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The main objective of this course is to increase the depth of understanding about RAMS methodsassessment. Sometimes the purpose of the assessment is to determine the reliability requirements or to determine the reliability of system functions in light of safety requirements. At other times, the purpose is to minimize costs or downtime of the systems, by considering different maintenance strategies.
| Think about a system. |
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| This system |
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| may constitute many different parts |
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| and together they shall perform many different functions. |
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| Systems may be production-critical, safety-critical, or even both. Safety |
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| -critical systems are used to protect personnel from injury and death, or to protect the environment from severe damages. |
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| Production-critical systems may, if they fail, cost "a whole lot", and have a severe effect on a manufacturer reputation, the quality of products developed, and the costs associated with correcting the system after failure. Critical infrastructures may be consideres as both production and safety-critical. Stable and safe public transportation, clean and stable water supply, power supply, and net supply are important for serving the society and business, and a failure of these could affect safety at a local level as well as at a national level. |
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Some key questions to ask in relation to such systems are shown in the figure below, and in many cases, they need to be solved using RAMS assessment and optimization methods.
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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 analyses of safety-critical systems
Lecture material is: |
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- Development of reliability requirements
"Safety integrity level (SIL) is a key reliability performance measure used for safety-critical systems. Reliability requirements are therefore often given as 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." - Reliability analyses: Extending with new analytical & 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 a step further and introduce the foundations for analytical formulas presented in IEC 61508 (a key standard for reliability of safety-critical systems), the PDS method (a method along with a set of analytical formulas widely adapted in the Norwegian oil and gas industry, but which has a wider application area), and dynamic modeling, using Petri Nets." - Special analysis challenges (possible candidates for under this heading):
- Partial and imperfect testing
- Follow-up of SIL requirements in the operational phase
- Hardware fault tolerance - Hardware design constraints of safety-critical functions
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- 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
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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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| Lecture material supporting this topic are: |
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- Age, block and minimal repair strategies.
The 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. The application of these methods are many. Two examples are maintenance planning of railway tracks and another is planning of intervention (for maintenance purposes) of subsea equipment.
Sub-topics also covered under the same "umbrella" are:
- Modeling
- of effective failure rate:
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:
**Say something here**
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- PF models and state based models:
**Say something here** - Spare part optimization:
Spare parts may be costly to have on the stock, but at the same time it is costly not to have a spare part available when it is needed. This topic concern how to calculate the probability of running out of spares, using simple formulas and Markov analyses. The use of Monte-carlo simulations for this purpose is also shown. This topic may not be some relevant for very specialized systems, where it is not possible to aquire a spare within short time. For a manufacturer that develops products, such as sensors, in a large scale to e.g. the oil and gas industry, it may be relevant to find the optimal number of spare parts for warranty and repair services.
- Prognostics and remaining useful life:
**Say something here** - Bayesian methods
- Counting processes
Tutorials & Project
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There will be mandatory problems/tasks to solve as part of the course.
Reliability analyses:
Tutorials will focus on the application of lectured methods, and in particular comparing results of using different approaches. Two or three case studies will be introduced and used as basis for the problem solving. Matlab, Maple and Grif (the latter is a rather recent software for reliability assessment in use here at the NTNU) will be preferred to assist the reliability analyses.
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