Cyclic flow cut-off characteristics of gas-liquid two-phase flow in pipeline-riser system and prediction of its occurring condition

Abstract

In offshore oil and gas fields, the prediction of gas-liquid two-phase flow pattern is of great importance for the design and operation of pipeline-riser systems. However, no special attention was put on the mechanism of periodic flow cut-off at the riser outlet, which is an inherent characteristic of certain flow patterns directly related to operation safety, in the study of flow pattern transition. In this study, differential pressure signals are used to explore the internal correlation between the flow cut-off of gas and/or liquid phase at the riser bottom and the riser outlet. The flow patterns are classified based on continuous flow or flow cut-off at the riser outlet. Then, the flow pattern transition mechanisms are studied from the view of the condition under which flow cut-off will appear. At high gas and low liquid velocities, the mechanism can be explained by a conventional theory; while at high gas and high liquid velocities, dissipation of hydrodynamic slugs becomes the major reason for flow pattern transition; and a unified model is introduced to predict the dissipation. At low gas and high liquid velocities, the condition for gas-liquid eruption can still describe the flow pattern transition, but it is modified by the slug dissipation model. At higher pressure, the lasting time of gas cut-off at the riser elbow becomes shorter, and a threshold can be set to decide whether it can be ignored. The predicted results are in good agreement with the flow pattern maps under different pipeline structures and fluid properties, and the reason why flow cut-off disappears in the experimental loop at high pressure of 10 MPa is successfully explained.