682 / 973
4
Conclusions
This paper describes error or weaknesses that can be found in safety application software
and preventative measures to avert reoccurrence. Like proof testing, the objective is to
identify residual errors before an instrumented function fails to perform its function in a real
demand situation, and to allow corrective action to be taken.
This paper advocates that IEC61511 code reviews should be repeated periodically during
the operation phase; starting after the first year of operation, where the risk of introducing
errors is highest. Theoretically the pre-start-up code review removes the risk of residual
software errors, while functional safety management procedures (such as the prohibiting of
all software forces during operation) reduce the risk of the introduction of errors. However,
reality may be different, not least in IEC61511 designs where large numbers of asset and
convenience trips are not fully segregated from personnel safety and environmental
protective functions.
In Fire & Gas and ESD systems, and also Process Shutdown Systems designed to API RP
14C, end-to-end testing of large numbers of safety instrumented function is neither always
practical nor required by some regulators. In such applications, periodic offline testing is
essential, providing a low-cost solution to eliminate errors with minimal disruption caused to
operations.
Application software code reviews, whether conducted pre-start-up, or during the operations
phase, tell you how effective functional safety management and verification procedures have
been in preventing the introduction of errors. This provides a valuable tool to pinpoint where
those procedures need strengthening for subsequent projects.
5
References
[1] IEC61508
Functional safety of electrical / electronic / programmable electronic
safety-related systems
[2] IEC61511
Functional Safety – Safety instrumented systems for the process
industry sector
[3] API RP14C
Recommended Practice for Analysis, Design, Installation, and Testing
of Basic Surface Safety Systems for Offshore Production Platforms
[4]
SPC/TECH/OSD/31
UK HSE: Safety instrumented systems for the overpressure protection
of pipeline risers