Evaluation of frictional pressure drop correlations for air-water and air-oil two-phase flow in pipeline-riser system

Accurate prediction of the frictional pressure drop is important for the design and operation of subsea oil and gas transporting system considering the length of the pipeline. The applicability of the correlations to pipeline-riser flow needs evaluation since the flow condition in pipeline-riser is quite different from the original data where they were derived from. In the present study, a comprehensive evaluation of 24prevailing correlation in predicting frictional pressure drop is carried out based on experimentally measured data of air-water and air-oil two-phase flows in pipeline-riser. Experiments are performed in a system having different configuration of pipeline-riser with the inclination of the downcomer varied from-2°to-5°to investigated the effect of the elbow on the frictional pressure drop in the riser. The inlet gas velocity ranges from 0.03 to 6.2 m/s, and liquid velocity varies from 0.02 to 1.3 m/s. A total of885 experimental data points including 782 on air-water flows and 103 on air-oil flows are obtained and used to access the prediction ability of the correlations. Comparison of the predicted results with the measured data indicate that a majority of the investigated correlations under-predict the pressure drop on severe slugging. The result of this study highlights the requirement of new method considering the effect of pipe layout on the frictional pressure drop.

Flow Pattern and Pressure Fluctuation of Severe Slugging in Pipeline-riser System

During the exploitation of offshore oil and gas,it is easy to form severe slugging which can cause great harm in the riser connecting wellheads and offshore platform preprocessing system.The flow pattern and pressure fluctuation of severe slugging were studied in an experimental simulation system with inner diameter of 0.051 m.It is found that severe slugging can be divided into three severe slugging regimes：regime I at low gas and liquid flow rates with large pressure fluctuation,intermittent flow of liquid and gas in the riser,and apparent cutoff of liquid phase,regime II at high gas flow rate with non-periodic fluctuation and discontinuous liquid outflow and no gas cutoff,regime III at high liquid flow rate with degenerative pressure fluctuation in form of relatively stable bubbly or plug flow.The results indicate that severe slugging still occurs when the declination angle of pipeline is 0,and there are mainly two kinds of regimes：regime I and regime II.As the angle increases,the formation ranges of regime I and regime III increase slightly while that of regime II is not affected.With the increase of gas superficial velocity and liquid superficial velocity,the pressure fluctuation at the bottom of riser increases initially and then decreases.The maximum value of pressure fluctuation occurs at the transition boundary of regimes I and II.

Characteristics of air-water upward intermittent flows with surfactant additive in a pipeline-riser system

The effect of the surfactant additive on the upward intermittent flows in a pipeline-riser system is studied experimentally, in a 3 m long horizontal pipe connected to a Perspex pipe of 2.0 m long and 25 mm in diameter, inclined to the horizontal plane by 7？, followed by the vertical PVC riser of 3.5 m high and 25 mm in diameter, operating at the atmospheric end pressure. Based on the analysis of the pressure signal and the visual observation of the riser, it is shown that the additive of surfactant to the carrying liquid makes bubbles smaller in size but much larger in number in the upward intermittent flows. In addition, the additive of surfactant to a two-phase flow does not have a significant impact on the in-situ gas fraction, the pressure drop and the frequency of the liquid slug, but it reduces significantly the velocity of the liquid slug. When the superficial liquid velocity is set, an exponential relationship between the dimensionless velocity of the liquid slug and the Webber number can be obtained. These results might be used for estimating the characteristic parameters of the upward intermittent flow based upon the input operating conditions.

An improved model for severe slugging stability criteria in offshore pipeline-riser systems

Flow assurance has been a major challenge in petroleum production systems especially in offshore operations where severe slugging is a dominant aching issue.This is characterized by pulsating flow behavior which impacts the reservoir productivity,pipeline-riser systems,and surface facilities(separator).The only means so far,of delivering reservoir fluids from offshore wells to the separator at the surface is by pipeline-riser systems where slugging is most prominent and oftentimes unavoidable.Though passive and active measures have been deployed over time to mitigate the associated problem,there is the need to develop a predictive tool for mitigating offshore slugging in the riser system.This study involves the use of a model and analysis approach to eliminate severe slugging based on the balance of forces acting on the pipeline-riser system.Modified stability criteria which consider accumulation and frictional pressure drops have been developed.The current approach is solely physicsdriven,to predict the corresponding liquid and gas velocities at which the fluctuations would be severe when producing multiphase fluids through pipeline-riser systems.By balancing the forces acting on a riser-pipeline system and accounting for frictional and accumulation effects,two hydraulic flow modeling approaches have been used in this study to improve the criteria for predicting the condition of severe slugging.The developed criteria were computed using MATLAB/Simulink and a severe slugging flow map was generated.The“new study 1”and“new study 2”showed a 16%and 13%increase respectively in predicted unstable cases than the Malekzadeh criteria.The criteria were tested at different separator pressures.The Taitel and Jansen criteria could not predict severe slugging at elevated separator pressure.Notwithstanding,the developed flow maps using the“new study 1”are more reliable than the“new study 2”.This is because the“new study 2”approach over defines the physics of severe slugging.The criteria were also tested at varying accumulation times.This was done by step increments in the order of magnitude of time.The separator pressure affects the optimal time for modeling the accumulation at the riser-base.The criteria are therefore useful for predicting optimal operating conditions for flow assurance for the pipeline-riser system in consideration.

Numerical Investigation on Severe Slugging in A Catenary Riser

A mathematical model is presented to study the main characteristics of severe slugging in a downward inclined pipeline followed by a catenary riser. In this model, both simplified transient model and phase distribution model are included so that the flow characteristics of each stage for severe slugging can be accurately reproduced, especially for blowout stage. The results show that the flow features of severe slugging can be simulated by the proposed mathematical model. A good agreement between the experimental data and the numerical results is observed. The model can predict the transient fluctuation of many respects, such as the superficial gas velocity at the bottom of the riser and the average velocity at the outlet of the riser.