Design of MRFT-based tuning strategies for gas loop PI controller of Liquid-Gas Separator

  • Hamdati Al Shehhi

Student thesis: Master's Thesis

Abstract

Horizontal type separators are widely used in the petroleum industry for separation of oil from gas and water contained in oil reservoirs. The separation process for the fluids is needed in order to treat each petroleum product individually. The amount of the oil and associated gas which is coming from different wells is high and needs a quick and reliable separation unit. For that reason, in order to maintain the production of the oil field, it is important to have efficient control of the gas and liquid inside the separator unit. For example, the control system in the offshore platform has to maintain the gas pressure in the separator at a specific set-point despite various disturbances coming from the inflow stream and the gas pressure manifold. Poor performance of the gas pressure loop may result in over -pressurization. In the case when the separator pressure grows above the trip point of 120 PSIG, depressurization has to be done by sending the gas to flaring, which leads to a negative environmental impact. On the other hand, it is also essential to consider the change of the crude oil amount inside the vessel when tuning the gas pressure controller for optimal performance. This research focuses on designing the controller tuning strategies for the gas loop of the horizontal type liquid-gas separator. The model of the gas loop is considered as a process in this system which is controlled by the Proportional-Integral (PI) controller. The aim of the controller is to maintain the pressure equal to the set point regardless of various disturbances. The Modified Relay Feedback Test (MRFT) is a continuous oscillation method for identification of process parameters and controller tuning. In this research, the MRFT was used for the design of robust and adaptive PI controllers for a gas loop in the two-phase separator. The gas normally found in the separator is the natural gas (mostly methane) which is contained in crude oil coming from the reservoir. In this study, the PI controller was developed based on sixty four modes of operation corresponding to certain ranges of gas inflow and liquid level values - the two parameters that characterize the operating modes. It was shown through the developed model and simulations that these operating modes affect the dynamics of the gas loop. Dynamic properties of the process in each mode were studied through MRFT. Robust and adaptive PI controllers were designed in order to maintain the pressure during the change of the operating conditions. The performance of the robust and adaptive PI controllers are demonstrated by the loop reaction to the three disturbances and set point combinations which are the sinusoidal changes of the inflow and level and, step changes of the separator pressure set point, the inflow, level and the manifold pressure.
Date of Award2014
Original languageAmerican English
SupervisorIgor Boiko (Supervisor)

Keywords

  • Applied sciences
  • Electrical engineering
  • 0544:Electrical engineering

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