Abstract:
A technique operates a valve system in a subsea test tree via a control system of a type suitable for gaining desired industry ratings. A monitoring system is utilized to monitor functions of the control system, but the monitoring system is independent from the control system.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is based on and claims priority to U.S. Provisional Application Ser. No. 61/174,005, filed Apr. 30, 2009. 
    
    
     BACKGROUND 
     In a variety of subsea well related applications, subsea test trees (SSTTs) are installed within subsea risers during completion operations. The subsea test trees enable the safe and temporary closure of subsea wells. Depending on the application, a control system is positioned either at a topside location or a subsea location and coupled to the subsea test tree. The control system is used to actuate valves in the subsea test tree by controlling the delivery of hydraulic fluid through a control line. The hydraulic fluid is selectively applied to cause a desired change in state, e.g. transition of a valve, on the subsea test tree. In some of these applications, it may be desirable to design the control system with simplicity to obtain a desired Safety Integrity Level (SIL) rating recognized by the industry. However, designing the control system with simplicity for certification as an SIL unit can limit the ability to monitor functionality of the control system. 
     SUMMARY 
     In general, the present application provides a system and methodology for controlling a subsea test tree via a control system of a type suitable for gaining desired industry ratings. A monitoring system is utilized to monitor functions of the control system, but the monitoring system is independent from the control system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Certain embodiments will hereafter be described with reference to the accompanying drawings, wherein like reference numerals denote like elements, and: 
         FIG. 1  is a schematic view of a well system used in a subsea application, according to an embodiment; 
         FIG. 2  is a schematic illustration of one example of a control system and an independent monitoring system positioned to monitor functions of the control system, according to an embodiment; 
         FIG. 3  is a schematic illustration of subsea components of the control system and the monitoring system illustrated in  FIG. 2 , according to an embodiment; 
         FIG. 4  is an orthogonal view of one example of a riser instrumentation module that can be utilized in the monitoring system, according to an embodiment; 
         FIG. 5  is another view of the riser instrumentation module illustrated in  FIG. 4 , according to an embodiment; and 
         FIG. 6  is a schematic illustration of a gauge monitor pressure sensing arrangement, according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, numerous details are set forth to provide an understanding of various embodiments. However, it will be understood by those of ordinary skill in the art that many embodiments may be practiced without these details and that numerous variations or modifications from the described embodiments may be possible. 
     The present application generally relates to a technique for utilizing subsea control devices in subsea applications. This technique also relates to instrumentation that involves sensors and/or monitoring in subsea control devices and applications. The subsea systems and methodologies can be employed in a variety of subsea applications with wells formed in many types of subsea environments. For example, wells may be formed as generally vertical wells or as deviated, e.g. horizontal, wells, and the equipment used in a given well application may be selected according to the type of well, subsea environment, surface equipment, and other factors that affect the specific well application. 
     According to one embodiment, a subsea well  20  extends below a subsea test tree  22  positioned at a subsea location  24  along, for example, a seabed  26 , as illustrated in  FIG. 1 . The subsea test tree  22  comprises a valve system  28  that may be selectively operated to open and shut off the subsea well  20 . In the example illustrated, subsea test tree  22  is connected with a surface structure  30  via a riser  32  or other suitable structure that provides a passage through the sea between surface structure  30  and subsea test tree  22 . The surface structure  30  may be at a surface location  33  and may be in the form of a surface vessel, a permanent structure or a semi-permanent structure depending on the type and location of subsea well  20 . 
     In the embodiment illustrated, a control and monitoring system  34  is employed in cooperation with the subsea test tree  22 . In this example, system  34  comprises a control system  36  operatively coupled with the subsea test tree  22  to control features of the subsea test tree, such as valve system  28 . System  34  further comprises a monitoring system  38  which is positioned and employed to monitor functions of control system  36 . In this example, monitoring system  38  comprises a riser instrumentation module system which is independent from and remains isolated from control system  36 . 
