The Safety Instrumented System Safety Lifecycle defines the necessary activities involved in the implementation of SIF occurring during a period of time that starts at the concept phase of a project and finishes when all of the SIF are no longer available for use. The standards IEC 61511 defines and explains all phases of SIS lifecycle which should be followed for reliable operation of the SIS. The SIS safety lifecycle forms the central framework which links together most of the concepts in IEC 61511.
All of lifecycle phases are mainly grouped into 3 “periods”:
1. Risk analysis
The project scope is defined, and basic engineering is completed to the point where the process risks are sufficiently clear to allow definition of a set of Safety Instrumented Functions (SIFs) along with their risk reduction requirements. This segment concludes when the preliminary version of the Safety Requirements Specification (SRS) is ready.
2. SIS design
The basic design of the Safety Instrumented System (SIS) is elaborated sufficiently for SIL verification, which confirms that the risk reduction and architectural requirements are met. Then the fully completed hardware SRS and application program SRS are developed and the detailed SIS design is completed. Procurement, construction, commissioning, validation (testing and inspection) and Functional Safety Assessment
(FSA) are then executed.
3. Operation
The plant is operated with the hazards present and the SIS in service. Testing and maintenance of the SIS are carried out. The performance of the SIS is monitored. Modifications to the SIS, including partial decommissioning, are carried out when required. This period concludes when the entire SIS is finally decommissioned.
Following are some briefly explained steps of SIS lifecycle in IEC-61511, with objectives, Inputs and Outputs of each step.
1. H&RA (Hazards and Risk Assessment)
The Hazards and Risk Assessment (H&RA) process identifies potential hazards and hazardous events associated with processes and equipment. It evaluates the sequence of events leading to hazardous incidents, assesses process risks, determines risk reduction needs, and identifies necessary safety functions for achieving the required risk reduction. This process relies on inputs such as process design, layout, manning arrangements, and safety targets, and produces outputs like descriptions of hazards, required safety functions, and associated risk reduction measures.
2. Allocation of Safety Functions to Protection Layers
Safety functions are then allocated to protection layers, with the Safety Integrity Level (SIL) defined for each Safety Instrumented Function (SIF). Using the descriptions of required SIFs and associated safety integrity needs, this step results in a detailed allocation of safety requirements.
3. SIS Safety Requirements Specification
The Safety Instrumented System (SIS) safety requirements are specified to ensure each SIS meets its intended functions and associated safety integrity. This involves defining requirements for both the SIS and its application programs, resulting in a comprehensive set of SIS safety and application program requirements.
4. SIS Design and Engineering
The design and engineering phase of the SIS ensures the system meets the SIF requirements and their associated safety integrity. Based on the specified safety requirements and application program safety needs, this phase produces the hardware and software designs and a plan for integration testing.
5. SIS Installation, Commissioning, and Validation
Installation, commissioning, and validation ensure the SIS is fully functional and compliant with safety requirements. By integrating and testing the SIS according to the design and validation plans, this phase confirms that all safety functions operate as intended, with results documented for integration tests, installation, and commissioning.
6. SIS Operation and Maintenance
During operation and maintenance, the SIS is monitored and maintained to preserve functional safety. Activities are guided by the SIS safety requirements, design specifications, and operational plans, ensuring that the system continues to perform as required.
7. SIS Modification
Modifications to the SIS involve making corrections, enhancements, or adaptations while maintaining the required SIL. These updates are implemented based on revised safety requirements, ensuring the system remains compliant and effective.
8. Decommissioning
Decommissioning processes ensure the safe and structured removal of Safety Instrumented Functions (SIFs) while maintaining proper safety measures. As-built safety requirements and process information guide this phase, leading to the controlled deactivation of SIFs.
9. SIS Verification
Verification activities evaluate outputs from each lifecycle phase, ensuring consistency and compliance with the initial inputs and established standards. A verification plan ensures thorough assessment, with results documented for each phase.
10. SIS FSA (Functional Safety Assessment)
The Functional Safety Assessment (FSA) provides an evaluation of the SIS’s achieved functional safety. Using the FSA plan and SIS safety requirements as references, this assessment results in a formal judgment on safety performance.
11. Safety Lifecycle Structure and Planning
Finally, the safety lifecycle structure and planning establish a comprehensive framework for executing all lifecycle steps. This phase results in a detailed safety plan to guide the entire process.
References:
- IEC-61511
- Functional Safety from Scratch by Peter Clarke
