Comparison between nonlinear model-based controllers and gain-scheduling Internal Model Control based on identified model

Instability and inverse response behaviour make anti-slug control at offshore oil-fields an interesting control problem where a robust solution considering the nonlinearity of the system is required. We tested three control solutions by experiments in this paper. First, we used state-feedback with state estimation by a nonlinear high-gain observer. Secondly, we applied feedback linearization with measured outputs. Finally, we designed gain-scheduling IMC (Internal Model Control) based on linear models identified from closed-loop step test. We compared these three solutions in terms of robustness and their range of operation. The high-gain observer was only applicable by using the top-side pressure measurement in a limited range; it was not stable when using the the subsea pressure measurement in closed loop. The IMC method was more robust against time-delay in the subsea pressure measurement compared to the feedback linearizing controller.