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Topics to be covered are as part of the course are (organized according to whether the application is mainly for safety-critical systems or production-critical systms, or both) :
presented below. Note that more than one lecture may be used to cover one particular topic. See the lecture plan for more details.
Reliability analysis of safetySafety-critical systems [Six lectures]:
- How to define requirements Methods for developing reliability requirements for safety systems and barriers, with basis in risk analyses
Safety integrity level (SIL) is a key reliability performance measure used for safety-critical systems. The SIL requirements are identified in an extension of the risk analysis, using methods often refered to as SIL allocation, SIL targeting and SIL classification. Key methods like Layers of protection analysis (LOPA), risk graph, and minimum SIL are presented and discussed.
Some examples showing the relevance of this topic may be found with consultancy companies, such as with Safetec, Lloyd's Register Consulting, and DNV-GL (link to the GL-part of the services), and Lilleaker Consulting. - Methods used to determine the reliability of safety instrumented systemsExtension of methods for quantifying the reliability of safety-critical functions.
In TPK 4120, some analytical formulas were introduced to calculate the average probability of failure on demand (PFD). It was also shown how the average PFD may be calculated using Markov methods and fault tree analysis. This reliability measure is of high importance in relation to SIL, as a relationship is established between a SIL requirement and the maximum PFD tolerated for a safety function. In this course, we go one step further and introduce :- Introduce some other methods for quantifying the average PFD: The analytical formulas presented in a standard called IEC 61508 (in part 6), which builds on slightly different assumptions than the analytical formulas from TPK 4120. In addition, we will introduce the PDS method and Petri Nets.
- Study reliability of "high demand systems", where another reliability measure, the average system failure rate (called PFH), is recommended rather than the average PFD. One example of a high demand safety system is a machine that carry out safety-critical functions. Also PFH is linked to SIL.
- Monitoring and maintaining SIL performance in the operational/use phase.
The reliability of a safety-critical function is influenced over time after the system has been put in operation. Just like if you buy a car: We may think that the car has some kind of inherent reliability performance in light of what it costs, the type of engine, manufacturer reputation, safety systems installed with the car and so on. Nevertheless, once you start to drive it, its performance may change over time depending on your driving habits, how much you drive, where you drive, how often you send it to the garage for maintenance and checks and os on. You may collect some data about the car's performance, such as how often it does not start "on demand", milage, and how often some of the safety-features fail, and based on this (often limited information as you should not have much failures) you may try to estimate the reliability. In fact, you are trying to estimate the reliability as it has been up till a certain point in time. It is the same thing we would like to do with a safety-critical system: With rare data we would like to estimate the reliability with the information we have. If the performance is not sufficient (SIL requirement is not met), we need to do something.
Relevance:
- Some examples showing the relevance of this topic may be found with consultancy companies, such as with Safetec, Lloyd's Register Consulting, and DNV-GL (link to the GL-part of the services), and Lilleaker Consulting. Manufacturers like ABB, Siemens, AkerSolutions, FMC, Kongsberg Maritime and many more need to design systems in light of SIL requirements, and also demonstrate (sometimes with assistance of the consultancy companies) that the SIL requirements are met. End users, like railway service providers like Jernbaneverket, oil companies like Statoil, Det Norske, GDF-Suez, Shell and Conoco-Phillips among some, and owners of smelting plants, owners of water power stations must demonstrate that the SIL requirements continue to be met throughout the life of the systems.
How to define requirements for safety systems and barriers, and how to assess the reliability of safety instrumented systems with background in IEC 61508 and related standards. This includes SIL allocation, risk acceptance criteria, requirements for design of technical and operational barriers, alternative strategies for treatment of common cause failures, various methods for determining proof test intervals, and trade off between safety and regularity. Within maintenance optimization the following topics are covered: Age, block, and minimal repair policies. Optimisation of intervals and intervention level in condition monitoring models. Optimum grouping of maintenance activities. Spare part optimisation. Reliability Centred maintenance. Data collection and analysis. In relation to technical safety we study how the result from the risk analysis may be utilized to assess the effect of various safety system configurations, and combination of these under various constraints.
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