[SystemSafety] Maintaining Safety Cases Over Time

[Carl Sandom]

I have an interesting challenge relating to the development and maintenance of an evolving Safety Case for a complex system over the medium to long term. I apologise in advance for the loose use of the terms 'system' and 'integrate' here.

A core system (call it System X) was developed over a number of years along with a Safety Case and it is now in service. Over time, other 'systems' are being connected to System X (Systems Y or Z) to either replace existing functionality or to introduce new functionality. I don't mean System X software updates here; I'm referring to the connection of other systems that are developed independently.

Even when Systems Y or Z have existing Safety Cases, the System X Safety Case requires significant effort to update each time a new system is integrated. Assuming a hazard analysis reveals no new hazards following integration, the impact of the integration on the System X Safety Case will still affect the quantitative aspects in particular (safety targets apportionment and integrity claims).

This can lead to a situation whereby System X and System Y can independently support a SIL x claim; but the integration of the two systems results in System X + Y not achieving the required safety target.

Does anyone have any experiences and/or advice to offer on how to deal with this scenario?

Best Regards
Carl Sandom
iSys Integrity Ltd.
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