Abstract:
A method of testing a control system including a valve which is operable by a solenoid is disclosed. The method comprises applying a current through a coil of the solenoid, the current being insufficient to cause the solenoid to operate the valve, and monitoring a response of the system to the current.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The field of the invention relates to testing a control system including a valve. 
     2. Description of the Prior Art 
     Control valves of control systems for subsea fluid production wells are often operated infrequently (e.g. every 6 months or less). A problem is that there may be a fault in the system which would not be apparent until the well operator tries to operate the valve. Discovery of the failure in an emergency situation could be disastrous. 
     Systems according to embodiments of this invention enable the well operator to be given advance notice of a failure, thus substantially reducing the possibility of discovery of a system failure only in an emergency situation. Systems according to embodiments of the invention frequently test the communications, control circuitry, monitoring circuitry, wiring to the valve and a solenoid coil of a solenoid for operating the valve, without actually operating it. Thus, if it is known that there is a problem before the function is actually needed, reliability can be increased by scheduling such a test during scheduled maintenance. 
     BRIEF SUMMARY OF THE INVENTION 
     According to an embodiment of the present invention, a method of testing a control system including a valve which is operable by a solenoid is disclosed. The method comprises applying a current through a coil of the solenoid, the current being insufficient to cause the solenoid to operate the valve, and monitoring a response of the system to the current. 
     According to an alternate embodiment of the present invention a control system including a valve which is operable by a solenoid is disclosed. The system comprises a power control configured to apply a current through a coil of the solenoid, the current being insufficient to cause the solenoid to operate the valve, and a monitoring arrangement configured to monitor a response of the system to the current. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate one or more embodiments and, together with the description, explain these embodiments. In the drawings: 
         FIG. 1  is a graph of current against time fir a typical current flow through a solenoid coil of a directional control valve (DCV) within a subsea control system when the valve is operated; 
         FIG. 2  is a schematic diagram of a control system for a subsea well operating in accordance with an embodiment of the invention; 
         FIG. 3  shows details of current sensing means of the system of  FIG. 2 ; 
         FIG. 4  is a schematic diagram of a control system for a subsea well operating in accordance with an embodiment of the invention; and 
         FIG. 5  shows details of voltage sensing means of the system of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the present invention improve the reliable operation of a subsea valve by testing the communications, control circuitry, monitoring circuitry, wiring and a solenoid coil at regular intervals, generally at a much greater frequency than the valve is used in operation. This is achieved by taking advantage of the fact that the current necessary to drive a valve operating solenoid ramps up relatively slowly. By firing the solenoid coil for only a short period of time, this current can be sensed to verify that all the necessary elements are working correctly. The power to the solenoid coil is turned off before current has ramped up enough to operate the valve. 
       FIG. 1  is a graph of current against time for a typical current flow through the solenoid coil of the solenoid of a directional control valve (DCV) at a tree within a control system for a subsea fluid extraction well when the valve is operated. As can be seen, the current rise from the application of the supply voltage to the solenoid coil is not instantaneous, due to the inductance of the solenoid coil, but ramps up to the point B on the graph, in, typically, between 50 and 300 milliseconds, when the solenoid mechanically operates. The current continues to rise until it reaches a steady state level determined by the resistance of the solenoid coil and the supply voltage, which is high enough to ensure that the solenoid is held in the ‘operated’ condition. In embodiments of the invention, the current in the DCV solenoid coil is allowed to rise to level A, typically after 10 to 20 milliseconds, which is insufficient to operate the solenoid but sufficient to be detected to provide confidence that at least the control communications, circuitry and wiring, including the DCV solenoid coil itself, are functioning correctly. In one embodiment, the current in the DCV solenoid coil is detected by monitoring the voltage across a small resistor connected in series with the solenoid coil. 
