This document makes use of a number of terms and concepts that are described in this section. OPC defined terms or terms defined in this document are in italics and camel case.

DCS (Distributed Control System) is the production facility control system provides a centralised control system for the facility for the operators and is used to monitor and control the subsea production system. The DCS system will normally host the OPC UA Client.

MCS (Master Control Station) is the central control node containing application software required to control and monitor the subsea production system.

Subsea Gateway provides a communications interface on the surface to the subsea control equipment over the subsea vendor’s communication system. The Subsea Gateway may form part of the overall MCS. The MCS or Subsea Gateway will normally host the OPC UA Server.

SCV (Subsea Controls Vendor) equipment refers to the subsea vendor supplied equipment and principally includes the subsea gateway in “Integrated” architecture or the subsea gateway and MCS in “Interfaced” architecture (see section 5.1 for additional details). It is intended to classify the functionality that is delivered by the subsea vendor whether it is implemented in the MCS or subsea gateway.

Note: The MCS may be supplied by a vendor other than the subsea or DCS vendor.

HPU (Hydraulic Power Unit) is the unit which provides low pressure and high-pressure hydraulic supplies for the control of subsea wells.

EPU (Electrical Power Unit) is the unit which provides power to the subsea system.

Interlocks (Permissive Logic) is permissive logic that needs to be satisfied before the action can initiate

The Customer is the end user, typically the “Operating Company”.

The Operator is the human being executing an operation, not to be confused with the “Operating Company”.

MDIS has standardised on the following definitions for Mandatory and Optional items. In all cases (Mandatory or Optional), if the item is available, the functionality described by the definition of the item must be correct and verifiable.


Objects specified as Mandatory will be required in all Objects and cannot be deleted. In OPC terms, a Mandatory item must exist on every Node of the NodeClass, for example if an MDISValveObjectType defines a Mandatory item Position then every instance of the MDISValveObjectType must have an item Position available.


Objects have functionality that may or may not be included; if they are included the OPC Client will know how to handle them. In OPC terms, an Optional item may or may not exist on every Node of the NodeClass, for example if an MDISValveObjectType defines an Optional item OpenTimeDuration then some instances of the MDISValveObjectType may contain an item OpenTimeDuration, but other instances may not. Clients are required to be able to handle the case where the item does not exist.

In regard to OPC Testing for Compliance and Certification the following concepts apply. The actual requirements for certification are defined on a project basis, but the standard third-party certification provided by the OPC Foundation provides the following:

Compliance - Assurance that the OPC UA Server or Client fulfils all functionality that it claims to support in terms of Profiles and that of all exposed interfaces function as defined in the specifications.

Interoperability - Testing of products against other products, this includes all functionality, data types and access rights.

Robustness - The testing of failure cases including handling of lost communication, communication recovery. All problems must not affect other connections, quality information must be correctly reported and audit entries are generated as needed. In short, end users are aware of any problem and problems are resolved automatically where possible.

Efficiency - The testing of products under load, forcing of noisy / bad communications and ensuring that products continue to work. Measuring CPU, RAM, threads, handles etc. and ensuring that even under the poor communications, heavily loaded Servers and Clients continue to function and not leak resources.

Usability - Verify that products are delivered with some level of documentation and that the documentation that is provided is accurate and understandable. Verify that the product functions as advertised and an end user would understand what is being provided.

Certification - Validation of Server or Client products. Certification includes compliance, interoperability, robustness, efficiency and usability testing and results in a seal of approval from an OPC Foundation test lab upon meeting or exceeding defined acceptance criteria.