From P&ID to SCD: Physical connections from various P&IDs with all logical relationships combined in one graphic.

© Aucotec

Traditionally, all devices and their physical connections are planned and mapped in the P&IDs (Piping & Instrumentation Diagrams) of the process specialists. Automation professionals use FBDs (Function Block Diagrams) to define the control and safety-relevant logic and signal paths for control system programming. But why redraw functions neutrally when the P&ID already contains so much basic data? With their graphical system overview, P&IDs are virtually the birthplace of automation, but they only show the interaction of the systems in a simplified form. An FBD, on the other hand, neutrally represents the processing of only a few signals from and to I/Os. Their relationship to the whole is not readily apparent.

Scandinavia shows the way

An SCD section: The connection from the tank to the control valve, including the safety-relevant shutdown function.

© Aucotec

German software developer Aucotec became aware of this format through customers such as Equinor, Kongsberg and Haldor Topsoe, all from Scandinavia and active in the oil and gas sector, and has incorporated it into its data-centric platform Engineering Base (EB). "Not only are SCDs particularly easy to create in EB, they also don't require the usual data transfers and reconciliations with other systems, which makes change management much easier," says Dr. Pouria Bigvand, Senior Product Manager at Aucotec. Until now, SCDs were separate documents with no reference to the overall system. EB is different.
There, each object exists only once. All core engineering disciplines access a common data model. The various specialists gradually add more and more details to objects and functions. Everyone involved can always see the current status of the digital twin of the (sub)plant. Changes in one discipline are immediately visible in all others. The respective discipline-specific views of the planning contain their own representations, but are fed from the same objects. This not only optimizes change management, but also ensures that no change is forgotten. The resulting data quality is an important pillar in the entire project lifecycle.

Benefits for all

An SCD cut-out: The connection between the level sensor and pump, including safety interlock.

© Aucotec

For these reasons, Bigvand is convinced that SCDs offer advantages to every plant manufacturer and operator. EB's data model makes it possible to create a comprehensive control logic graphic for each P&ID in an SCD in order to really clarify all system interrelationships. EB transfers the programming specifications directly from the SCD to the control system manufacturers. Subsequent P&ID changes are automatically visible in the SCD, and the specifications or changes to the control system programming are also available in EB - thanks to import via AML. These are all components that keep a digital twin up to date at all times. FBDs, on the other hand, are often no longer updated once a project has reached a certain status.

C&E tables automatically

Another advantage of EB's comprehensive knowledge of physical and logical connections is the ability to output cause & effect (C&E) tables at the touch of a button. They can be used to check the correctness of the system function definitions during the commissioning phase. They show, for example, whether a valve remains closed because it has been defined as such or because it is wired incorrectly.

The author: Leon Hanke is Solution Expert Plant at Aucotec.

© Aucotec

Without EB, the P&IDs, FBDs or SCDs and other documents from the engineering disciplines involved, and therefore often from different sources, would have to be laboriously searched through and the data entered manually for these tables - depending on the system scenario. This ties up highly qualified experts for several weeks, with no guarantee of reliability. EB, on the other hand, retrieves the data for the C&E or safety matrices from the central model and can display them in an instant. As the model is always up to date, all data is 100 percent consistent. "No other engineering system can do that," says Product Manager Bigvand.