Nonlinear adaptive observer control for a riser slugging system in unstable wells

This paper presents a nonlinear observer-based control scheme to stabilize the downhole pressure and eliminate riser slugging. A simple empirical model is developed that describes the qualitative behavior of the downhole pressure in case of severe riser slugging. A new nonlinear adaptive observer is developed for state estimations with the new parameter adaptation law. The controller is designed by an integrator backstepping approach to stabilize the downhole pressure and achieve asymptotic tracking. The stabilization in the unstable region is demonstrated by the proposed control. For the design and implementation of the controller, no knowledge is assumed on the system parameters. It is shown that the proposed controller not only can guarantee asymptotic stability, but also transient performance.