The use of distributive control and processing elements for industrial automation applications has become widespread during the past decade This has been an evolutionary process driven mainly by the need to effect control and process data locally over increasingly large application sites, and the proliferation of smart electronic systems and communication technology Although the trend continues, especially through the introduction of advanced serial communications protocols implemented physically as serial bus systems, the application of the technology to subsea production control systems has yet to become fully established In the process industry, including offshore oil and gas, the availability of smart sensors able to exist as elements of a distributed topography has resulted in extensive uptake, and focuses attention on extending these systems and technologies to include the subsea components
Typically, a present day subsea control system comprises a subsea control module (SCM) which provides hydraulic control to a number of remote valves and accepts measurements from a number of remote transducers The remote transducers typically interface to a subsea electronic module (SEM) located within the SCM via individual 4–20 mA links Each transducer requires a dedicated jumper and connector to the subsea module
An alternative concept is to replace the discrete wiring of the transducer links with a distributed send bus which connects a large number of uniquely addressed remote control or sensor elements via a pair of conductors Such a configuration is referred to as a "distributed" system
This paper reviews the design of a distribution bus to a level sufficient to define the system hardware from which a quantitative analysis of the reliability can be made This is compared with the present topography and hardware used in subsea systems
In a common subsea configuration, a cluster of Xmas trees may be located around a central manifold Xmas trees can also be located in distant satellite clusters Inter-tree separation is typically 10 meters with satellite Xmas trees up to 1000 meters from the central manifold Figure 1 shows a typical subsea arrangement In the model of the present subsea system configuration, one SCM controls and monitors the valves and transducers associated with one Xmas tree as shown in Figure 1
An alternative arrangement to the present configuration is to replace the individual wiring of the transducer links with a distributed bus connecting all units with a single cable Such a system contains 3 main items
Master Terminal, controls bus traffic and the remote terminals connected to the distribution bus
Distribution Bus, connects all the remote terminals to the master terminal. It can comprise a go and return line or ring and/or dual redundant wire distribution system or a combination
Remote Terminal, comprises the circuit which interfaces a remote device (transducer or actuator) to the distribution bus
In general terms this bus interface is digital and the remote devices are analogue Functionally the terminal comprises interface circuits, A/D converters, control and multiplexing circuit. Contemporary devices can manage 256 remote devices making it possible to replace a SEM but for the purposes of this discussion it is assumed that the terminal serves one device.
