| The pipeline-riser system, including a downward inclined pipeline and a riser, isneeded to transport oil and associated gas from subsea wellheads up to offshoreplatform systems in the exploitation of offshore oil and gas. One important problemexperienced in such a pipeline-riser system is a severe slugging phenomenon that isby the definition the buildup of liquid slugs equal to or longer than one riser height.This phenomenon is a considerably harmful flow pattern in offshore petroleumproduction systems due to the fact that it can not only damage the productionequipment but greatly reduce the production capability also. With the oil and gasexploration stepping into the deep sea, such a severe slugging phenomenon hasbecome one of the key problems in the exploitation of deep-sea oil and gas, and hencethe research on its deeper formation mechanism and flow characteristics has been ofconsiderably academic and practical importantance.On the basis of reviewing and summarizing the recent research progress about thissubject, a classical pipeline-vertical riser system and a novel pipeline-catenary risersystem in the exploitation of deep-sea oil and gas were chosen as the objectsrespectively, and a comprehensive investigation was performed for the formationmechanism, gas-liquid flow patterns and the flow parameter characteristics ofgas-liquid severe sluggings in such combined pipeline-riser systems based ontheoretical, numerical and experimental methods. The main work of the doctoraldissertation is as follows.1) Based on the consistence principle for the severe slugging formation condition,a CFD (computational fluid dynamics) method is proposed for numerically simulatingthe gas-liquid severe slugging in a pipeline-riser system by means of translating the3D severe slugging into a2D equivalent one, where the Brackbill surface tensionmodel is applied to simulate friction forces, and the VOF method is used to capturemoving interfaces between gas-liquid phases. Numerical simulations were conductedfor the gas-liquid flow patterns of the severe slugging in a declinationpipeline-vertical riser system according to the experimental cases and results presented in the reference, variation characteristics of the flow parameters wereobtained, including period, pressure fluctuation, as well as blowout time, andnumerical results are in good agreement with the experimental ones. On basis of thesenumerical simulations, the formation mechanism was further analyzed and presentedfor such a gas-liquid severe slugging in the declination pipeline-vertical riser system,and variation characteristics for these flow parameters which are difficulty to directlyacquire by the experimental method were obtained, including void fractions in thepipeline and the riser, liquid slug velocity, gas-liquid mixture velocity and flow rate inthe riser outlet.2) The proposed2D equivalent CFD method was also applied to simulate andanalyze the influence of gas-liquid physical parameters and pipeline geometrydeformations on the severe slugging in a pipeline-riser system. For gas-liquid physicalparameters, the liquids are chosen as water, crude oil and kerosene, the gases arechosen as methane and air respectively, results show that the physical parameters ofthe liquid, including viscosity, density and surface tension, have remarkable influenceon the characteristics of the severe slugging, including the flow pattern and its period,the pressure fluctuation, as well as the void fraction. However, the influence of the gasphysical parameters on the severe slugging is not significant. Moreover, for the caseof pipeline geometry deformations, results show that there exist some remarkablevariations for the flow formation and its parameter characteristics in the pipeline-risersystem due to the pipeline fluctuations, including multiple liquid slugs occuring, thepressure amplitude, liquid slug velocity as well as void fraction.3) A transient phase-separation theoretical model is presented for predictingflow parameter characteristics of the gas-liquid severe slugging in a pipeline-verticalriser system. The theoretical model real-timely sloves various flow parameters basedon the phase-separation flow characteristics of the gas and liquid for the severeslugging where the corresponding computational models are chosen by transientlyidentifing gas-liquid flow patterns for the severe slugging. The theoretical model cannot only predict the occurring and the type of the severe slugging in thepipeline-vertical riser system, but also obtain its various flow parameters, includingpressure fluctuation and period, blowout time, void fractions in the pipeline and theriser, liquid slug velocity, gas-liquid mixture velocity in the riser outlet, gas bubblerising velocity, liquid slug lengths in the pipeline and riser, gas-liquid superficialvelocities in the riser inlet. Moreover, the theoretical results agree with CFD ones. 4) Experiments were performed for the two-phase flow characteristics in ahorizontal/declination pipeline-catenary riser system with the gas-liquid mixturetransportation, and flow patterns were identified by series combinations of gas andliquid superficial velocities, including severe slug, intermittent and oscillation flows.In particular, hydrodynamic severe slugging in such a pipeline-riser system was firstobtained by the present experiments. The formation mechanisms of these flowpatterns were presented and the conditions where the severe slug flow can beproduced in the catenary riser were obtained. The results show that the severe slugflow in a catenary riser has remarkably periodic characteristics and consists of fourstages in a circle, including slug formation, slug production, slug blowout and liquidfallback respectively, where the characteristics of the flow parameters in each stagewere given. Moreover, the formation mechanism of the severe slugging in bothcatenary and vertical risers were compared and analyzed, a remarkable difference inthe slug formation stage for the two types of risers was observed. For the catenaryriser, a mixture liquid slug with both gas and liquid firstly occurs before the pureliquid slug, while there is only pure liquid slug for the vertical riser.5) According to the formation mechanism of the severe slugging in thepipeline-catenary riser system, the2D equivalent CFD method was further developedto simulate and analyze flow characteristics for the severe slugging in such apipeline-riser system. In particular, a gas-space optimization scheme is proposed toreduce the computational resource and time by means of adding a gas-collection tank.Numerical simulations were conducted for gas-liquid characteristics of the severeslugging in a horizontal/declination pipeline-catenary riser system according to theexperimental cases and results in the present doctoral dissertation, including gasbubble length, bubble catching up, bubble coalescence and trailing bubble amount,bubble position, as well as flow patterns in the catenary riser. Moreover, variationcharacteristics for these flow parameters were obtained, which are difficulty todirectly acquire by the present experiments, including void fractions in the pipelineand the riser, liquid slug velocity, gas-liquid mixture velocity and flow rate in the riseroutlet.The paper first developed a2D equivalent CFD method with a remarkablereduction of the computational resource and time for numerically simulating flowcharacteristics of the severe slugging in composite pipeline-riser systems.Theproposed transient phase-separation theoretical model is a quick and efficient prediction method for the severe slugging in pipeline-vertical riser systems. These twomethods provide new ways for analyzing flow characteristics of the severe slugging inpipeline-riser systems in the exploitation of deep-sea oil and gas. Moreover, the paperfirst conducted series experiments and2D equivalent CFD simulations for the severeslugging in a pipeline-catenary riser system, some achievements that are ofconsiderably theoretical and practical imortance for the safety exploitation of deep-seaoil and gas were obtained. |
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