| Natural gas?NG?pipeline transportation is the most common NG energy transmission mode due to its high efficiency and low cost.However,major accidents?jet fire major accidents occur the most frequently?occur occasionally in the operation process of NG pipelines,resulting in a mass of casualties,property loss and/or serious environmental pollution,giving rise to major concerns of the governments and the public.Meanwhile,in order to meet the growing energy demand,it becomes increasingly common that parallel laying pipelines over long distances are closely separated between them,considering limited space for the required corridor or high construction and maintenance cost.A Loss of Containment?LOC?in one pipeline can lead to jet fire accident,subsequently triggering the failure of adjacent pipelines,i.e.,the accident domino effect,thereby resulting in more serious consequences.Therefore,the safety problem of parallel pipelines should not be ignored.Furthermore,it is imperative and important to study the prevention and control of domino effect caused by jet fire in the operation process of parallel pipelines,in terms of its potential practical significance and its high usefulness for engineering application.Previous studies on the domino effect caused by jet fire accident in parallel pipelines mainly focus on the simplified failure determination of target pipeline and the accident domino effect occurrence probability.Very few attentions are paid to explore the occurrence rules of domino effect caused by jet fire accidents from the perspective of the prediction on hazard characteristic parameters of jet fire accident and thermal failure mechanism of target pipeline.Moreover,current studies on the domino effect pre-control are limited to qualitative or empirical analysis,and investigations on systematic quantitative pre-control methods for the domino effect are also rarely found.Therefore,in order to reveal the occurence mechanism of domino effect caused by jet fire accident and effectively prevent and control the domino effect,this paper concentrates on the domino effect scenario caused by jet fire accident in parallel pipelines.Main studies have been carried out successively as follows:?i?Experimental investigation on geometrical characteristics and thermal radiation distribution characteristics of NG jet fire.In order to study geometrical features and thermal radiation distribution features of NG jet fire due to a LOC of source pipeline,a a series of medium scale circular and rectangular source jet fire experiments with heat release rate up to 3.5 MW have been carried out on the NG jet fire experimental platform.The flame Froude number considered for circular source jet fires and rectangular source jet fires are up to 4.46 and 3.06,respectively.The CCD flame image and IR flame image are processed using the method of appearance probability distribution contour based on gray threshold,and the process results of the two type of images are compared.The experimental data of flame geometry parameters and thermal radiation in the near field are obtained,which provide data basis for the study of theoretical models in the following chapters.?ii?Study on prediction method of the impact length and lift-off distance of jet fire due to a LOC of source pipeline.The correlation for predicting the impact length of circular source jet fire is proposed using the dimensionless group considering the combustion dynamic parameters.The dependence of the lift-off distance on the nozzle diameter is explored.The correlation describing the variation of lift-off distance of circular source jet fires with exit condition parameters is put forward using the dimensionless groups derived from the mixedness-reactedness flamelet theory.Based on the refinement value of the entrainment strength constant C1 obtained,the application of the classic correlation of Quintiere&Grove can be extended to predict the visual flame length of rectangular source jet fire in transition regime from buoyancy-controlled to momentum-controlled.A novel correlation using the dimensionless groups derived from the mixedness-reactedness flamelet theory is proposed for predicting lift-off distance of rectangular source jet fires covering a wide range of nozzle aspect ratios.The above prediction correlations of geometrical characteristics can provide important input parameters for determining if flame contacts with pipeline in the scenario of flame direct impingement and predicting the radiant heat flux in the scenario of only thermal radiation,respectively.?iii?Study on prediction method of thermal radiation in the near field of jet fire due to a LOC of source pipeline.The Chicken Swarm Optimization?CSO?algorithm is utilized to invert and optimize the weight distribution parameters of heat sources in the weighted multi-point source thermal radiation model?WMPM?and explore the influence of test conditions on the weight parameters.The weight distribution characteristics of heat sources along the axial position of the flame are studied,and the correlation for weight distribution of heat source associating with the flame Froude number is proposed.Based on the correlation,a novel weighted multi-point source thermal radiation model is developed,which can effectively solve the problem of the very simplified weight distribution of heat sources in the original model and significantly improve the prediction accuracy of thermal radiation in the near field.In addition,the influence of nozzle aspect ratio on the thermal radiation of rectangular source jet fire in the near field is investigated.The correlation for thermal radiation correction factor associating with nozzle aspect ratio is established.The correction thermal radiation model for the case of a wide range of nozzle aspect ratios is put forward.The above study on the thermal radiation prediction model can be used to accurately predict the radiant heat flux received by target pipeline in the near field of jet fire.?iv?Study on thermal failure mechanism of target pipeline in the scenario of jet fire accident.Considering the unsteady characteristic of jet fire in real accident scenario,the gaseous mixture state parameters in the transient release process of NG in the source pipeline are deduced and obtained based on the Peng&Robinson equation of state.On the basis of foregoing study on prediction correlations of geometrical and thermal radiation characteristics,the prediction method of the transient radiant heat flux received by the outer wall of the target pipeline is proposed.The dynamic thermal response model for the target pipeline is established using the finite difference method,which can solve the deficiency of static thermal response analysis in previous studies.The method of dynamic thermal failure determination for target pipeline is put forward systematically for the case of aerial parallel pipelines and buried parallel pipelines,respectively.Based on the method of dynamic thermal failure determination proposed,two realistic parallel pipeline accidents involving two typical domino effect scenarios in parallel pipelines,i.e.,flame direct impingement and only thermal radiation impact without any direct contact with flames,are investigated in detail,respectively.Furthermore,the thermal failure mechanism of target pipeline is revealed from the perspective of total stress?including thermal stress and pressure stress?of pipe wall and the decline of mechanical strength of pipe wall.According to the calculated results of tangential total stress of the target pipe wall,dynamic thermal failure process and longitudinal rupture failure mode of the target pipeline are illustrated properly.?v?Study on prevention and control of domino effect caused by jet fire accident in parallel pipelines.Based on the method of dynamic thermal failure determination established,the effects of five kinds of key design or operation parameter optimization on the failure result of target pipeline are analyzed comprehensively and quantitatively.The five kinds of key design or operation parameters include four kinds of one-parameter,i.e.,distance between upstream and downstream block valve stations,inter-axis distance between source pipeline and target pipeline,operating pressure of target pipeline,flow velocity in target pipeline,and one kind of combination parameters,i.e.,the combination of operating pressure of target pipeline and wall thickness of target pipeline.From the perspective of the variation extent of parameters needed for reducing the maximum elastic failure stress and the maximum rupture failure stress,the mitigation effects of these five kinds of parameter optimization on the thermal failure results of target pipeline are compared.Considering the independence among these five kinds of parameter optimization,the prioritization for all kinds of parameter optimization and the proper optimization variation extent are obtained.Based on the principle and method of system engineering and the feasibility of engineering practice,pre-control strategies for the domino effect caused by jet fire accident in parallel pipelines at the design stage and the operation stage are proposed using the method of multi-level parameter optimization,respectively,which will provide novel insights into practical engineering application in safety design and operation safety management of parallel pipelines. |
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