     Control system  36  may be constructed in a variety of configurations with various components depending on the specific application. However, one specific example of a type of control system for controlling subsurface test trees is a subsea test tree control system available from Schlumberger Corporation and known as SenTURIAN. As noted previously, however, this type of control system employs limited or no monitoring to ensure sufficient simplicity for certification as a Safety Integrity Level (SIL) unit having a desired SIL rating, e.g. a SIL 2 rating. The SenTURIAN control system and similar systems may be defined as Safety Instrumented Systems (SAS) per IEC Standard 61508. In the present system, however, addition of the independent riser instrumentation module system  38  enables the overall system  34  to monitor functions of the primary control system  36  while maintaining isolation from the SIL system, i.e. control system  36 . This allows the control system to be designed in a manner that maintains the desired SIL certification and promotes compliance with the applicable International Organization for Standardization (ISO) standards. 
     To maintain the desired SIL rating on control system  36  while adding monitoring capabilities, the control functions are isolated from the monitoring functions. To accomplish the isolation, the riser instrumentation module system  38  contains separate components, such as separate acquisition circuits, modem, communication lines, e.g. cable, and/or other independent components. 
     As discussed in greater detail below, monitoring system information may be communicated between the subsea location  24  and the surface structure  30  via a separate communication line  40 , e.g. cable, relative to a communication line  42  of control system  36 . By way of example, communication line  42  may comprise a plurality of hydraulic lines used to deliver fluid for actuating valve system  28  and/or other systems of subsea test tree  22 . Creation of independent monitoring and control systems means that any problem with the monitoring system  38  causes no effect on the ability of control system  36  to effectively carry out its safety functions with respect to actuation of valve system  28  and/or other systems of subsea test tree  22 . 
     Referring generally to  FIG. 2 , the relationship between control system  36  and riser instrumentation module system  38  is illustrated. In this embodiment, control system  36  comprises a subsea control module  44  and a topside control system  46  that are connected with each other via communication line  42 . By way of example, communication line  42  may comprise a multicore cable having one or more hydraulic control lines. In this example, the monitoring system  38  comprises a subsea monitoring module  48  and a topside monitoring system  50  that are connected with each other via communication line  40 . By way of example, communication line  40  may comprise one or more electric, fiber-optic, wireless, or other suitable signal communication lines able to convey signals between the subsea location  24  and the surface location  33 . The subsea monitoring module  48  is designed to measure and monitor desired parameters, such as temperature and pressure in hydraulic control lines used to manipulate valve system  28  and/or other systems of subsea test tree  22 . 
     Communication line  40  and monitoring communication line  42  may be routed as two completely separated cables, or the communication lines  40 ,  42  may be combined in a common umbilical  52 . If a common umbilical  52  is utilized, the communication lines  40 ,  42 , e.g. cables, are maintained as independent paths for communicating signals between the subsea and surface locations. Accordingly, the isolated communication layout of the overall system is maintained. Additionally, data can be observed and/or input to control system  36  and/or monitoring system  38  via a display system  54 . By way of example, display system  54  may utilize a graphical user interface  56  for displaying information to a user and for allowing the user to input control commands or other system data. 
     As illustrated in  FIG. 3 , parameters of control system  36  are monitored with appropriate sensors  58  of subsea monitoring module  48 . The sensors  58  may comprise, for example, a temperature sensor and/or pressure sensor associated with individual hydraulic lines  60  extending between subsea control module  44  and controlled components of subsea test tree  22 , e.g. valve system  28 . In some applications, other sensors, e.g. vibration sensors, also may be employed to detect parameters related to operation of control system  36 . 
     The sensors  58  may be associated with individual hydraulic lines or with a plurality of hydraulic lines, and the output from sensors  58  is directed to acquisition circuitry  62  that is completely independent of componentry of control system  36 . Acquisition circuitry  62  may be part of subsea monitoring module  48  or may be positioned at other suitable locations in monitoring system  38 . In the particular example illustrated, parameter data is directed to one or more sensors  58  by providing a “T” in the corresponding hydraulic line  60  to measure, for example, pressure and temperature of the hydraulic control line  60  without obstructing its function. Use of the “T” coupling enables observation of the desired parameter at a specific location  63  along the hydraulic line; however other systems may be used to observe the desired parameter. 
