Abstract:
A subsea electronic information system for managing data related to a characteristic of subsea equipment locatable subsea. The system includes sensors locatable subsea and in communication with and capable of measuring a characteristic of the subsea equipment. A sensor interface box (SIB) separate from the sensors and locatable subsea includes a processor and a memory device capable of receiving and storing sensor measurement data. Additionally, the SIB is in data and power communication with the sensors. 
     The system further includes a subsea retrievable data capsule capable of recording all system data over a long period. The capsule may be recovered from subsea (independently of the other elements of the information system) for forensic analysis of the recorded data.

Description:
BACKGROUND 
     In most offshore drilling operations, a wellhead at the sea floor is positioned at the upper end of the subterranean wellbore lined with casing, a blowout preventer (BOP) stack is mounted to the wellhead, and a lower marine riser package (LMRP) is mounted to the BOP stack. The upper end of the LMRP typically includes a flex joint coupled to the lower end of a drilling riser that extends upward to a drilling vessel at the sea surface. A drill string is hung from the drilling vessel through the drilling riser, the LMRP, the BOP stack, and the wellhead into the wellbore. 
     During drilling operations, drilling fluid, or mud, is pumped from the sea surface down the drill string, and returns up the annulus around the drill string. There are a number of functions and operating parameters of the LMRP, BOP, and other subsea well equipment that may need to be monitored and controlled. 
     As an example, in the event of a rapid invasion of formation fluid into the annulus, commonly known as a “kick,” the BOP stack and/or LMRP may actuate to help seal the annulus and control the fluid pressure in the wellbore. In particular, the BOP stack and the LMRP include closure members, or cavities, designed to help seal the wellbore and prevent the release of high-pressure formation fluids from the wellbore. Thus, the BOP stack and LMRP function as pressure control devices. 
     For most subsea drilling operations, hydraulic fluid for operating the BOP stack and the LMRP is provided using a common control system physically located on the surface drilling vessel. However, the common control system may become inoperable, resulting in a loss of the ability to operate the BOP stack. As a backup, or even possibly a primary means of operation, hydraulic fluid accumulators are filled with hydraulic fluid under pressure. The amount and size of the accumulators depends on the anticipated operation specifications for the well equipment. 
     An example of an accumulator includes a piston accumulator, which includes a hydraulic fluid section and a gas section separated by a piston moveable within the accumulator. The hydraulic fluid is placed into a fluid section of the accumulator and pressurized by injecting gas (typically Nitrogen) into the gas section. The fluid section is connected to a hydraulic circuit so that the hydraulic fluid may be used to operate the well equipment. As the fluid is discharged, the piston moves within the accumulator under pressure from the gas to maintain pressure on the remaining hydraulic fluid until full discharge. 
     The ability of the accumulator to operate a piece of equipment depends on the amount of hydraulic fluid in the accumulator and the pressure of the fluid. Thus, there may be a need to know the volume of the hydraulic fluid remaining in an accumulator so that control of the well equipment may be managed. Measuring the volume of hydraulic fluid in the accumulator over time can also help identify if there is a leak in the accumulator or hydraulic circuit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a detailed description of the preferred embodiments of the invention, reference will now be made to the accompanying drawings in which: 
         FIG. 1  shows a schematic view of an offshore system for drilling and/or producing a subterranean wellbore with an embodiment of an electronic data collection and control system; 
         FIG. 2  shows an elevation view of the subsea BOP stack assembly and electronic data collection and control system of  FIG. 1 ; 
         FIG. 3  shows a perspective view of the subsea BOP stack assembly and electronic data collection and control system of  FIGS. 1 and 2 ; and 
         FIG. 4  is a schematic view of the electronic data collection and control system, including the information system. 
     
    
    
     DETAILED DESCRIPTION 
     The following discussion is directed to various embodiments of the invention. The drawing figures are not necessarily to scale. Certain features of the embodiments may be shown exaggerated in scale or in somewhat schematic form and some details of conventional elements may not be shown in the interest of clarity and conciseness. Although one or more of these embodiments may be preferred, the embodiments disclosed should not be interpreted, or otherwise used, as limiting the scope of the disclosure, including the claims. It is to be fully recognized that the different teachings of the embodiments discussed below may be employed separately or in any suitable combination to produce desired results. In addition, one skilled in the art will understand that the following description has broad application, and the discussion of any embodiment is meant only to be exemplary of that embodiment, and not intended to intimate that the scope of the disclosure, including the claims, is limited to that embodiment. 
