Stabilization of desired flow regimes in pipelines

Elimination of slugging by changing the design or the operating point (e.g. increasing the pressure) has been suggested by many authors. The objective for them was to get to an operating point where the desired flow regime is (open-loop) stable. In this paper the objective is to stabilize the unstable (optimal) operating point using feedback control. A simple two-phase flow model is used for analysis and controller design. The model captures the main dynamics of severe slugging, and is first order continuous, thus suited for linear analysis and controller design. We have shown that a system exhibiting severe slugging is a Hopf bifurcation with the choke valve position as free variable. The pressure sensor used as measurement for control should be placed in the lower part of the system. With the pressure sensor located in the riser, RHP-zeros close to the imaginary axis limits the bandwidth of the control system, making stabilization of the system difficult. The task of eliminating severe slugging in pipelines consists of two sub-problems; 1. Breaking the limit cycle and bringing the system to the desired oper-ating point 2. Stabilize the system at the desired operating point In this work we show that it is possible to break the limit cycle manually by closing the choke valve and then bring the system to its desired closed-loop stable operating point using a simple PI-controller