Unrestricted Horizon Predictive Controller Applied in a Biphasic Oil Separator under Periodic Slug Disturbances

Abstract

Multi-phase flow, characterised by the presence of both liquid and gas phases, often occurs in riser pipes during oil extraction. These flows can be problematic because they can cause oscillations due to the formation of bubbles within the pipes, which can negatively impact the safety and efficiency of offshore production operations. One solution to this problem is to use a gravitational oil separator, which is designed to dampen these oscillations. The separator is equipped with a control system that uses liquid level and gas pressure sensors to stabilise the flow by adjusting the positions of its valves. This paper presents the use of a specific type of model-based predictive controller to control the level and pressure of a biphasic oil separator, particularly in the presence of slug disturbances. The designs of the separator model and controller are discussed in detail, with a focus on the advantages of using an unrestricted horizon predictive controller, such as its ability to make predictions over a long horizon and its relatively low computational requirements. For the sake of comparison, a linear quadratic regulator is also evaluated. The simulation results demonstrate that the proposed control system is able to effectively regulate the separator’s liquid level and gas pressure within a magnitude range of 10−4 m for the liquid level and 10−3 bar for the internal pressure. Aside from that, the dynamics of the closed-loop system is six times faster than the plant’s for the liquid behaviour and 30 times faster for the pressure, while also presenting sharp attenuation characteristics for the input disturbances of nearly 50 dB for the pressure output and 68 dB for the liquid level.