     Certain terms are used throughout the following description and claims to refer to particular features or components. As one skilled in the art will appreciate, different persons may refer to the same feature or component by different names. This document does not intend to distinguish between components or features that differ in name but not function. The drawing figures are not necessarily to scale. Certain features and components herein may be shown exaggerated in scale or in somewhat schematic form and some details of conventional elements may not be shown in interest of clarity and conciseness. 
     In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . .” Also, the term “couple” or “couples” is intended to mean either an indirect or direct connection. Thus, if a first device couples to a second device, that connection may be through a direct connection, or through an indirect connection via other devices, components, and connections. In addition, as used herein, the terms “axial” and “axially” generally mean along or parallel to a central axis (e.g., central axis of a body or a port), while the terms “radial” and “radially” generally mean perpendicular to the central axis. For instance, an axial distance refers to a distance measured along or parallel to the central axis, and a radial distance means a distance measured perpendicular to the central axis. 
     Referring now to  FIG. 1 , an embodiment of an offshore system  10  for drilling and/or producing a wellbore  11  is shown. In this embodiment, the system  10  includes an offshore vessel or platform  20  at the sea surface  12  and a subsea BOP stack assembly  100  mounted to a wellhead  30  at the sea floor  13 . The platform  20  is equipped with a derrick  21  that supports a hoist (not shown). A tubular drilling riser  14  extends from the platform  20  to the BOP stack assembly  100 . The riser  14  returns drilling fluid or mud to the platform  20  during drilling operations. One or more hydraulic conduit(s)  15  extend along the outside of the riser  14  from the platform  20  to the BOP stack assembly  100 . The conduit(s)  15  supply pressurized hydraulic fluid to the assembly  100 . Casing  31  extends from the wellhead  30  into the subterranean wellbore  11 . 
     Downhole operations are carried out by a tubular string  16  (e.g., drillstring, production tubing string, coiled tubing, etc.) supported by the derrick  21  and extending from the platform  20  through the riser  14 , through the BOP stack assembly  100 , and into the wellbore  11 . A downhole tool  17  is connected to the lower end of the tubular string  16 . In general, the downhole tool  17  may comprise any suitable downhole tool(s) for drilling, completing, evaluating, and/or producing the wellbore  11  including, without limitation, drill bits, packers, cementing tools, casing or tubing running tools, testing equipment, perforating guns, and the like. During downhole operations, the string  16 , and hence the tool  17  coupled thereto, may move axially, radially, and/or rotationally relative to the riser  14  and the BOP stack assembly  100 . 
     Referring now to  FIGS. 1-3 , the BOP stack assembly  100  is mounted to the wellhead  30  and is designed and configured to control and seal the wellbore  11 , thereby containing the hydrocarbon fluids (liquids and gases) therein. In this embodiment, the BOP stack assembly  100  comprises a lower marine riser package (LMRP)  110  and a BOP or BOP stack  120 . 
     The BOP stack  120  is releasably secured to the wellhead  30  as well as the LMRP  110 . Likewise, the LMRP  110  is releasably secured to the BOP stack  120  and the riser  14 . In this embodiment, the connections between the wellhead  30 , the BOP stack  120 , and the LMRP  110  include hydraulically actuated, mechanical wellhead-type connections  50 . In general, the connections  50  may comprise any suitable releasable wellhead-type mechanical connection such as the DWHC or HC profile subsea wellhead system available from Cameron International Corporation of Houston, Tex., or any other such wellhead profile available from several subsea wellhead manufacturers. Typically, such hydraulically actuated, mechanical wellhead-type connections (e.g., the connections  50 ) include an upward-facing male connector, or “hub,” that is received by and releasably engages a downward-facing mating female connector or receptacle  50   b . In this embodiment, the connection between LMRP  110  and the riser  14  is a flange connection that is remotely controlled, just as the connections  50  may be remotely, hydraulically controlled. 
