1. Safety Assessment: Safety Integrity Levels
ITV Model-based Analysis and Design of Embedded Software
Techniques and methods for Critical Software
Anders P. Ravn
Aalborg University
September 2011

2. Safety Assessment Find hazards that may lead to incidents or mishaps,
analyze their relations,
and estimate their consequences.
May involve probabilistic reasoning (Reliability Engineering).
Is PFH < per hour (h-1 ) ?

3. Process & Documents

4. System Analysis and Definition

5. Model the context Hazards!
Problem domain: That part of a context that is administrated, monitored, or controlled by a system
Application domain: The organization that administrates, monitors, or controls a problem domain
Hazards!

6. System Definition (FACTOR)
Functionality: The system functions that support the application-domain tasks.
Application domain: Those parts of an organization that administrate, monitor, or control a problem domain.
Conditions: The conditions under which the system will be developed and used.
Technology: Both the technology used to develop the system and the technology on which the system will run.
Objects: The main objects in the problem domain.
Responsibility: The system’s overall responsibility in relation to its context.

7. Determining the Hazards and risks

8. Hazard Determination HAZOP (hazard and operability) -study

9. Determine Event Sequences Fault Trees
Primary Events:
Basic event – fault in atomic component
Undeveloped Event – fault in composite component (may be analyzed later)
External event – expected event from environment
Intermediate event:
Nodes inside a fault-tree

10. Fault Tree - Gates
...
...
condition
Inhibit gate

11. Determining the SIL

12. Methods ALARP (As Low As Resonably Possibble)
with Quantitative method (Appendix C, D)
Risk Graphs (Appendix E)
LOPA (Layer of Protection Analysis) (Appendix F)
Hazardous Event Severity Matrix (Appendix G)

13. ALARP – Frequency and Consequence

14. C a weight of the consequence, eg on a scale [0,1].
ALARP – to SIL
C a weight of the consequence, eg on a scale [0,1].
Fnp frequency in h-1 or y-1.
Fp (Ft) tolerable frequency = PFHavg  Fnp

15. Risk Graphs - SIL

16. Data for Risk Graph