System Safety Assessment process specification

Having identified the appropriate risks with respect to the design under consideration, each risk should be the subject of a specific study to examine and document the simultaneous or cascading effect(s) of each risk. The objective is to ensure that any SR effects are either eliminated or the risk is shown to be acceptable. A particular risk assessment is required for aircraft airworthiness certification by the FAA the process is documented in SAE/ARP-4761, Guidelines and Methods for Conducting the Safety Assessment Process on Civil Airborne Systems and Equipment, 1996. 

Alternatively we may decide to create more specific claims, perhaps that the system performs at a level that is conducive to practical operation in a clinical environment and/or with a defined degree of availability. Whatever measure is selected it should be clear and concise and, most importantly, be capable of being demonstrated within the scope of the planned CRM assessment process. In other words it should be possible to draw lines of logical inference between each claim and the arguments set out in the body of the safety case. 

This internationai standard addresses the appiication of safety instrumented systems for the Process industries, it aiso requires a process hazard and risk assessment to be carried out to enabie the specification for safety instrumented systems to be derived. Other safety systems are oniy considered so that their contribution can be taken into account when considering the performance requirements for the safety instrumented systems. The safety instrumented system inciudes aii components and subsystems necessary to carry out the safety instrumented function from sensor(s) to finai eiement(s). 

Functional Safety Assessments can utilize checklists to facilitate the assessment at the various lifecycle stages. This qualitative assessment is a continuous process carried out at each life-cycle phase, i.e., specification, design, manufacture, commissioning and operation of the system. The use of a checklist ensures that ... 

A number of standards require a system to be accompanied by a safety case, and a SoS should be no different. The safety case communicates an argument, supported by evidenee, that a system is acceptably safe in a given operational context. The safety ease eaptures the underlying reasoning and evidence that support claims made about the safety of a system. We believe that the hazard assessment process we have proposed is compatible with the creation of a safety case. We also believe that it will be possible to produce a manageable SoS safety case which takes ae-count of SoS specific issues and which conforms to relevant standards. 

When system safety is performed, it may become obvious that the NDI is not the best alternative. Specifically, hazards must be identified, risks assessed, and the risk made acceptable regardless if the component/function is provided as NDI. The decision to use NDI or COTS items does not negate SSRs. A very important rule-of-thumb for NDI safety is that the use of NDIs does not eliminate the requirement for a safety analysis and risk assessment COTS/ NDI items must be evaluated for safety as an integral part of the entire system. COTS/NDI items are not exempt from the system safety process. The decision to use COTS/NDI items does not negate the need for SSRs and processes for COTS/NDI applications. 

Every model relevant to safety assessment has to be validated. Validation should give reasonable assurance that the model is applicable to the specific disposal system. It will largely depend on the barrier concept and the specific geological situation for which models have to be validated. As a tentative list of models that are prime candidates for validation, the following processes and models can be mentioned ... 

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