Flexible solutions

Safety Instrumented Systems perform a critical role in providing safer, more reliable process operations and are widely adopted by oil and gas companies both onshore and offshore. A SIS can be used for emergency shutdown applications to prevent hazardous situations from occurring, and can initiate the shutdown of a plant, unit, or piece of equipment should it be required. A SIS can also be used for fire and gas systems (FGS) to mitigate the effects of fires or chemical releases when they occur. Sensors are used to detect abnormal operating conditions and send a signal to a logic solver. The logic solver is configured to make decisions based on the inputs and then give an output that will bring the plant to a safe state, by closing a valve for instance.

The latest safety instrumented systems help to reliably protect assets and improve plant performance. Bulky logic solvers and multiplexers are replaced by state-of-the-art logic solvers that support digital communications with smart instruments. Increased safety integrity is provided by continuously monitoring the ability of sensors, logic solvers, and final control elements to perform on demand, with faults diagnosed before they cause spurious trips. In other words, the system applies predictive intelligence to increase process availability and reduce lifecycle costs. Logic solvers that have integrated field instrument diagnostics can actively monitor safety loop performance.

The IEC61511 standard insists upon physical separation and independence of control and safety functions to eliminate failures that might affect both layers of protection. An integrated SIS platform can meet this requirement whilst still answering end-users' desire for an integrated configuration, maintenance, and operations environment for easy access to all safety-related information. This visibility into the process enables plant personnel to respond quickly and make informed decisions to avoid unplanned shutdowns.

The latest process safety systems, such as Emerson Process Management's DeltaV SIS, have modular architectures that move away from a centralised logic controller, to instead offer a flexible safety instrumented function (SIF) based approach. A SIF-based SIS architecture means that certain functions can be separated within the safety system. Each logic solver is a container for a small number of SIFs and there is no unplanned interaction between them. This approach isolates SIFs, eliminates single points of failure, and simplifies change management. It also allows incremental expansion and simplifies system maintenance by allowing units to be isolated.

For offshore applications, there is commonly a very strong integration of the emergency shutdown (ESD) system and FGS. In most cases, a confirmed FGS trip will automatically result in a trip of the ESD system to isolate the problem from potential sources. Many companies utilise an integrated approach and interface the FGS with the ESD system to initiate plant shutdown if hazardous events occur. Class Societies, such as Det Norske Veritas (DNV), recommend separate FGS and ESD systems to maintain independence between protection layers. Previously, where a single logic solver was applied it was not permitted to execute both an ESD and FGS system in the same logic controller. A modular SIF-based architecture enables both applications to be integrated into a single system due to the ability to isolate logic across completely separate logic solvers.

A modular, distributed architecture enables the process safety system to be customised for each application as it is scalable in size and allows you to add memory and processing as I/O is added; ensuring adequate processing power is available. A distributed architecture using a high-speed SIL-rated communications network also provides flexibility in the field. Modular SIS logic solvers can be placed close to the process unit being protected; this then offers fewer opportunities for maintenance errors and limits the amount of wiring required.

Biography

Alan Baird is PlantWeb Marketing Manager, Middle East & Africa at Emerson Process Management, responsible for promoting Emerson's PlantWeb digital architecture and Smart Wireless technology.