     Referring to  FIGS. 1-4 , the LMRP  110  includes a riser flex joint  111 , a riser adapter  112 , one or more annular BOPs  113 , and a pair of redundant control units or pods  114 . A flow bore  115  extends through the LMRP  110  from the riser  14  at the upper end of the LMRP  110  to the connection  50  at the lower end of the LMRP  110 . The riser adapter  112  extends upward from the flex joint  111  and is coupled to the lower end of the riser  14 . The flex joint  111  allows the riser adapter  112  and the riser  14  connected thereto to deflect angularly relative to the LMRP  110  while wellbore fluids flow from the wellbore  11  through the BOP stack assembly  100  into the riser  14 . The annular BOP  113  comprises an annular elastomeric sealing element that is mechanically squeezed radially inward to seal on a tubular extending through the LMRP  110  (e.g., the string  16 , casing, drillpipe, drill collar, etc.) or seal off the flow bore  115 . Thus, the annular BOP  113  has the ability to seal on a variety of pipe sizes and/or profiles. 
     In this embodiment, the BOP stack  120  comprises at least one annular BOP  113  as previously described, choke/kill valves  131 , and choke/kill lines  132 . Choke/kill line connections  130  connect the female choke/kill connectors of the LMRP  110  with the male choke/kill adapters of the BOP stack  120 , thereby placing the choke/kill connectors of the LMRP  110  in fluid communication with the choke lines  132  of the BOP stack  120 . A main bore  125  extends through the BOP stack  120 . In addition, the BOP stack  120  includes a plurality of axially stacked ram BOPs  121 . Each ram BOP  121  includes a pair of opposed rams and a pair of actuators  126  that actuate and drive the matching rams. In this embodiment, the BOP stack  120  includes four ram BOPs  121 —an upper ram BOP  121  including opposed blind shear rams or blades for severing the tubular string  16  and sealing off the wellbore  11  from the riser  14  and the three lower ram BOPs  121  including the opposed pipe rams for engaging the string  16  and sealing the annulus around the tubular string  16 . In other embodiments, the BOP stack (e.g., the stack  120 ) may include a different number of rams, different types of rams, one or more annular BOPs, or combinations thereof. The control pods  114  include subsea electronics modules (SEMs) and operate the valves  131 , the ram BOPs, and the annular BOPs  113  of the LMRP  110  and the BOP stack  120 . 
     As best shown in  FIG. 3 , the BOP stack  120  also includes at least one set or bank  127  of hydraulic accumulators  127   a  mounted on the BOP stack  120 . While the primary hydraulic pressure supply is provided by the hydraulic conduits  15  extending along the riser  14 , the accumulator bank(s)  127  may also be used to support operation of the rams, the choke/kill valves  131 , the connector  50   b  of the BOP stack  120 , and the choke/kill connectors  130  of the BOP stack  120 . As will be explained in more detail below, the accumulator bank(s)  127  serves as a backup means to provide hydraulic power to operate the rams, the valves  131 , the connector  50   b , and the connectors  130  of the BOP stack  120 . 
     Although the control pods  114  may be used to operate the BOPs  121  and the choke/kill valves  131  of the BOP stack  120  in this embodiment, in other embodiments, the BOPs  121  and the choke/kill valves  131  may also be operated by one or more subsea remotely operated vehicles (ROVs). 
     As previously described, in this embodiment, the BOP stack  120  includes one annular BOP  113  and four sets of rams (one set of shear rams and three sets of pipe rams). However, in other embodiments, the BOP stack  120  may include different numbers of rams, different types of rams, different numbers of annular BOPs (e.g., annular BOP  113 ), or combinations thereof. Further, although the LMRP  110  is shown and described as including one annular BOP  113 , in other embodiments, the LMRP (e.g., LMRP  110 ) may include a different number of annular BOPs (e.g., two sets of annular BOPs  113 ). Further, although the BOP stack  120  may be referred to as a “stack” since it contains a plurality of ram BOPs  121  in this embodiment, in other embodiments, BOP  120  may include only one ram BOP  121 . 
