Abstract:
A method of controlling an electric submersible pump, comprising: a) monitoring the pressure at the suction and discharge of the pump; b) calculating the pressure difference between the discharge and suction pressure; and c) controlling the pump to maintain a constant pressure difference between the discharge and suction pressure.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/605,795, filed Mar. 2, 2012, which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to a method of controlling an electric submersible pump located in a subsea caisson. 
       BACKGROUND 
       [0003]    Electric submersible pumps are used in deepwater oil and gas production settings to provide artificial lift such that the oil and gas can be raised to the surface for further processing, storage and/or transport. During fluid property changes of the oil/gas mixture and/or introduction of water into the pump, large swings in pump operation can occur. 
         [0004]    These swings in pump operation can result in increased downtime, damage to the pump components, seals and motor and decreased pump longevity. These problems are often encountered during startup of the pump when the fluids in and around the pump suction side have been allowed time to separate into distinct phases. For example, a separate water phase, gas/liquid phase and hot oil phase may be encountered in the electric submersible pump during startup. 
       SUMMARY OF THE INVENTION 
       [0005]    The invention provides a method of controlling an electric submersible pump, comprising: a) monitoring the pressure at the suction and discharge of the pump; b) calculating the pressure difference between the discharge and suction pressure; and c) controlling the pump to maintain a constant pressure difference between the discharge and suction pressure. 
         [0006]    The invention further provides a method of starting up an electric submersible pump, comprising: a) starting the pump on differential pressure control, comprising: i) monitoring the pressure at the suction and discharge of the pump; ii) calculating the pressure difference between the discharge and suction pressure; and iii) controlling the pump to maintain a constant pressure difference between the discharge and suction pressure; and b) switching the pump control to pressure control, comprising: i) monitoring the pressure at the top of the caisson; and ii) controlling the pump to maintain a constant pressure at the caisson. 
         [0007]    The invention also provides a method of controlling an electric submersible pump, comprising: a) monitoring the pressure at the caisson top; b) switching the pump control to differential pressure control during the start-up phase when fluid density inside the pump is changing; and c) switching the pump control to pressure control when the system has stabilized in a specified operational pressure range. In certain embodiments, the method may further comprise: d) during steady operation, switching to constant pressure difference control automatically when an instability is detected, comprising: i) monitoring a performance variable that indicates the stability of the caisson operation; and ii) switching the control automatically to constant pressure difference control when the performance variable exceeds a threshold value. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  depicts an embodiment of the electric submersible pump showing the different fluid phases present on startup. 
       
    
    
     DETAILED DESCRIPTION 
       [0009]    Electric submersible pumps are designed to operate in a range of conditions, but they are not designed for large pressure swings that can occur when the fluids passing through the pump change significantly. For example, slugs of water, changing oil/gas ratios and rapid changing of distinct fluid phases passing through the pump cause rapid swings in pressure as the pump operation characteristics vary with different fluid densities. These pressure swings can cause damage to pump components and is believed to be at least a contributing factor of current electrical failures of electric submersible pumps in the field. 
         [0010]    The speed of an electrical submersible pump may be set via a Variable Frequency Drive (VFD); the speed of the pump may be directly-related to the frequency. Caisson pressure may be controlled by manipulation of the demand to the VFD. Manipulating the electric submersible pump based on caisson pressure is normally satisfactory, but does not provide a rapid response during changing fluid conditions. 
         [0011]    The invention provides an alternate process control scheme that can be used to more effectively manipulate the electric submersible pump during startup and when fluid properties are rapidly changing. This process control scheme is referred to as constant boost mode because it manipulates the pump speed based on the differential pressure across the pump. The differential pressure is calculated as the difference between the discharge pressure and the suction pressure of the pump. 
         [0012]    Manipulating the pump in constant boost mode provides a more rapid response to fluctuations in fluid density to avoid instabilities on the system. 
         [0013]    One example of the issues encountered during startup is described further. When starting up an electric submersible pump located in a subsea caisson, the fluid phase that is typically in the pump is an oil/gas mixture. As the pump starts up, the oil/gas mixture is pumped out the discharge and after some time, a water phase that has separated from the oil/gas mixture reaches the pump suction. The water is pumped and then another oil/gas phase is encountered. Then a hot oil phase is passed through the pump while the wells are ramped up and start producing and this is followed by the oil/gas mixture being produced from the field. With each change in phases, the density changes; increases with water, decreases with oil/gas, increases with hot oil and decreases with field oil. 
         [0014]      FIG. 1  depicts electric submersible pump system  100 . Electric submersible pump system  100  may comprise a pump  110  and a shroud  120  surrounded by a caisson  130 . Electric submersible pump system  100  may further comprise an inlet  101 , a gas outlet  102 , and a liquid outlet  103 . 
         [0015]    In certain embodiments, fluid within the pump may be an oil/gas mixture. A separate water phase may be located at the bottom of the caisson and an additional oil/gas phase is floating on the water phase outside of the shroud. After these phases have been pumped out, a new oil/gas mixture from the formation is pumped through the pump. Once the pump has started up manually, the system is switched to caisson pressure control. 
         [0016]    In the alternative, the pump can be switched to constant boost control if slugs of water or other distinct phases are encountered as observed by pressure swings in the pump during operation. The constant boost mode will help the pump through the phase changes and the pump can then be returned to caisson pressure control when system is in stable operation. 
         [0017]    In one embodiment, the differential pressure control is used to manipulate the electric submersible pump during startup and at any time during operation when the fluid density in the Caisson changes abruptly. As the caisson pressure is brought back into a more stable condition by controlling the differential pressure control, the pump can be switched back to manipulation via caisson pressure control. 
         [0018]    The stability of the pressure can be detected by comparing the differential pressure in the Caisson to the last measured differential pressure or to a running average of a series of differential pressure measurements. The specific characteristics of a stable system with regards to differential pressure should be determined based on the specific characteristics of the formation and the performance characteristics of the electric submersible pump. 
         [0019]    The differential pressure control scheme described herein provides a more rapid response to fluctuations in pressure caused by changing fluid density, especially those encountered during startup.