Abstract:
A method of increasing the maximum offset distance for underwater sensors, the method including the steps of: providing at least one sensor on a sensor bus, and emulating at least one additional sensor on the sensor bus. This has the effect of lowering the total network resistance, which increases the maximum possible offset distance. The additional sensor(s) by be emulated by one of: a smart plug-in module, a printed circuit board assembly, and a virtual sensor.

Description:
BACKGROUND 
       [0001]    Embodiments of the invention relate to a method for increasing the maximum offset distance for underwater sensors compliant with subsea instrumentation interface standard (SIIS) Level 2, for example CANbus Fault Tolerant, connected to subsea modules, for example subsea control modules (SCMs) or power and communication distribution modules (PCDMs). A sensor bus which implements the method is also claimed. 
         [0002]    SIIS is the Subsea Instrumentation Interface Standard being defined and in use by the offshore oil and gas Industry. It seeks to create a common platform for implementation of subsea sensors. SIIS Level 2 applies to a family of digital serial devices. The current implementation of SIIS Level 2 is a CANbus Fault Tolerant physical Layer (with CANOpen protocol). 
         [0003]    The implementation of a SIIS Level 2 sensor network is impacted by a number of parameters, such as: number of sensor nodes, baud rate, bit sampling point, cable capacitance, and termination resistance. 
         [0004]    These parameters constrain the maximum offset distance the sensor may reach away from the subsea control module (or similar subsea module). The ‘offset distance’ is the length of the cables connecting a sensor to the subsea control module. In a star network topology this is equivalent to the sum of the length of cable from the sensor to the cable split plus the length of cable from the cable split to the subsea control module. 
         [0005]    Each parameter is considered in turn below: 
         [0006]    Controlled Parameters 
         [0007]    The number of nodes is defined by the customer on a project-by-project basis. It is dependent on the subsea field layout. Each node represents a sensor in the SIIS Level 2 network. 
         [0008]    The baud rate is the communication speed. SIIS Level 2 has defined, fixed and controlled this within the SIIS Recommended Practices. It is set to 50 kbps (default) with higher speed option of 125 kbps. 
         [0009]    Uncontrolled Variables 
         [0010]    Bit sampling point is defined but prone to variations between 75% and 92.5% (87.5% nominal). The higher the sample rate, the further the maximum possible offset distance; conversely, the lower the sample rate, the shorter the maximum possible offset distance, for SIIS Level 2 sensor networks. 
         [0011]    Cable capacitance is defined but prone to variations. These variations arise from total cable length, tolerances, supplier quality and environmental effects (especially pressure). SIIS Level 2 defines this to 40 pF/m, 120 ohms at 1 MHz. The influence of pressure increases the pF/m capacitance value. For longer runs of cable, this becomes even more significant. The lower the total capacitance, the further the distance; the higher the capacitance, the shorter the distance, for SIIS Level 2 sensor networks. 
         [0012]    Controlling Variable 
         [0013]    Termination resistance impacts the total resistance in the system. The total resistance has the ability to mitigate the adverse effects of cable capacitance and bit sampling on offset distance. More termination resistances in the system should lead to lower total resistance and an increase in the maximum possible offset distance; conversely, fewer termination resistances in the system generally leads to a higher total resistance, and a shortening of the maximum possible offset distance. 
         [0014]    A termination resistance is inherent within each sensor. The more sensors, the greater the maximum possible offset distance. Once sensors are installed, the total resistance of the system becomes a fixed and known entity. The full capacity of sensors is almost never utilised in practise, as underwater sensors are expensive and so it is not cost-effective to include additional sensors over the minimum number required in the system. 
       SUMMARY OF THE INVENTION 
       [0015]    The problem solved by embodiments of the present invention is that it may be desirable to place an underwater sensor further away from a subsea control module than is practicable with the number of sensors on the bus, due to environmental factors which adversely affect integrity of the digital signal in the network. 
         [0016]    Embodiments of the present invention overcome this problem by emulating sensors, i.e. adding termination resistances into the SIIS Level 2 sensor network without the need for the expensive sensors that go with it. It uses the spare unused capacity of the sensor network to boost the maximum offset distance achievable by the actual installed sensors. 