     Both the LMRP  110  and the BOP stack  120  comprise re-entry and alignment systems  140  that allow the LMRP  110 -BOP stack  120  connections to be made subsea with all the auxiliary connections (i.e., control units, choke/kill lines) aligned. The choke/kill line connectors  130  interconnect the choke/kill lines  132  and the choke/kill valves  131  on the BOP stack  120  to the choke/kill lines  133  on the riser adapter  112 . Thus, in this embodiment, the choke/kill valves  131  of the BOP stack  120  are in fluid communication with the choke/kill lines  133  on the riser adapter  112  via the connectors  130 . However, the alignment systems  140  are not always necessary and need not be included. 
     As shown in  FIGS. 3 and 4 , the subsea BOP stack assembly  100  further includes a subsea electronic data system, which includes at least one sensor  210 . In this embodiment, the sensors  210  measure operating conditions of various equipment on the BOP stack  120  or other subsea well equipment such as the LMRP  110 . For example, the sensors  210  may measure the position of movable pistons within the hydraulic accumulators  127   a  that separate hydraulic fluid from gas. There may be a sensor  210  for each accumulator  127   a  or there may be a sensor  210  (or sensors) that measures the position of more than one piston. In this manner, there may be fewer sensors  210  than accumulators  127   a , with the potential for as little as one sensor  210  for the entire data system. As shown schematically in  FIG. 4 , the sensors  210  may be located at various positions on the LMRP  110  and the BOP stack  120 . In addition to measuring position, the data system may also include sensor(s)  210  for measuring temperature within the internal volume of the accumulators  127   a , for example. Sensors  210  may also be used to measure the position of the rams in the BOPs  113 ,  121 . Sensors  210  may also be used to measure other conditions, such as pressure, flow rates, temperature, BOP lock status, connection status, etc., of the equipment on the LMRP  110  or the BOP stack  120 . 
     As shown in more detail in  FIG. 4 , the electronic data system further includes at least one sensor interface box (SIB)  220  capable of receiving the measurement information from the sensor(s)  210 . In this embodiment, the data system includes more than one SIB  220  networked together. However, the SIBs  220  may also be separate from each other. The SIBs  220  collect and store and/or retransmit the information from sensors  210  on the BOP stack  120  and the LMRP  110 . If the SIBs  220  are used as local data historians for the sensors  210  to store sensor data, the SIBs  220  may include a suitable memory device, such as a flash memory hard drive for example. The SIBs  220  may also include equipment to provide power supply management and power conditioning, including secondary elements of wireless data and power (WDP) units described further below. The SIBs  220  may also include the subsea elements of a data over power system as described more fully below. The SIBs  220  include processors and electronic components capable of handling data manipulation, such as real-time reporting, reporting on demand, or reporting by exception, for example. The SIBs  220  also include electronic components for various communication protocols as described further below. Additionally, the SIBs  220  may house control cards for the sensors  210  when the control cards are not integral to the sensors  210  themselves. The SIBs  220  also include any required networking/addressing components and capability based on the design of the system. 
     Data and power communication between the SIBs  220  on the BOP stack  120  and the SIBs  220  on the LMRP  110  may be accomplished by using wireless data and power (WDP) units  224  made up of receiver plates  226  when the LMRP  110  is in proximity to the BOP stack  120 . The WDP receiver plates  226 , or wireless stingers, are located on each of the BOP stack  120  and the LMRP  110 . When located sufficiently close to each other, wireless data and power communication may be established between the BOP stack  120  and the LMRP  110 . Data may be transferred wirelessly using any suitable protocol, such as Ethernet running Modbus TCP, RS232 and RS485. Also, power may be communicated by inductive methods. 
     The data system is designed for efficiency, modularity, and multiple redundancy for accessing data. As a measure of redundancy, more than one SIB  220  may be used as shown in  FIG. 4 . Each SIB  220  is dedicated to receive information from only some of the sensors  210 . Which SIB  220  a sensor  210  communicates with may be designed specifically as a redundancy feature in case communication with one of the SIBs  220  becomes unavailable. For example, sensors measuring the ram position of the BOPs  121  may send data to one SIB  220  while sensors measuring the lock status of the same BOPs  121  may be sent to another SIB  220 . However, each of the SIBs  220  is networked together as shown by the solid and dotted line connections between the SIBs  220 . In this manner, every SIB  220  can record all of the data from all of the sensors  210  even though different sensors  210  may be connected with different SIBs  220 . Additionally, communication with all of the SIBs  220  may be established by establishing communication with only one SIB  220 . Additionally, historical data stored in one SIB  220  may be accessed by another SIB  220 . 
