In the oil and gas industry, wellhead applications span many areas from closed-loop control and monitoring, to safety shutdown functions. Due to size and budget factors, designers often try to combine all three functions in one box. While this approach might save on cost, it carries an inherent risk as hardware and software are shared across applications. The conundrum is that a failure in the control function will likely also affect the safety function, therefore affecting safety functionality.
Operations at a well site can be described as follows:
• Control function: the control function is responsible for maintaining the well's target production by regulating the choke valve. Other auxiliary control functions such as chemical injection and hydraulic pump control are also used to maintain the production within the well.
• Monitoring function: some process variables monitored may require that an action be taken to keep the well running within desired limits, thus preventing the well shutdown and a halt in production.
• Safety: these are the critical safety functions normally carried out by safety systems designed to the IEC 61508:2010 international safety standards and providing risk reduction capability up to SIL3.
Keeping the safety and non-safety functions separate while achieving seamless integration
In Emerson's DeltaV SIS with Electronic Marshalling Platform, the safety function is allocated into the safety hardware. This hardware is designed to the IEC 61508 standard, achieves the targeted SIL performance (usually SIL3), and is approved by third-party certification bodies TUV and Exida. This layer is physically and logically independent from the non-safety functions. With DeltaV SIS platform the I/O related to the Safety Instrumented Functions (SIFs) with targeted SIL (SIL1 -SIL3) can be wired to the SIL3 CHARMs Smart Logic Solver (CSLS) where the safety functions are carried out separately from the non-safety functions.
The non-safety functions, on the other hand, can still be handled by the DeltaV SIS, but in a separate yet integrated manner. The non-safety I/O can be wired to the non-safety hardware and executed in a separate controller. These non-safety controllers and I/O are seamlessly integrated within DeltaV SIS with zero data mapping or integration configuration requirements.
With this modern Safety Instrumented Systems (SIS) approach, seamless integration can be achieved without compromising the segregation between functions required to meet international safety standards IEC 61508 and IEC 61511.
For more information contact Mohamed Salman, Emerson Automation Solutions, mohamed.salman@emerson.com or emrsouthafrica@emerson.com, www.emerson.com
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