Anti-slug control of two-phase flow in risers with: Controllability analysis using alternative measurements

Riser slugging is a problem in multiphase pipeline systems. Slugging is a flow regime that causes a lot of problems, due to rapid changes in gas and liquid rates entering the separators and large variations in system pressure. Solutions to avoid these slugs are economically feasible and under continuous research. Slug control solution has become an important topic in recent decades. Different control design based on single or multiple measurements can be used for stabilizing control to counteract slugging. A controllability analysis was performed in order to investigate which measurements that are most suited for stabilizing control, and this analysis can be used on both SISO and MIMO systems. A linear model was used to perform analysis, and the model used was a simple low-order dynamical model for severe slugging in pipeline-riser systems. This analysis use the lower bounds on the closed-loop transfer functions S, T, KS, SG, KSGd and SGd to obtain information about achievable performance and possible robustness problems. The peak values for the closed-loop transfer functions were evaluated. Topside pressure should not be used for stabilizing control because of high peak values on all transfer functions. This is due to its RHP-zeros. These RHP-zeros, in combination with RHP-poles, time delays, disturbances, noise and inputs constraints will give limitations in stabilizing control. Inlet pressure has proved to effectively stabilize flow, but unfortunately this subsea measurement is not always available. A difficult task would then be to find other control solutions that use more available measurements and achieve equal results. A combination of topside pressure and volumetric flow rate as measurements is a conclusive solution. As flow measurements should not be used in single-loop stabilizing control, due to low steady-state gain that results in low-frequency performance. Different control designs were tested in Simulink, on both linear and nonlinear model. Single-loop PI control based on inlet pressure as measurement, managed to stabilize the system quickly after remarkable change in one disturbance when Z >10%. A PI controller based on volumetric flow rate did also stabilize the system, but response was slow and new disturbances probably occur prior to system stabilization. Topside pressure as measurement was unable to stabilize the system when a single-loop PI controller was used. A Kalman filter based on topside pressure and topside flow rate was used to estimate inlet pressure. These estimated values were used as measurements in a single-loop PI controller. It proved impossible to stabilize the flow when this control solution was used. LQG controller with only proportional action based on both single and multiple measurements was tested. A LQG controller based on topside pressure and volumetric flow rate managed to stabilize the linear model and nonlinear model. Deviations between real and estimated values occur, and deviations are largest when LQG control was used on the nonlinear model. There was also a steady state offset when LQG control with only proportional action was used.