         [0017]    Embodiments of the present invention provide a way to utilise added termination resistances to compensate for the unknown and uncontrollable elements which adversely impact maximum offset distance achievable for the sensors in the SIIS Level 2 network. 
         [0018]    According to the present invention from a first aspect, there is provided a method of increasing the maximum offset distance for underwater sensors comprising the steps of providing a sensor bus comprising at least one sensor, and emulating at least one additional sensor on the sensor bus. 
         [0019]    The at least one additional sensor could be emulated by a smart plug-in module, a printed circuit board assembly, or a virtual sensor. 
         [0020]    The step of emulating at least one additional sensor on the sensor bus could comprise emulating additional sensors up to the maximum capacity of the sensor bus. 
         [0021]    The sensor bus could be in accordance with the subsea instrumentation interface standard. 
         [0022]    The at least one additional sensor could be emulated as a sensor connected in a star topology network, or as a sensor connected in a daisy chain topology network. 
         [0023]    The sensor bus could be connected to a subsea control module. The at least one additional sensor could be emulated between the subsea control module and the sensor bus, or within the subsea control module. The subsea control module could comprise a subsea electronic module. The at least one additional sensor could be emulated within the subsea electronic module. 
         [0024]    The step of emulating at least one sensor on the sensor bus could comprise connecting at least one termination resistance to the sensor bus in parallel. At least one termination resistance could be switchably connected to the sensor bus. At least one termination resistance could be a variable resistance. 
         [0025]    The method could be performed in an underwater hydrocarbon extraction facility. 
         [0026]    According to the present invention from a second aspect, there is provided a sensor bus comprising: at least one sensor, and an emulator for emulating at least one additional sensor on the sensor bus. 
         [0027]    The at least one additional sensor could be emulated by a smart plug-in module, a printed circuit board assembly or a virtual sensor. 
         [0028]    The emulator could emulate additional sensors up to the maximum capacity of the sensor bus. 
         [0029]    The sensor bus could be in accordance with the subsea instrumentation interface standard. 
         [0030]    The at least one additional sensor could be emulated as a sensor connected in a star topology network, or as a sensor connected in a daisy chain topology network. 
         [0031]    The sensor bus could be connected to a subsea control module. The emulator could be disposed between the subsea control module and the sensor bus, or within the subsea control module. The subsea control module could comprise a subsea electronic module. The emulator could be disposed within the subsea electronic module. 
         [0032]    The emulator could comprise at least one termination resistance connected to the sensor bus in parallel. At least one termination resistance could be switchably connected to the sensor bus. At least one termination resistance could be a variable resistance. 
         [0033]    The sensor bus could be used in an underwater hydrocarbon extraction facility. 
         [0034]    The apparatus and methodology proposed by the present invention is compatible to SIIS Level 2 with CANbus physical layer implementation. The protocol choice does not impact the operation of the invention. It is independent of chosen protocol, but dependent on physical layer. 
         [0035]    A subsea field with fewer sensors (say two) can span distances typical of more sensors (say six) by using an emulator to ‘pretend’ to be four sensor slots in the network. The emulator is not restricted to set number and may fill between zero to the maximum capacity of SIIS Level 2 sensor slots, as required. 
         [0036]    The SIIS Level 2 emulator must be connected in parallel to other SIIS sensor nodes. 
         [0037]    SIIS Level 2 emulator is not constrained to a single method of packaging. The item may take the form of a printed circuit board assembly (PCBA), a module suitable for direct exposure to seawater at depth (i.e. marinised), or a module contained within the subsea control module (or a similar subsea module). 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0038]      FIG. 1  is a schematic diagram of a prior art star topology sensor bus. 
           [0039]      FIG. 2  is a schematic diagram of a prior art daisy chain topology sensor bus. 
           [0040]      FIG. 3  is a graph showing the ideal voltage profile for a communications signal in a subsea network and the actual voltage profile encountered in practice. 
           [0041]      FIG. 4  is a graph showing increased adverse effects on the actual voltage profile. 
           [0042]      FIG. 5  is a graph showing the effects of decreasing the total network resistance on the actual voltage profile. 
