MUX 101: Surface-Subsea Communications | Lloyd's Register ...

The previous discussion had stated that the next entry in this series would cover the surface control system. Before delving into the surface controls, an understanding of how the surface and subsea systems communicate with each other should be understood.

The Umbilical Link

The link that connects the surface controls with the subsea electronics is the MUX umbilical cable. Along with the necessary power being supplied from the surface to the subsea electronics, the MUX cable contains copper conductors that are used to provide a communications link.

The GE/Hydril systems (for the most part) utilize a form of serial Modbus protocol in order to communicate between the surface and subsea components. There are a few GE/Hydril systems and systems from other original equipment manufacturers (OEMs) that also utilize a fiber link between the surface and subsea components. There is also a system that was being developed that utilizes an acoustic link as the primary means of communicating between the surface and subsea components.

Each means of communicating between surface and subsea components has its advantages and detractions.

• The acoustic option has a major advantage of not requiring a physical connection between surface and subsea but suffers in speed of communications. Typical acoustic link speeds are 4800/9600 baud. This limits the amount of data that can be sent back and forth.
• The copper option has been the mainstay in this industry. It has proven reliable, and had adequate bandwidth capabilities. But, as operators and contractors begin requiring more data to be sent back and forth, the fiber solution is becoming the option of choice.
• The fiber option has large bandwidth capabilities that allow for extra data such as video to be shared between surface and subsea. Fiber typically has had issues with cable failures, however this is improving as operators and the industry as a whole become more aware of its additional maintenance and operating requirements.

For this discussion, as there are more copper systems in use, we will concentrate on the serial communications solution and in particular the GE/Hydril version.

SEMs, Pods and Control

The subsea components are the blue and yellow pods with each pod containing redundant subsea electronic modules (SEMs): SEM A & SEM B. Each pod (blue/yellow) is connected to the surface via separate MUX umbilical cables with each cable providing communication conductors for both SEM A & B. Each SEM provides independent command and control capabilities for the subsea blowout preventer (BOP) stack. Thus as long as one SEM on one pod is functioning and there is sufficient hydraulic supply, the surface has control of the subsea BOP stack.

The SEM by itself does not perform any control functions on its own other than to monitor subsea sensors such as pressure transducers, solenoid currents, angle inclinometers, etc. The SEM will not operate any functions (energize solenoids) unless commanded to do so from the surface. The SEM will also not send any sensor data to the surface unless commanded to do so from the surface.

Master and Standby Explained

The surface control system is in charge and will constantly poll the subsea SEM for sensor data as well as constantly send the commanded state (on/off) of all solenoids. Since there are two SEMs per pod and both SEMs should not be energizing and de-energizing solenoids concurrently, the surface will tell each SEM whether it is in a Master role or a Standby role. Each pod will have a Master and a Standby SEM.

Both SEMs within a pod will be sent the commanded state (on/off) of all solenoids, however only the Master SEM will actually energize/de-energize the solenoids. The Standby SEM will save the solenoid commanded states so when it becomes Master it can immediately place all the solenoids in the correct on/off state. Since the Master/Standby role is sent from the surface, if the Master were to fail the Standby SEM will remain in Standby role until commanded to change roles from the surface. The surface system might have logic in place to detect a Master SEM failure and immediately command the Standby SEM to become Master, however the SEM will not change roles on its own.

Hopefully by now you can see that the SEM does nothing unless told to do so from the surface.

Poll Cycles

Once it establishes communications with a SEM, the surface control system will constantly be communicating with that SEM. The surface goes into a poll cycle where each poll cycle consists of sending commands and retrieving data. The poll cycle will be different depending on whether the SEM is in Master or Standby role. Once a poll cycle is completed the next poll cycle is immediately started. Thus constant communications are occurring between the surface and a SEM.

The Master SEM poll cycle consists of sending command states and master/standby role and retrieving digital data (solenoid states: armed/fired/over current), analog data, accumulator data, and error data. The analog data consists of high priority analogs (pressure transducers) and low priority analogs (solenoid currents, voltages, electronic riser angle data, temperature/water monitor data, ground fault data, etc.).

The high priority analogs are retrieved every poll cycle and the low priority analogs are retrieved every 10 poll cycles. The separation of high and low priority analog data is to reduce the demand placed on the bandwidth. Even though the bandwidth is better than the acoustic solution, it is limited and much lower than that achieved with fiber. The accumulator data is the flow meter counts.

The Standby SEM poll cycle consists of sending command states and master/standby role and retrieving error data.

The data packets that are transmitted back and forth between the surface and SEM are encoded typically with some sort of cyclic redundancy check (CRC) error-detecting code to prevent erroneous commands being sent to the SEM and corrupted data being retrieved from the SEM.

Another safety measure is provided to prevent erroneous commands from being sent to the SEM. Each function that is operated from the surface requires first an Arm (prepare to energize solenoid) command to be sent to the SEM and Arm state confirmed back from the SEM before the actual Fire (energize solenoid) command is sent.

Now that you know what is subsea and how the surface and subsea systems communicate, in the next discussion we should be ready to begin looking at the surface MUX BOP control system.

Source: http://blog.lrenergy.org/mux-101-surface-subsea-communications/

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