The functional elements of the system either reside in the DCS or SPCS depending on a particular vendor’s solution or customer’s requirements. The “Operating Company” should specify where each of these optional functions should reside. See Figure 8 for an illustration.

Data Arbitration is the system function that manages the reception and transmission of dual / redundant SPCS data.

If the Subsea system performs this function, only a single process value or operator command is typically passed between the SPCS and DCS system. If the DCS performs this function, both the A and B data values would typically be passed across the interface.

There are multiple types of data that could require arbitration. Instruments can be redundant; SEMs can be redundant and it is possible that the different types of data maybe arbitrated in different locations. I.e., in some projects, sensor data may be arbitrated by the DCS while the SEM may be arbitrated by the SPCS. Data Arbitration choices can also affect redundancy.

Certain subsea instruments may only be powered by one SEM at a time, selectable by the operator. Also, a SEM may have various modes, such as ROV mode or maintenance mode, which can be selected.

An Interlock is a control permissive that exists to prevent or warn an operator against potentially undesired operator commands being issued to the subsea system. Depending on an operator’s access level, he / she may be able to override the interlock in order to perform the desired operation. Interlocks can be categorised into two types: process interlocks and product / system interlocks, though not all customers or SCV’s make this distinction.

Process interlocks are interlocks which are specific to a particular project dependent on field layout, tree functionality, etc. These are often defined by the customer’s process requirements or by regulatory agencies; e.g., prevention of opening the tree crossover valve if the production master valve and annulus master valve are open.

Interlocks defined by the SCV for the protection of the subsea system; for example, low hydraulic pressure inhibiting opening (pressurising) of a tree valve. These interlocks are typically not able to be overridden by an operator.

These are defined subsea valve operation sequences that take the subsea system to a safe state. They are initiated either by subsea process conditions, operator intervention or emergency conditions triggered from external interfaces such as the facility Safety Instrumented System (SIS).

These are multi-step control sequences triggered by the issuance of a single operator command, such as smart well (interval control valve) controls, hydrate prevention or preparation of a tree for start-up.

This refers to determination of the status of a subsea valve by evaluating some or all of the following: hydraulic output function line pressure, hydraulic flow and last command received.

This refers to the calculation of the assumed choke position based upon the number of step commands issued to the subsea choke. It may be maintained in percentage open or step position and is compared to the position transducer on the choke for calibration.

The HPU interface may include HPU control capability, data monitoring and configuration such as Motor control setpoint changes.

The interface to the EPU may include monitoring of the power supply to the subsea equipment including input voltage / current, umbilical voltage(s) / current(s), line insulation monitoring data and power alarm statuses (over-voltage and over-current).

A valve profile, or signature, is a representation of the performance of a subsea valve in terms of its hydraulic fluid pressure and flow characteristics as measured at the subsea control module. Valve Profile / Signature Validation is a software function that compares a current valve profile/signature to a baseline or template signature recorded previously, typically at subsea system commissioning. Not all systems have this functionality.

The chemical injection interface may include control and monitoring capability. Typically, the interface includes verification to the subsea system of chemical delivery (flow rate and / or pressure) from the topsides chemical injection system.