Consequences Prediction And Protective Optimization Subjected To H₂S-containing Natural Gas Leakage And Chain Effects For Offshore Platforms

This dissertation is supported by "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 safety requirements of offshore oil and gas development for China,a systematic research concerning consequences prediction and protective optimization subjected to H2S-containing natural gas leakage and chain effects for offshore platforms is conducted.The research has made significant progress in small-scale gas leakage dispersion experiment and numerical modeling method,dynamic assessment of consequences for H2S-containing natural gas leakage,dynamic assessment of consequences for multi-hazard accident due to the chain accidents of H2Scontaining natural gas release and explosion,protection optimization design and evacuation route planning for H2S leakage scenarios.Through the research,the escalation path and catastrophic evolution mechanism of chain accidents originating from the release of H2Scontaining natural gas on offshore platforms is deconstructed,the consequences assessment and protection optimization methodology for chain accidents originating from the release of H2Scontaining natural gas is developed,which could provide support for the safe and efficient development operations in offshore sulfur-bearing oil and gas fields.The detailed progresses are summarized as follows:1.The dispersion behavior and numerical modeling of gas leakage on offshore platformsAiming at the dispersion and migration of leakage gas on offshore platforms,a small-scale experiment system is designed and built.A series of gas release experiments are conducted,in which both constant leakage rate and time-varying leakage rate are concerned.The dispersion and migration characteristics of the released gas are studied,and influence of leakage rate on the accumulation of the released gas is analyzed.A CFD(computational fluid dynamics)-based numerical model concerning gas leakage and dispersion is established.The availability of the numerical model is tested and validated by reproducing the experiments.In order to provide support for follow-up research,the influence of parameters,such as wind direction and leakage direction,on the dispersion behavior and accumulation characteristics of released gas are explored systematically.2.Dynamic assessment of consequences for H2S leakage on offshore platformsA dynamic assessment method for the consequences of H2S leakage accidents on offshore platforms is proposed considering the dynamic characteristics of H2S leakage accidents on offshore platforms.A numerical model for gas leakage and dispersion of offshore platforms is established.According to the engineering practice,a fully transient leakage rate is adopted by considering the interference of emergency shutdown system(ESD)and blowdown system,and the spatial-temporal development of the released H2S is predicted.The emergency evacuation behavior of operators was considered based on the emergency response timing sequence and emergency evacuation trajectory.The damage degree of the operator is evaluated with a modified dose-response model,in which the spatial-temporal variation of the location of the operator and the distribution of the released H2S are comprehensively considered.The"dynamic" evaluation results are compared with the "static" and "semi-dynamic" assessment results to demonstrate the accuracy and applicability of the proposed method.Furthermore,the grid-based concept is introduced to consider the randomness of the initial position of operators,and the hazard area identification and classification is performed in combination with the proposed dynamic assessment method.3.Dynamic assessment of consequences for multi-hazard accident due to the H2Scontaining natural gas release and explosion on offshore platformsAiming at the chain accidents of H2S-containing natural gas leakage and explosion on the offshore platform,the upgrading trajectory and the necessary conditions for the occurrence of the chain accidents are identified.The chain accidents of H2S-containing natural gas leakage and explosion are simulated based on the data-dump technology.The distribution of explosion overpressure load is obtained.The combustible gas leakage is considered as the trigger event for the emergency evacuation,and the real-time location of operators are determined based on the grid-based concept and the emergency response time sequence.The maximum explosion overpressures in the real-time position of the sufferers are extracted to predict the explosion impact.Furthermore,the inevitability of the existence of toxic impact and explosive impact in the chain accident of H2S-containing natural gas leakage and explosion is revealed.The toxic impact during the emergency evacuation and the explosion impact at the time of ignition are predicted respectively,and the consequences of multi-load accidents are evaluated by introducing the risked-based concept.4.Research on protection strategy of H2S leakage on offshore platformsThe intervention mechanism of the process protection system on the leakage rate is analyzed,and the influence of the response time of the process protection system on the spatial distribution of the dangerous load is explored.The mitigation effect of the process protection system is quantitatively evaluated by considering the emergency evacuation and nonemergency behavior.The dependencies among the ventilation speed,leakage rate,concentration of H2S and dispersion behavior are discussed.The most dangerous initial position in each accident scenario is identified based on the grid concept.The“as low as reasonably practicable"(ALARP)guideline is employed to judge the effectiveness of ventilation in mitigating the worst toxic impact.Some safety-related results can be obtained,such as accident scenarios where ventilation can effectively mitigate the toxic impact.Furthermore,the effect of evacuation response time on the disaster degree of the operator is analyzed.The "safe time window" for emergency evacuation in each leakage scenario is obtained.The results could provide support for protection optimization design for major accidents due to H2S-containing natural gas release.5.Evacuation route planning for H2S release accident on offshore platformsAiming at the evacuation route planning in H2S release accident on offshore platforms,an emergency evacuation response model for offshore platforms is established,and the characteristics of evacuation route planning in H2S release accident are analyzed.An evacuation route planning model is developed,in which the time-varying characteristics of the dangerous load is considered.The dispersion behavior of H2S and evacuation behavior are simultaneously considered in each step based on a uniform time dimension.The moving direction is adjusted in real time according the variation of the spatial distribution of H2S toxic load.Through the comparative analysis of engineering examples,the applicability of the proposed evacuation route planning model to real accident scenarios is demonstrated.Relevant studies can provide support for evacuation path planning in accident scenarios with time-varying dangerous loads.