     Operation of the DCVs of a well control system for a subsea well is normally controlled from a surface platform via a complex communication system, a typical example of which (modified in accordance with an embodiment of the invention) is illustrated in  FIG. 2 . The command signal to operate a DCV, as a well operating function, is generated at a surface platform  1  by a well control and monitoring arrangement  2  and is transmitted via a modem  3  at the platform through a surface to subsea well umbilical  4  to a subsea control module (SCM), which houses a subsea electronics module (SEM), which in turn houses a subsea modem  5  and an Ethernet switch blade (ESB)  6  and a single board computer (SBC)  7 . The ESB  6  and SBC  7  are features of modern subsea well communications, and handle the control signals to the multiplicity of drivers for the DCVs on the well. The SBC  7  is located in a directional control valve support module (DSM) containing a power supply  8 , power control  9  and control logic  10  to output supply voltage to the solenoid coils of DCVs located in the SCM via an H bridge  11  and connectors and wiring  12 . In the system diagram  FIG. 2 , a DCV solenoid coil  13  of only one DCV is shown for simplicity. 
     Testing of the control system routing comprising the communications, control circuitry, monitoring, wiring and solenoid coil, by a current flow sensing method is effected by the well control and monitoring arrangement  2  being programmed to command power control module  9 , via the communication route described above, to output a power supply pulse to the solenoid coil  13 , for a limited duration of typically 10 to 20 milliseconds. Although the route of the command signal is the same as the normal operational route, the test command message includes a ‘test tag’ in the message so that the SBC  7  produces only the required 10 to 20 ms current drive through the DCV solenoid coil  13  rather than the continuous DC input to the coil under operational conditions. The current passing through the solenoid coil also passes though a small value, typically 1% of the coil resistance, resistor  14 , as shown in  FIG. 3  which is an expansion of current sensing means  15  between bridge  11  and connectors and wiring  12 . The small voltage produced across the resistor  14  due to the current flowing through the coil  13 , is amplified by an operational amplifier  16 , with the gain set by resistors  17  and  18 , to provide a suitable voltage level to an analogue to digital converter (A/D)  19 . The digital output  20  of the A/D  19  represents the current flow through the coil  13  and is fed to the SBC  7 , where it can either be sent back to the surface platform via the communication system for verification, or it can be compared, in the SBC  7 , with stored values of correct expected test values and a verification or otherwise message sent to the surface platform  1 . Thus, confidence is provided that the complex control signal routing is functioning correctly, and that the solenoid coil drive and coil continuity is correct. Operation of the test routine can be effected by the well operator and/or automatically at regular intervals by suitable programming of the well control and monitoring arrangement  2  at the surface platform. 
     The current sensing means  15  could be elsewhere than as shown. For example, it could be immediately before the solenoid coil  13 , after the coil  13 , before the H bridge  11  or anywhere suitable in the control system. 
     As an alternative to monitoring current by monitoring the voltage drop across a resistor, current sensing means  15  could be provided using a Hall effect device. 
     An alternative method of securing confidence that the control of DCVs via the well communication system is functioning correctly, without operating a valve, is to monitor the voltage pulse appearing across the coil  13 , when the test pulse is applied. This is illustrated in  FIGS. 4 and 5 , there being voltage sensing means  21  in  FIG. 4  between bridge  11  and connectors and wiring  12 . The voltage sensing means  21  is detailed in  FIG. 5 , the voltage across the coil  13  being connected to an A/D converter  22 . Items in  FIG. 4  which correspond with items in  FIG. 2  have the same reference numerals as in  FIG. 2 . The digital output  23  of the A/D converter  22  represents the test voltage across the coil  13  and is fed to the SBC  7  where it again can either be sent to the surface platform for verification, or compared in the SBC  7 , with stored values of correct expected test values and a verification, or otherwise, message sent to the surface platform. This method of verification of the correct functioning of the DCV coil drive does have the disadvantage over the current flow method of not providing confidence in the continuity of the solenoid coil itself. 
     The revenue generated from an oil well can be millions of U.S. dollars per day, which would be lost if the production ceased because of a faulty valve circuit. The cost to fix this problem can be hundreds of thousands of U.S. dollars per day for the boat/submarine hire to recover and replace the faulty equipment from the seabed. Worse still, if the fault occurs outside an operational weather window, then the maintenance might not be possible for several months with the corresponding several months of lost production. Embodiments of the invention enable detection of faulty equipment involved with the well valve control system before it is needed, thus allowing any detected faulty component to be changed as part of scheduled maintenance.