Comparison of Model-Based Control Solutions for Severe Riser-Induced Slugs

Control solutions for eliminating severe riser-induced slugs in offshore oil & gas pipeline installations are key topics in offshore Exploration and Production (E&P) processes. This study describes the identification, analysis and control of a low-dimensional control-oriented model of a lab-scaled slug testing facility. The model is analyzed and used for anti-slug control development for both lowpoint and topside transmitter solutions. For the controlled variables’ comparison it is concluded that the topside pressure transmitter (Pt) is the most difficult output to apply directly for anti-slug control due to the inverse response. However, as Pt often is the only accessible measurement on offshore platforms this study focuses on the controller development for both Pt and the lowpoint pressure transmitter (Pb). All the control solutions are based on linear control schemes and the performance of the controllers are evaluated from simulations with both the non-linear MATLAB and OLGA models. Furthermore, the controllers are studied with input disturbances and parametric variations to evaluate their robustness. For both pressure transmitters the H∞ loop-shaping controller gives the best performance as it is relatively robust to disturbances and has a fast convergence rate. However, Pt does not increase the closed-loop bifurcation point significantly and is also sensitive to disturbances. Thus the study concludes that the best option for single-input-single-output (SISO) systems is to control Pb with a H∞ loop-shaping controller. It is suggested that for cases where only topside transmitters are available a cascaded combination of the outlet mass flow and Pt could be considered to improve the performance.