           [0043]      FIG. 6  is a schematic diagram of a star topology sensor bus incorporating an emulator according to an embodiment of the invention. 
           [0044]      FIG. 7  is a schematic diagram of a daisy chain topology sensor bus incorporating an emulator according to an embodiment of the invention. 
           [0045]      FIG. 8  is a schematic view of schematic diagram of a star and daisy chain combined topology sensor bus incorporating an emulator according to an embodiment of the invention. 
           [0046]      FIG. 9  is a schematic diagram of an emulator connected between a subsea control module and a network according to an embodiment of the invention. 
           [0047]      FIG. 10  is a schematic diagram of an emulator located within a subsea control module according to an embodiment of the invention. 
           [0048]      FIG. 11  is a schematic diagram of an emulator located within a subsea electronics module within a subsea control module according to an embodiment of the invention. 
           [0049]      FIGS. 12A and 12B  each show a schematic diagram of an emulator according to an embodiment of the invention. 
           [0050]      FIGS. 13A and 13B  each show a schematic diagram of an emulator according to an embodiment of the invention. 
           [0051]      FIGS. 14A and 14B  each show a schematic diagram of an emulator according to an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0052]      FIGS. 1 and 2  show two different prior art network topologies of sensor buses used in subsea networks. 
         [0053]      FIG. 1  shows a star topology, in which a subsea control module (SCM)  1  is connected to a cable split  2 . From the cable switch two sensors  3  are connected. 
         [0054]      FIG. 2  shows a daisy chain topology. The SCM  1  is connected to two cable splits  2  in series. The cable splits  2  have sensors  3  connected to them. 
         [0055]    A typical digital signal for use in such subsea networks is represented in  FIG. 3 . An idealised signal is represented by the square wave shown in solid line. The dotted line represents the actual signal encountered in practice due to environmental factors. 
         [0056]    As the cable capacitance increases—for example, due to pressure, aging or increased distance—the digital waveform is skewed even more. Variances in bit sampling exacerbate the effect. This skewing of the signal adversely impacts the signal integrity and results in information loss. These increased adverse effects are illustrated by the arrow A in  FIG. 4 . The actual signal, shown in solid line, is moved even further away from the idealised square wave towards the signal shown in dotted line. 
         [0057]    One way to recover this signal is to decrease the total resistance in the system. This can be done by either adding additional sensors in the communications network (an expensive and impractical solution) or through the emulator used in the present invention. 
         [0058]    The cumulative effect is to minimise the negative impact on the RC time constant or establish a measure of control on the RC time constant of the system, by counteracting the uncontrolled variable changes with changes to the controlled variable. 
         [0059]    The emulator used in the present invention adds termination resistances (for example, those defined by subsea instrumentation interface standard (SIIS) to the network. Since it is connected in parallel the overall system resistance decreases. This has an effect of improving the signal integrity, and prevents information loss over longer distances. 
         [0060]      FIG. 5  shows the effect of decreasing total network resistance. Arrow B shows the actual signal, shown in solid line, moving back towards the idealised square wave towards the signal shown in dotted line. 
         [0061]    Typical SIIS defined termination resistance is 1500 ohms This value is defined in the SIIS Recommended Practice. The emulator is able to mirror subsequent changes in the standard defined values. 
         [0062]    The emulator is not restricted to using the standard defined values. The emulator may break from the SIIS umbrella to develop a customer focused bespoke solution if required to meet the project needs. 
         [0063]      FIGS. 6 to 8  show different exemplary placements of the emulator in the network. 
         [0064]      FIGS. 6 and 7  show network topologies based on  FIGS. 1 and 2  respectively, each modified to include the emulator of the present invention. Like components retain their reference numerals where appropriate. 
         [0065]      FIG. 6  shows a star topology, in which a subsea control module (SCM)  1  is connected to a cable split  2 . From the cable switch two sensors  3  are connected, as well as an emulator  4 . This will be described in more detail below. 
         [0066]      FIG. 7  shows a daisy chain topology. The SCM  1  is connected to three cable splits  2  in series. The first and third cable splits  2  have sensors  3  connected to them, and the second cable split  2  has an emulator  4  according to an embodiment of the invention connected to it. 
