The safety requirements specification is carried out after SIL selection in the safety life-cycle.
In order to create a comprehensive SRS it is important that the required information is accessible for the personnel dealing with the SRS documentation.
A typical set of input information includes:
Process information and process conditions
The process itself shall be described in order to give detailed information regarding the process parameters to the personnel dealing with the SRS documentation.
Drawings that support the description of the process itself are useful. Later on this process information is important for the personnel dealing with implementation of SIS and SIF.
Specific process conditions that are important for the safety must be addressed.
Process and hazard report (PHA)
The PHA report is needed. This report gives valuable information about the hazards and the hazardous events for the intended Safety Instrumented System.
Important information are also the hazard frequencies and hazard consequences.
Required Safety Instrumented Systems
A specification of the required Safety Instrumented Systems
Required Safety Instrumented Functions
A specification of each individual Safety Instrumented Function.
The target SIL shall be defined for each SIF.
If there are any regulatory requirements that affect the design of the SIS, the SRS shall include these requirements.
Common cause failures
The possibilities of common cause failures must be taken in account. These failures could reduce or eliminate the redundant safety measures applied in the SIF or SIS.
Sometimes it is tricky to find the common cause failures that affect the safety measures. The personnel involved in the design of the SIS or SIF must identify possible common cause failures.