Investigation On Two-Phase Flow Characteristics In Pipeline-Riser Systems

During the exploitation of offshore oil and gas, gas-liquid mixed transportation, which has become one of mainly transportation way, always uses seabed pipelines and risers as a combined system. Because of the difference physical characteristics between gas and liquid, gas-liquid mixed flow is easy to cause irregular flow phenomenon during transportation such as severe slugging. Severe slugging can cause drastic fluctuation of pressure and velocity in riser that will damage the pipeline systems and production equipment. With the oil and gas exploration stepping into the deep sea, a deep research on gas-liquid mixed irregular flow phenomenon, especially severe slugging, becomes more and more important. In resent paper, pipeline-vertical/catenary riser systems are chosen as the research objects. By proposing a theoretical model and numerical simulation method, a further study about gas-liquid flow characteristics is presented in the resent paper as following.Based on the separated flow theory in the riser, a one-dimensional transient model is presented to simulate the characteristics of gas-liquid flows in a pipeline-riser system, where the frictional pressure drop is taken into consideration, and the drift flux correlation is used as a closure law. In particular, a new method for the initial condition is proposed for solving the transient model. For pipeline-vertical/catenary riser systems, the numerical results about periods and pressure amplitudes are in good agreement with the experimental results presented in the references.Based on numerical results, four types of gas-liquid flows are successfully simulated which are subdivided into SSI, SSII, SSIII and stable flow. Moreover, the flow pattern maps in pipeline-vertical/catenary riser systems are presented. The results indicate that SSI occurs for the case of low gas and liquid inlet velocity. The area where SSII and SSIII occur is between SSI and stable flow so they can be considered as transitional flow pattern. According to SSI flow characteristics, a modified transform boundary between SSI and SSII is presented and proved more accurate after comparing with Pots’ one.Finally, based on resent numerical results, a further study about pressure, period, velocity, void fraction and mass distributions is presented, and some conclusions are shown as following. Severe slugging causes drastic fluctuation of gas-liquid flow parameters in pipeline-riser systems. Amplitudes of pressure fluctuation, velocity and average void fraction in riser of SSI are larger than other flow patterns, and the largest condition occurs on transform boundary between SSI and SSII. Gas-liquid flow in catenary riser system is smoother than that in vertical riser system because of different geometry.