Modified smith predictor for slug control with large valve stroke time in unstable systems

Abstract

Slugging as a flow assurance challenge is an upsetting condition to the oil and gas industry due to the instabilities it poses on the system. The negative repercussions associated with slug flow stems from the inlet through to the topside facilities where processing is done. Active control has been established as one of the best techniques to eradicate slug and its accompanying challenges, however the controller robustness and some setbacks make improvement a necessity. Again, the robustness of the controller has been in question.Due to the potential of active control, several other researchers have delved into it. Many outcomes from pilot scale experiments (usually 2–4 inch valves), that shows promising and improved benefits fail to replicate when emulated on real offshore facilities (over 8 inch valve diameters). This is because of the difference in the valve stroke time (time taken for a valve to move from fully open to fully close or vice versa). Comparatively, larger diameter valves are designed to have larger stroke time than smaller diameter.In this paper, an advancement and extension of an active slug control strategy that uses measurement signals from the topside of the riser, will be used to deal with systems with time-delay as a result of large valve stroke time. The Smith predictor model was specially modified not just to deal with time-delay but also to deal with variable time-delay in both stable and unstable systems. Thus, a design of a modified smith predictor where the modification mitigates the direct estimation of the time delay itself. This is supposedly due to the internal structure of the proposed modification which, accounts for plant-model mismatch and makes small changes to the control input based on a reference controller.