     The SIBs  220  may also retransmit the sensor information through any one or more transmission paths. One possible transmission path is using the riser  14  ( FIG. 3 ) to communicate information to the sea surface. In this configuration, the SIBs  220  communicate through the riser  14  to a communication center  230  on the sea surface. The communication center  230  may be connected to a human/machine interface (HMI)  231  such as a computer display and various controls as well as a data historian  237  for recording the data. 
     Communication using the riser  14  may also be established with any other suitable data connection or connections between the SIBs  220  and the communication center  230 , such as using other existing power and data communication connections. For example, as shown the LMRP  110  includes control pods  114  with subsea electronics modules (SEMs) that communicate through a wired connection with the communication center  230  using typical so-called “yellow” and “blue” umbilicals. 
     Power for the data system instrumentation and electronics may be provided by a power unit  233  on the rig  20  that communicates with the system via umbilicals connected with the LMRP  110 . Power from the power unit  233  is transferred from the LMRP  110  to the BOP stack  120  instrumentation and electronics through the WDP receiver plate  226  connections between the LMRP  110  and the BOP stack  120 . The power unit  233  may also work in conjunction with a data over power surface unit  235  to provide data communication capability with the subsea elements of the data over the power system. The data over power surface unit  235  includes processors and electronic components for encoding and decoding communications protocols as well as any required networking/addressing components and capability based on the design of the system. 
     The data received by either the communication center  230  or the data over power surface unit  235  may be recorded and stored in a suitable memory device  237 , or historian, such as a flash memory hard drive for example. The memory device  237  is the primary information repository for the control system HMI  231  and provides data to be displayed on the HMI  231 . 
     Another transmission path may be through the use of a remotely operated vehicle (ROV)  250 . For ROV access, the data system may include at least one WDP receiver plate  228  at a location on the BOP stack  120  and/or LMRP  110  accessible by the ROV. The data system may also include multiple WDP receiver plates  228 , located in multiple places on each of the LMRP  110  and the BOP stack  120  for added accessibility and redundancy. To establish communication with the sea surface, the ROV  250  equipped with a counterpart WDP plate  228  is maneuvered into proximity with the WDP plate  228  on the LMRP  110  or the BOP stack  120 . Alternatively, other communication interfaces such as ROV stabs  229  or wet-mate connectors may be provided to establish direct communication between the SIBs  220  and the ROV  250 . Using either the WDP or ROV stab connection to establish a communication link, information may be transmitted from the SIBs  220  to an ROV support vessel (ROVSV)  280  at the sea surface through the transmission line of the ROV  250 . The ROVSV  280  may include an ROV interface  282  and another human/machine interface (HMI)  284  for displaying the transmitted data as well as inputting commands. The HMI  284  may optionally be connected with a portable acoustic unit for use of a common display. It should be appreciated that the ROV communication equipment may be provided instead of or in conjunction with the riser or umbilical communication equipment described above. 
     Another possible communication path is for the SIBs  220  to communicate with a vessel of opportunity (VoO)  286  through acoustic telemetry. Optionally included on the LMRP  110  and/or the BOP stack  120  are acoustic communication and power units  260  that communicate with an acoustic transducer  288  connected with a portable surface acoustic unit  290  located on the VoO  286 . The portable surface acoustic unit  290  is an autonomous, self-supporting unit that is battery powered and capable of controlling and receiving data from the acoustic transducer  288 . The information sent or received using the acoustic communication and power units  260  may be processed in a similar manner as described above. Additionally, the acoustic communication and power units  260  may be used to provide power to the subsea equipment of the data system as and when required. It should be appreciated that the portable acoustic communication equipment may be used by itself with or without the LMRP and/or ROV communication equipment described above. 
     In conjunction with or alternative to the portable surface acoustic unit  290 , the data system may also include a fixed surface acoustic unit  295  installed on the rig  20  and connected with its own acoustic transducer  288 . The fixed surface acoustic unit  295  is powered by electrical power from the rig  20  and the acoustic transducer  288  may either be a dunking transducer or a fixed hull transceiver. The fixed surface acoustic unit  295  is capable of communicating with the subsea acoustic communication and power units  260  to both receive information from the SIBs  220  as well as send operator instructions. The fixed surface acoustic unit  295  may also interface directly with the surface HMI  231  to display received information as well as input of commands to be sent to the SIBs  220 . It should be appreciated that the fixed acoustic communication equipment may be used by itself with or without the other communication equipment described above. 