     Subsea monitoring module  48  may be constructed in various configurations with components selected to enable independent monitoring of control system functions. In one example illustrated in  FIG. 4 , the subsea monitoring module comprises a modular monitoring hub  64  that may be mounted at a variety of locations along the subsea test tree  22  and riser  32  to monitor a desired parameter or parameters related to control system  36 . For example, the modular monitoring hub  64  may be constructed as a pressure and/or temperature monitoring hub utilized in cooperation with the control system  36  to monitor pressure/temperature in control lines at the desired location. The modular monitoring hub  64  may be mounted on a mandrel  66 , such as a 10 ksi or 15 ksi mandrel of the type used in a variety of offshore, well related applications. 
     In one example, modular monitoring hub  64  is designed to slide over and attach to mandrel  66 , as illustrated in  FIG. 4 . As further illustrated in  FIG. 5 , the modular monitoring hub  64  may comprise a plurality of hydraulic flow ports  68  designed to enable measuring and monitoring of the desired parameter at specific locations  63  along subsea test tree  22  and/or riser  32 . In this manner, monitoring hub  64  can be designed as a modular component for utilization in many types of riser systems to monitor hydraulic lines or other pressure lines. 
     The modular monitoring hub  64  may be designed with a first, e.g. top, interface  70  and a second, e.g. bottom, interface  72 , as illustrated schematically in  FIG. 6 . The top interface  70  provides a hydraulic interface designed for connection to many types of hydraulic control lines  60  by providing appropriate adapters to form the connection. Similarly, bottom interface  72  also provides a hydraulic interface that may be connected to many types of hydraulic control lines  60  by providing the appropriate adapters. Multiple individual pressure and/or temperature sensors  58 , e.g. gauges, are connected between top interface  70  and bottom interface  72  to detect parameters of the control fluid moving through individual ports  68 . For example, individual sensors  58  can monitor corresponding hydraulic lines  60  at ports  68  through a “T” engagement as described above. 
     As a result, modular monitoring hub  64  enables the independent monitoring of multiple hydraulic control lines in control system  36 . In some applications, it may only be necessary to monitor an individual hydraulic line; although monitoring hub  64  simplifies the monitoring of greater numbers of control system hydraulic lines  60 . 
     The control and monitoring system  34  also may be designed to automatically detect the presence of riser instrumentation module system  38 , e.g. subsea monitoring module  48  or specific components of the system, such as modular monitoring hub  64 . For example, when monitoring hub  64  is installed in the string along riser  32  or subsea test tree  22 , the system  34  automatically detects its presence and enables control of the monitoring functions conducted with respect to control system  36 . In one specific embodiment, a topside system, such as topside monitoring system  50  and/or topside control system  46  may be utilized to detect the presence of modular monitoring hub  64  or other portions of riser instrumentation module system  38 . Once detected, the graphical user interface  56  on display  54  may automatically be updated to include data related to monitoring system  38 . In one example, the topside system accomplishes updating of the graphical user interface by monitoring a modbus port associated with the riser instrumentation module system  38 . When the riser instrumentation module is detected, the topside system reads communication frames from the module to ensure the topside system sets up appropriate graphics on the graphical user interface  56 . 
     System  34  may be constructed in a variety of configurations for use in many types of subsea wells. For example, many types of topside processing systems may be incorporated into the topside control system and topside monitoring system, respectively. Additionally, various sensors may be employed at the subsea test tree  22  or at other suitable subsea locations, and the mechanical structures used in mounting the sensors can be adjusted according to the configuration of the corresponding subsea components. Furthermore, various parameters and combinations of parameters may be measured to monitor the control system without compromising the SIL rating of the control system. This is accomplished by maintaining the monitoring system as a separate, independent system which does not utilize common sensors, common control circuitry, common communication lines, or other common components with the control system. Thus, the monitoring system is not able to interfere with operation of the control system. 
     The subsea test tree  22  and riser  32  also may be constructed in a variety of sizes and configurations. Depending on the specific subsea application, control system  36  may be utilized in a variety of safety controls, such as closing off the subsea well  20  at subsea test tree  22 . However, control system  36  also may be designed to control other or additional functions within subsea test tree  22  and/or along riser  32 . 
     Although only a few embodiments have been described in detail above, those of ordinary skill in the art will readily appreciate that many modifications are possible without materially departing from the teachings of this application. Accordingly, such modifications are intended to be included within the scope defined in the claims herein and subsequent related claims.