Deep offshore production in the petroleum industry requires a better knowledge of multiphase flow, as the design of pipelines may cause the flow to become strongly unstable. For instance, for low flow rates and when a sea line ends at a riser, the riser base may accumulate liquid and stop the flow of gas. Then, the upstream gas is compressed until its pressure is large enough to push the liquid slug downstream. Under such conditions, a cyclic process occurs which is called severe slugging. Severe slugging is an unstable flow pattern where large and fast fluctuations in pressure and flow rates can damage installations or reduce the production of the field. In this study, we propose a new method to stabilise this undesirable phenomenon using an active bypass. Based on a theoretical description of the severe slugging, a model has been designed and used to work out an algorithm to control a valve bypassing the flow between the sea line and the riser. This paper first details the controller based on a static equilibrium forces assumption, dynamic balance modelling and two-time scale control techniques. Then, improvement compared to others systems is presented. Finally, results of experiments using a small size loop designed to reproduce severe slugging instabilities are provided. Good agreement is found between data and simulation. Moreover, the algorithm used to control a pneumatic valve in real time shows the elimination of severe slugging. So, active bypass stabilises the flow and allows the riser base pressure to remain constant in time at a minimum value.
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