     Through one or more of these different communication paths, the SIBs  220  may receive various inputs, including data from the sensors  210  and interrogation requests from the HMIs  231 ,  290 , and  284  as well as other communications from the sea surface via umbilicals from the surface, the WDP units of the system, or through the acoustic communication and power units  260 . The SIBs  220  may also receive power and communications via the various WDP units of the system. The SIBs  220  may also receive data from other SIBs  220 , potentially as part of a communication path to the surface equipment. The SIBs  220  may also receive power via the acoustic communication and power units  260 , the control pods  114 , the ROV WDP units  228 , or from the ROV stab or wet-mate connector  229 . 
     The SIBs  220  may also produce certain outputs, including power to the sensors  210  and pass-through of command/control signals from the surface equipment (e.g., HMI  231 ) such as interrogation requests for example. The SIBs  220  may also produce processed data from the sensors  210  as live or historical values. The processed data may include reporting by exception, in which case the exception conditions may be flexible, e.g., above or below a value, in or out of a range, percentage change, and rate of change, and all of these conditions may be configurable by an operator via the surface HMIs  231 ,  290 , and  284 . The SIBs  220  may also output all processed sensor data going to the surface from the data system. 
     The LMRP SIBs  220  also receive various inputs, including interrogation requests from the HMIs  231 ,  290 , and  284  from the sea surface and data from the sensors  210  on the LMRP  110 . The SIBs  220  may also include power via the data over power unit  233 , ROV WDP units  228 , ROV stab or wet-mate connector  229 , or via the acoustic communication and power units  260 . The SIBs  220  may also produce certain outputs, including processed data from the sensors  210  on the LMRP  110 . The processed data may include reporting by exception, in which case the exception conditions may be flexible, e.g., above or below a value, in or out of a range, percentage change, and rate of change, and all of these conditions may be configurable by an operator via the surface HMIs  231 ,  290 , and  284 . 
     As an example of the data gathering functionality of the data system, a sensor  210  may be located to measure the position of the piston of the ram of a BOP  121 . The sensor  210  sends a signal to a control board mounted in an SIB  220  located on the BOP stack  120 . The SIB  220  also provides power to the sensor  210  as well as handles electrical power management for other sensors  210  on the BOP stack  120 . The SIB  220  includes a local data historian so that in the event that the communications that pass through the LMRP  110  are unavailable, the sensor data will be available either through the acoustic unit(s)  260  connected to the SIB  220  or via the ROV connection(s)  228 ,  229  to the SIB  220 . 
     Assuming that the LMRP  110  is in position and connected to the BOP stack  120 , the ram position sensor data passes from the SIB  220  on the BOP stack  120  through a WDP unit  224  to a SIB  220  on the LMRP  110 . Each SIB  220  on the LMRP  110  also includes a local data historian so that in the event that the communications that pass through the main umbilicals are unavailable, the sensor data will be available via either the acoustic unit(s)  260  connected to the SIB  220  on the LMRP  110  or via the ROV connection(s)  228 ,  229  to the SIB  220  on the LMRP  110 . 
     The SIB  220  on the LMRP  110  includes the subsea elements of the data over power sub-system and the sensor data are then communicated to the lines that connect the LMRP&#39;s control pods  114 . From the control pods  114 , the data passes up the power conductors in the blue and yellow umbilicals to the surface. The surface data over power unit extracts and processes the sensor data from the power line and, using network protocols, makes the information available to the surface data historian  237 . The ram position sensor data are then accessed by the data historian  237  and displayed on the HMI  231 . 
     Other sensor data are handled in a similar manner, depending on whether the sensor is on the LMRP  110  or the BOP stack  120 . Regardless of where the sensor is mounted, however, there are multiple routes available to retrieve the data using at least one of four different technologies: (1) direct connection; (2) acoustic; (3) wireless ROV coupling; and (4) wet-mate ROV coupling. 
