Optimal Control Strategy and Experimental Investigation of Gas/Liquid Compact Separators
- S. Wang (U. of Tulsa) | R. Mohan (U. of Tulsa) | O. Shoham (U. of Tulsa) | J.D. Marrelli (ChevronTexaco E&P Technology Co.) | G.E. Kouba (ChevronTexaco E&P Technology Co.)
- Document ID
- Society of Petroleum Engineers
- SPE Journal
- Publication Date
- June 2002
- Document Type
- Journal Paper
- 170 - 182
- 2002. Society of Petroleum Engineers
- 1.10 Drilling Equipment, 5.3.2 Multiphase Flow, 5.5.1 Simulator Development, 5.3.4 Integration of geomechanics in models, 4.4.3 Mutiphase Measurement, 4.6 Natural Gas, 4.1.5 Processing Equipment, 1.7.5 Well Control, 4.5.9 Subsea Processing, 5.6.4 Drillstem/Well Testing, 4.1.2 Separation and Treating
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The deployment of the new technology of gas/liquid compact separators such as the Gas/Liquid Cylindrical Cyclone (GLCC, copyright, U. of Tulsa, 1994) often requires dedicated control systems for field applications. The control strategy implementation is crucial for process optimization and adaptation, especially when GLCCs are operated at a wide range of liquid and gas flow rates. In this study, a unique and simple control strategy, which is capable of optimizing the operating pressure and adapting to liquid and gas inflow fluctuations, has been developed. Detailed simulations and experimental investigations also have been conducted to evaluate the performance of the proposed control systems. The significant advantages of this strategy are threefold: the system can be operated at optimum separator back pressure, the system can adapt to the changes of liquid and gas flow rates, and the strategy can be easily implemented using simple PID controllers available on the market. This provides the oil and gas industry with a simple, robust compact separator control technique that has the potential for offshore and subsea applications.
Compared to conventional separators, compact separators such as the GLCC are simple and compact, possess low weight, require little maintenance, are low-cost, and are easy to install and operate. GLCCs have been used to enhance the performance of multiphase meters, multiphase pumps, desanders, slug catchers, partial separators, portable well testing equipment, preseparators, and primary separators for offshore and onshore operations. They also have the potential for applications as flare gas scrubbers and downhole separators, as well as for subsea processing.
Presently, more than 350 GLCC units have been installed and put into use in the field for various applications. The size of these GLCCs varies from 3 in. to 5 ft in diameter and 7 to 30 ft in height. Fig. 1 shows the largest GLCC in the world, a 5-ft-diameter, 20-ft-tall GLCC field unit operating in Minas, Indonesia, in a bulk separation/metering loop configuration.1
The GLCC separator is a vertically installed pipe/vessel mounted with a declined tangential inlet, with outlets for gas and liquid provided at the top and bottom, respectively. It has neither moving parts nor internal devices. The two phases of the incoming mixture are separated by the centrifugal/buoyancy forces caused by the swirling motion and gravity. The heavier liquid is forced radially toward the walls of the cylinder and is collected from the bottom, while the lighter gas moves to the center of the cyclone and is taken out from the top.
A GLCC in a metering loop configuration, in which the gas and liquid outlets are recombined, is capable of self-regulating the liquid level for small flow variations. However, for large flow variations, it is essential to have a control sytem for proper operation. Also, GLCCs for other applications, such as bulk separation, must have suitable control systems so as to prevent the liquid overflow through the gas leg and gas blowout through the liquid leg. There is an increasing need to develop appropriate control strategies, design tools, and simulators for GLCC control, because its residence time is very small, and the range of applications for the GLCC is quite large. Also, the performance of compact separators could be enhanced considerably by incorporating suitable control systems.
The objectives of this study are to develop a mathematical model, establish the optimal and adaptive control strategy, perform dynamic simulations, and conduct experimental investigations to evaluate the control strategy. The features of the GLCC dynamic model and the dynamic simulators are developed using Matlab/ Simulink (MathWorks, Natick, Massachusetts, 1997) software. A unique optimal control strategy is proposed for the first time for GLCC control. This strategy has the capability of minimizing the operating pressure at any liquid and gas flow rate and dynamically adapting the controller for different operating conditions. This provides the petroleum industry with an effective tool for GLCC control system design, control system implementation, and dynamic simulation.
Several investigators have realized that the performance of compact separators could be improved by incorporating suitable control systems. Kolpak* and Wang2 developed hydrostatic models for passive control of compact separators in a metering loop configuration. These models demonstrate the sensitivity of the liquid level to the gas and liquid inflow rates.
Gas-liquid separators may operate under slug flow conditions in the field. The system dynamics are significant for such applications, especially when a control system is added to the separator. Genceli et al.3 developed a dynamic model and a simulator for a slug catcher. They proposed a liquid level control and pressure control configuration and PI controllers for both control loops. The slug catcher program was primarily used to optimize the slug catcher size.
Mohan et al.4 conducted detailed experimental investigations on a newly developed GLCC passive control system. They demonstrated that the passive control system improved the GLCC operational envelope for liquid carry-over in a limited range of flow rates. As a continuation of this work, Wang et al.5 developed a dynamic model for GLCC control system design and dynamic simulation. Wang et al.6,7 conducted detailed experimental investigations to evaluate the improvement in the GLCC operational envelope for liquid carry-over with the integrated level and pressure control system for a wide range of flow rates. They have also developed different control strategies and control system simulators for GLCC field applications.
Previous studies also demonstrate that the performance of compact separators could be enhanced considerably by incorporating suitable control systems. However, there is an increasing need to develop optimal control strategy for GLCC control for offshore and subsea applications. The overall objective of this investigation is to expand the state of the art of compact separation technology through development of suitable control strategies and simulators and control system design.
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