OPC UA, through the MDIS Companion Specification, has been specified as a controls interface between the MCS and DCS. This normative guideline provides details on other potential applications of the MDIS standard and additions to standard MDIS implementations that are outside the original scope of MDIS. Concepts covered include:
- Application of OPC UA and standard objects and concepts defined within the MDIS Companion Specification as “read only” interfaces for historian applications.
- Additions to OPC UA MDIS implementations that can facilitate data transfer that are not specifically control related.
The MCS can provide “read only” interfaces to facilitate data to external databases, as specified on a project basis. In these cases, OPC UA can be specified and use of standard objects and concepts taken from the MDIS standard can be utilized based on project specific requirements.
To use the OPC UA interface as a “read only” interface it is recommended to that a Server be able to indicate that all writeable points (by definition have access restriction that denies write for all users and that no writeable ObjectType instance will exist in the address space. For all methods that allow writing, the user access rights should indicate not executable. The Server should be configurable to support this “read only” configuration. The server should not be able to be changed from this configuration without a restart.
Valve signatures are files that contain high sample rate pressure profiles recorded during individual valve operations subsea exposed by the subsea system. There are cases where valve signatures are required to be transferred from the SPCS to either the DCS or external historian interfaces.
A Server that supports transferring the valve signature via OPC UA shall expose a FileObject as part of the Valve Object Instance via the HasSignature ReferenceType. The valve Signature will also be provided in the MDIS folder under the MDISInformation Object. File formats and specific information captured within the valve signature files are outside the scope of this specification and should be specified on a project basis.
The items required are provided as part the individual object types. This functionality is optional and covered in a separate profile.