Observer design for gas lifted oil wells

Unstable regimes occuring for multiphase flow in vertical risers have successfully been stabilized using conventional linear control techniques. However, the control systems rely on downhole measurements, which are at best unreliable, if at all available. We design a nonlinear observer for the states of the multiphase flow that relies on topside measurements only, and apply it to estimate downhole pressure for feedback control. A key feature of the design is that it exploits the structure of the model to obtain robustness with respect to the internal flows in the system. The performance of the observer is demonstrated in simulations.