Controllability analysis of two-phase pipeline-riser systems at riser slugging conditions

A PDE-based two-fluid model is used to investigate the controllability properties of a typical pipeline-riser system. Analysis of the model reveals a very interesting and challenging control problem, with the presence of both unstable poles and unstable zeros.It is confirmed theoretically that riser slugging in pipeline-riser systems can be avoided with a simple control system that manipulate the valve at the top of the riser. The type and location of the measurement to the controller is critical. A pressure measurement located upstream of the riser (that is, at the riser base or pipeline inlet) is a good candidate for stabilizing control. On the other hand, a pressure measurements located at the top of the riser cannot be used for stabilizing control because of unstable zero dynamics. A flow measurement located at the top of the riser can be used to stabilize the process, but, because the steady state gain is close to zero, it should in practice only be used in an inner control loop in a cascade.