The following sequence diagrams indicate the intended SPCS and DCS interface operational steps. The sequence diagrams are used only to visualise different choke operations and to provide helpful information for implementation of the MDISChokeObjectType in OPC UA.

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Figure 28 - Choke Move to Position

Sequence description; the above sequence details a successful execution of a Move to Position command [open or close] from the DCS to SPCS in addition to intermediate acknowledgements and states.

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Figure 29 - Choke Move Fault

Sequence description; the above sequence details a Fault during the execution of a Move to Position command [open or close] from the DCS to SPCS in addition to intermediate states.

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Figure 30 - Choke Move Interlocked

Sequence description; the above sequence details a failed execution of a Move to Position command [open or close] from the DCS to SPCS due to an Interlock being active.

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Figure 31 - Choke Move Abort

Sequence description; the above sequence details abort of a Move to Position command [open or close] from the DCS to SPCS.

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Figure 32 - Choke Move Interlock Override

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Figure 33 - Choke Fault

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Figure 34 - Choke Step Success

Sequence description; the above sequence details a successful execution of a Step Open / Close command from the DCS to SPCS in addition to intermediate acknowledgements and states. The sequence diagram also includes information from instruments such as the Linear Variable Displacement (Differential) Transmitter (LVDT) to help illustrate what the actual information flow is.

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Figure 35 - Choke Step Fault

Sequence description; the above sequence details a failed execution of a Step Open / Close command from the DCS to SPCS due to a choke fault.

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Figure 36 - Choke Step Abort

Sequence description; the above sequence details a successful execution of a Choke Abort command from the DCS to SPCS in addition to intermediate acknowledgements and states.

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Figure 37 - Choke Set Position

Sequence description; the above sequence details a successful execution of a Set Calculated Position command from the DCS to SPCS.

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Figure 38 - Choke Enable / Disable

Sequence description; the above sequence details a successful execution of an Enable / Disable Choke from the DCS to SPCS in addition to intermediate acknowledgements and states.

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Figure 39 - Choke Step Interlock Override

Sequence description; the above sequence details a successful execution of a Defeat / Override Interlock Choke command from the DCS to SPCS in addition to intermediate acknowledgements and states