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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 safety-critical systems [Six lectures]:
- 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." - Extension of methods for quantifying the reliability of safety-critical functions - analytical approaches & dynamic modeling approaches.
"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 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.
- The analytical formulas in the PDS method
- Petri Nets, which is an alternative and more flexible approach than e.g., Markov method state models.
- 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.
- Introduce some other methods for quantifying the average PFD:
- 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 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 using the information that we have. If the performance is not sufficient (in light of e.g. the SIL requirement is not met), we need to do something. This "something" is also discussed as part of this topic.
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.
Maintenance optimization Maintenance optimization (production critical systems):
- Age, block and minimal repair strategies.
Choice of maintenance strategy is an important issue, in particular where maintenance is costly or the equipment is not easily accessable. The big question isThe intervals of maintenance for safety-critical systems are normally determined from the reliability analyses. For other systems, we may use decide upon intervals of testing using different maintenance strategies. These models include parameters like costs, failure rates, and aging. The models come of with the answer to the following two questions: When should we do maintenance and what tasks and equipment should be included. Maintenance . The application of these methods are many. Two examples are maintenance planning of railway tracks and another is one such example of an application where this is an important (and cost-driving) decision and planning of intervention (for maintenance purposes) of subsea facilities is another. Analyses of the the costs associated with these maintentance strategies define what is sometimes called the maintenance cost significant itemsequipment.
- Modeling of effecitve effective failure rate:
Identifying the maintenance cost signifant items is not the only parameter of interest on a decision about when to doe maintenance Maintenance significant items
Maintenance interval and and intervention level (extensiveness of maintenance) is obviously influencing the failure rate of the components. This topic concerns the modeling of the relationship between these two parameters and what we can refer to as the effective (or resulting) failure rate. - Weibull renewal
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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