     As shown in  FIG. 4 , the subsea electronic data system may also include one or more retrievable data capsules (RDCs)  302 , which operate as so-called “black-box” data recorders to log all LMRP  110  and BOP stack  120  data. By taking advantage of the communications network and by utilizing available interfaces on the SEMs (within the control pods  114 ) the data captured and held subsea can be expanded from just the sensor data to include all the communications passing up and down the main control umbilicals discussed above. This information typically includes, but is not limited to: single valve, regulate, and multiple valve commands received (from the operator on the surface), commands acknowledged (the SEM confirming receipt of a given single valve, regulate, or multiple valve command), valve states (open or closed?) and analogue values such as pressures and temperatures. By placing one or more RDCs  302  subsea, the data system offers unprecedented levels of redundancy and potential access to critical data due to the networked nature of the system providing the opportunity to add the RDCs  302 . 
     Preferably, two RDCs are used: one RDC  302  on the LRMP  110  and one RDC  302  on the BOP stack  120 . The RDCs  302  may be mounted on the LMRP  110  and the BOP stack  120  in locations that are considered to be readily ROV accessible and connected to the data network using, preferably, the same WDP units  224  as previously discussed. The WDP units  224  establish communication between the RDC  302  and a network junction unit  300  for establishing communication with the data system network. In the case of the RDC  302 , however, the whole unit will be detachable for retrieval to the surface for data download and analysis. To this end, the RDCs  302  may be packaged in ROV retrievable housings to allow recovery from subsea to surface for data download and analysis. 
     Given that the RDCs  302  are regarded as a black box data recorder, reliability and robustness are important. Preferably, the RDC  302  internal components and functions will be of an equivalent standard to the data storage used in aircraft flight data recorders. The RDC  302  units may include control electronics for the RDC&#39;s WDP unit internal to the housing. The RDCs  302  may also include a combined data storage, processing, and networking unit internal to the housing. However, these functions may be split out in separate units as well. Additionally, a dry-mate external connector may be included for downloading data from the RDC  302  after retrieval from subsea. 
     The RDCs  302  may also include interfaces to the data system Ethernet network for access to the SIB  220  and SEM data. The RDCs may also include interfaces to data download and analysis equipment (on a surface vessel or elsewhere). The RDCs  302  may also accept all data logged by SIBs  220 , i.e., sensors connected directly to the network. The RDCs  302  may also accept all communications traffic passing through main control umbilicals, i.e., commands sent from the surface, plus all information sent topside from subsea, via SEMs. Power for the RDCs  302  may be provided from the data network, i.e., from the umbilicals, ROV connections, or acoustic units as described above. 
     Normally, while in-situ on LMRP  110  or BOP stack  120 , the RDCs  302  will not have any outputs. When recovered to the surface for interrogation, the RDCs  302  may output all available logged data. 
     The ability to log all communications traffic through the main umbilicals requires data storage. As an example only, the system may require the capacity to store more than 8 GB of data. The data may be stored on any suitable data storage means, such as a flash drive. One means of maximizing data logging without an undue overhead of large storage is by using the reporting on exception strategies. Although intended primarily for use in the event that the only communication channel available is acoustic and, therefore, low bandwidth, reporting on exception strategies would also allow much larger volumes of information to be stored in a given memory capacity than when all data are recorded. Preferably, the RDCs will support data logging of multiple preceding days of operation. Once the data recording capacity is full, the RDCs  302  may follow a rolling logging/deleting process such that the latest data are always recorded and the oldest data deleted. 
     In recording data, the RDCs  302  may also log the data entries with time stamps in a manner that accurately correlates with all other entries. In this manner, the timestamp for an event such as an operator pressing a command button is synchronized with the timestamp of the log entries of the receipt and acknowledgement of that action subsea. 
     To ensure accurate and consistent time synchronisation between all the RDCs  302 , a network time protocol (NTP) server and associated techniques may be used. The timing derived from the NTP server will ensure that the timestamp applied to all data logged in the system&#39;s historians (including the RDCs  302 ) is done in a consistent, accurate, and controlled manner. Where a reference signal, such as a GPS input, is available on the rig  20 , the NTP servers may also be equipped to connect to this signal. 
     Although the present invention has been described with respect to specific details, it is not intended that such details should be regarded as limitations on the scope of the invention, except to the extent that they are included in the accompanying claims.