         [0067]      FIG. 8  shows a topology consisting of star and daisy chain combined. The arrangement is similar that shown for  FIG. 2 , except that in this case the third cable split  3  has a further sensor  5  connected to it. 
         [0068]    Again, like components retain their reference numerals where appropriate. 
         [0069]    It should be noted that emulators may be placed in various locations in connection with the network. 
         [0070]      FIG. 9  shows a SCM  1  connected to a network N. The network can be any of star topology, daisy chain topology, or a combined star and daisy chain topology as shown in  FIGS. 1 to 3 . In this embodiment, the emulator is connected between the SCM  1  and the network N. 
         [0071]      FIG. 10  shows a SCM  1  connected to a network N. The SCM  1  houses a subsea electronics module (SEM)  6 . The emulator  4  is connected between the SEM  6  and the network N, and is also housed within the SCM. 
         [0072]      FIG. 11  show a SCM  1  connected to a network N. The SCM  1  houses a SEM  6 , and the emulator  4  is housed within the SEM  6 . 
         [0073]      FIGS. 12A to 14B  show various methods of implementing the detailed design, respectively using: 
         [0074]    Fixed termination resistances 
         [0075]    Switchable fixed-value termination resistances 
         [0076]    Switchable variable-value termination resistances 
         [0077]    Each of  FIGS. 12A-14B  show a possible emulator design,  FIGS. 12A ,  13 A and  14 A showing each of the termination resistances R 1  to R n  connected individually in parallel to a sensor network  8 , and  FIGS. 12B ,  13 B and  14 B showing the termination resistances R 1  to R n  connected to a single parallel connection to a sensor network  8 . 
         [0078]    The switchable elements within the emulators shown in  FIGS. 13A ,  13 B,  14 A and  14 B may be controlled remotely through a wired or wireless controller  7 . Each termination resistance, R 1  to R n , in the network forms an additional parallel connection in the total sensor network, further reducing the total resistance. In subsea applications, n will typically be less than, or equal to  9 , this number being derived from SIIS recommended practice. 
         [0079]    The emulator may optionally have a common ground for the termination resistances R 1  to R n  to allow for discharge path for the signal (not illustrated). Similarly, the device may optionally have a common line for the termination resistances R 1  to R n  connecting to a return power line of the network, in place of a common ground (not illustrated). The emulator can be considered as a permanent-set-to-recessive-device as per CANbus Physical Layer, SIIS Level 2 implementation. 
         [0080]    The emulator may be further expanded to allow for SIIS approved power supplies to form part of the system loop. 
         [0081]    The above-described embodiments are exemplary only. Various modifications will be apparent to those skilled in the art. 
         [0082]    There are other benefits which arise from embodiments of the invention, including: 
         [0083]    A safety error margin is introduced in the commissioning of new fields. For example, if three sensors are deployed near the cusp of their maximum distance limit, pressure or other effects may affect the system adversely so that this system does not work. The emulator can add a termination resistance into the system to increase the maximum distance limit, thereby allowing for the system to operate. This removes the need for retrieving equipment from subsea or changing equipment. 
         [0084]    An installed field is subject to degradation over the course of its operating life. This has typically been 25 years but is increasing to 40 years. System elements including those in the SIIS Level 2 network degrade over time affecting timing and communication parameters and may render previously operating sensor network inoperable. The emulator may be able to compensate for field degradation without the need for recovery of subsea elements and costly down-time. The method of an embodiment allows the recovery of communication over field life as the uncontrolled variables degrade over time, or the recovery of communication during installation due to variances in uncontrolled variables. 
         [0085]    In summary, the SIIS Level 2 emulator addresses three concerns facing digital serial sensor devices utilising the SIIS Level 2 standard being implemented in the subsea industry: 
         [0086]    Removes the offset distance limitation encountered when fewer than the maximum sensor nodes are installed. 
         [0087]    Provides a safety margin during commissioning for unaccounted effects of cable capacitance due to environmental pressure effects and supplier tolerances; and due to variances in bit sampling. 
         [0088]    Allows for asset integrity management by providing margin for cable degradation. 
         [0089]    This written description uses examples to disclose the invention, including the preferred embodiments, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.