events is consistently a challenging task due to data scarcity.In this paper,a new methodology comprising of Hierarchical Bayesian analysis(HBA)implemented with the Bayesian network(BN)is proposed to assess the risk of subsea pipelines leak.This methodology capture both data and model uncertainty.HBA is utilized to handle the uncertainty among failure data from different sources;BN is used to capture the dependencies among primary events and safety barriers.Failure probabilities from HBA approach are used as prior beliefs,and the occurrence probabilities of different scenarios are derived from BN reasoning in uncertain conditions.Furthermore,the proposed methodology makes use of a concept of fuzzy loss ratio to evaluate the consequence of different scenarios.Integrating probability with consequence gives a reasonable risk assessment result.As new observation data becomes available,the assessed risk can be updated to generate a dynamic risk profile.An industrial case study demonstrates the application of the methodology.This methodology can serve as a helpful tool for risk early warning and decision-making of subsea pipelines leak.3.Dynamic leak characteristic of subsea oil and gas pipelinesConsidering the operational characteristic of subsea oil and gas pipelines,and the effect of process control measures on leak process,a OLGA based dynamic leak model for subsea oil and gas pipelines is proposed,and the corresponding calculation procedure is formed,to predict the leak process of subsea oil and gas pipelines.The response process of leak pressure and medium release rate are studied.The effect of leak orifice size,position and orifice number on leak process is analyzed.In addition,the effect of leak control measures such as pressure maintaining and emergency shutdown on leak process with different orifice size are studied.Based on the analysis results,the leak mechanism of subsea oil and gas is revealed,and the process control strategy of subsea pipelines leak is proposed.4.Safety assessment of underwater gas dispersion from subsea gas pipelines leakFacing the underwater gas dispersion due to subsea gas pipelines leak,a small-scale experiment system is designed and built,and a matrix of underwater gas release experiments are conducted.Simulation and assessment flowchart for underwater gas release and dispersion resulting from subsea gas pipelines leak is proposed,and a numerical model based on Eulerian-Lagrangian method is established.The numerical model is tested and validated by reproducing the experiments.Then,this numerical model is applied to study subsea gas plume and free surface behavior,analyze gas dispersion process under the action of current,assess the critical risk parameters,reveal the effect of gas release rate,current rate,water depth and leak position on underwater gas plume behavior.5.Enviorment effect of gas dispersion above sea from subsea gas pipelines leakConsidering the hazards of gas dispersion and deflagration above sea from subsea gas pipelines leak,a simulation and assessment procedure is proposed,and a CFD-based model is established to predict the development process of flamambe gas cloud.A jack-up platform is introducted in the model to study the effect of flammable gas cloud on offshore structure,and assess the distribution of dangerous area.Considering the possibility of ignition,the flagration of flammable gas is simulated and analyzed.Based on deflagration damage criteria,the effect of deflagration overpressure,flame termperature and heat radiation on offshore platform and operators is evaluate.The results could provide a powerful support for risk assessment and emergency response of major accidents due to underwater gas release.6.Maintenance planning and leak emergency,prevention and control frameworkFor the corrosion degradation of subsea pipelines,a risk-based maintenance planning methodology is proposed.In this methodology,BN is used to determine the probability of pipeline faiure.Then,the risk state of pipeline is judged,and the required failure probability is obtained by probability updating.Based on the required failure probability and the failure probability distribution,the maintenance interval is determined.In addition,markov process is utilized to build the evolution model of corrosion state.The optimal maintenance scheme is obtained to get a balance between pipeline availability and maintenance cost.A methodology for maintenance operation safety analysis is developed,to find the causation factors of operation failure,and assess operation reliability,and determine the critical risk factors in operation procedures,which could contribute to improve the safety level of subsea pipelines maintenance operation.A integrated risk and emergency framework of subsea pipelines leak is established from the systematical perspective.Prevention and early-warning mechanism of leak is analyzed,and emergency response procedures and the corresponding implementation principle are formed.Eventually,the main modules for the digital system of subsea pipelines leak emergency are discussed. |
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