It's the control system engineer's objective to design a control system that fulfills the requirements of the process functionality, as well as those imposed by the overall operating and control philosophy and human resources levels. Lacking proper documentation of overall control and monitoring functions, an operator's evaluation of the plant's operational efficiency is a difficult task. Often, operational problems within different systems cannot be identified until the system is in operation, which, in a worst-case scenario, can lead to major modifications at a late stage of a project.

The distributed control system (DCS) supplier's logic and arithmetic functions available for implementing the required control system functionality are typically accurate, but they are also proprietary. In-depth system knowledge is required to understand both the available functions of the DCS as well as their interconnections. However, no intuitive link exists between the control system and the process flow itself.

Due to this missing link between functions implemented in the DCS and the P&ID's definition of process flow, the process engineer's ability to verify that all process aspects have been properly addressed in the control system implementation is limited.

Using the system control diagram (SCD) approach—a supplier- and project-independent structured methodology—can eliminate this missing link.

Based on the core of the P&ID—the process schematic—the SCD concept, in effect, removes all information not required for the design of the control system. The SCD focuses on representing system and functional relationships, and documents the process control functions on a high-level basis. However, the concept also contains definition of functions and method of work. In many cases, the SCD approach can incorporate all information given by functional specification documents commonly used for specifying advanced control functions.

A level transmitter is connected to an MA template (LT040239) located in controller C18. Event high (BXH) starts the pump template SBE (PA0001A39) by the connection to start priority 1 (XP1H). Event low (BXL) stops the pump (XP1L). Control signal to the electrical motor control center (MCC) is start (YH) and stop (YL). Besides normal control, there is a safety level measurement via an MA template (LST040139) located in controller P21 where a low-low alarm trip (ALL) is connected to a single binary control block (SB). Output signal is connected directly to the MCC but also split (S) down to lock the SBE template into manual mode with low signal to the motor (LSL).

Control functions in an SCD are represented by a limited number of high-level function templates. By interconnecting these templates, complex control and interlocking strategies can be achieved. Additional elementary logic and arithmetic functions may also be used. The standard has also introduced a set of definitions, states, and transition rules within the function block frame.

As an example of an SCD standard, consider the NORSOK SCD standard, which specifies a set of supplier-independent templates. In this standard, 85% of process control logic is developed based on the five main blocks. These five blocks are: monitoring of analog signal (MA); monitoring of binary signal (MB); control of electrical equipment (motors); control of pneumatic/hydraulic equipment, such as valves (SBV); and modulating control, such as PID (CA).

The SCD approach can also be based on any high-level templates, even proprietary DCS supplier templates. Higher levels in template functionality result in more readable and understandable control structures laid out on the SCD.

Using the NORSOK SCD approach allows reuse of functional monitoring and control solutions from one plant development project to the other, even if different engineering contractors or control systems are used to implement the functions.

This SCD methodology has been used in most of the larger Norwegian offshore oil/gas platform developments for the last 10 years. It has become a de facto standard used by the operators, engineering contractors, and DCS suppliers. All major system suppliers represented on Norwegian platforms, such as Honeywell, ABB, and Siemens have developed function blocks within their official function block library in accordance with the SCD templates defined by the NORSOK SCD standard. The standard is free and can be accessed at www.standard.no/imaker.exe?id=390.

Idar Ingebrigtsen is an oil and gas industry consultant serving as SCD committee leader; Jan Even Hjelmtvedt is senior project engineer with Honeywell Industry Solutions.

For related reading from the Control Engineering Resource Center, look at "How to read P&IDs" http://resource.controleng.com/article/CA401414