Preliminary reliability and safety assessment methodology for trans-atmospheric transportation systems

Purpose This paper aims to propose a methodology for a safety and reliability assessment for the conceptual and preliminary design of very complex and disrupting innovative systems like trans-atmospheric vehicles. The proposed methodology differs from existing ones because it does not rely on statistical data at aircraft-level but exploits the statistical population at components-level only. For the sake of clarity, the paper provides some preliminary results of the application of the methodology at system level. The example deals with the safety and reliability assessment of a very complex propulsion system aimed at guaranteeing vertical take-off and landing capabilities of a suborbital vehicle. Design/methodology/approach The proposed methodology is strongly based on a systems engineering approach. It exploits safety and reliability assessment analyses which have already been developed in both aeronautical and space engineering domains, but it combines them in an innovative way to overcome the lack of statistics at aircraft level. The methodology consists of two different steps: a qualitative top-down process, allowing a functional and physical decomposition of the transportation system and a following quantitative bottom-up approach, which provides the estimation of system-level reliability and safety characteristics starting from the statistical estimation of the components’ characteristics. Findings The paper presents a new methodology for the preliminary reliability and safety assessment of innovative transportation systems, such as hypersonic transportation systems. The envisaged methodology will overcome the poorness of statistical data that is usually affecting the conceptual design of breakthrough systems. Research limitations/implications The paper shows the application of the articulated methodology to a limited case study. A complete example of application of the methodology to estimate safety and reliability characteristics at vehicle level will be provided in feature works. Practical implications The methodology has been proposed to be exploited in international research activities in the field of hypersonic transportation systems. Furthermore, a massive application of this approach would allow to create a database for the generation and the update of semi-empirical models focused on high-level estimations of reliability, availability, maintainability and safety (RAMS) characteristics. Moreover, the proposed safety assessment has been conceived to be fully integrated within a typical conceptual design process. Originality/value The existing literature about safety and reliability assessment at the early design stages proposes pure statistical approaches which are usually not applicable to highly innovative products, where the statistical population is not existing, for example, in the case of trans-atmospheric vehicles. This paper describes how to overcome this problem, through the exploitation of statistical data at components-level only through the combination of these data to estimate RAMS characteristics at aircraft-level thanks to functional analysis, concept of operations and typical safety assessment tools, like functional hazard analysis, failure mode and effect analysis, reliability block diagram and fault tree analysis. </jats:sec

A Model based Safety Assessment for Multirotors

Unmanned Aerial Vehicles (UAVs) must be safe and reliable to prevent fatal accidents in densely populated areas. This research makes the first steps to create a framework which can integrate safety and reliability considerations in the design process. The conceptual design process should consider creating design models coupling sizing with system architecture. Additionally, the multirotor has safety challenges from the propulsor configuration. They lose flight control and show erroneous flight behaviour when propulsors fail. Hence, the design models of multirotor should also incorporate a controllability assessment method to identify and isolate uncontrollable events. For this matter, an appropriate tool should be considered to create such design models. A combination of OpenAltarica, System Analyst and Python is used to create design models of multirotor in a model-based safety assessment framework. These models are developed by integrating system architecture and controllability assessment following the etiquettes of the process. A case study is used to validate the framework and to demonstrate its ability to explore innovative designs. The reliability analysis confirms that the multirotors are fault-tolerant except quadrotor and some configurations are potentially highly reliable. The results demonstrate the feasibility of the multirotor system modelling methods in terms of reliability and pave the way to further develop the model-based safety assessment framework with sizing methodologies. The models can also be further enhanced with the addition of a component fault library, additional failure modes and implementation of diagnosability analysis, fault detection and identification analysis. Fault libraries and failure modes can help in foreseeing uncontrollable cases. In contrast, diagnosability analysis, fault detection and identification analysis can integrate detect, isolate and recover mechanisms, and ensure redundancy optimization effectively. Additionally, the framework should also be combined with multidisciplinary design optimization for sizing. Such design models can contribute to the emergence of UAVs for safety-critical applications

Center and Characteristic Seismic Reliability as new indexes for accounting uncertainties in seismic reliability analysis

Seismic reliability analysis is a powerful tool to assess structural safety against ground shaking actions induced by earthquake occurrences. The classic approach for computing seismic reliability of a structural system requires a seismic hazard curve and a fragility function and leads to the estimation of the failure probability of the investigated damage state. However, resulting failure probability is strongly related to the preliminary assumptions in both hazard and fragility analyses, and slight changes in the input model parameters may cause relevant variability of seismic reliability estimates. The present work formalizes a general approach to be followed when dealing with seismic reliability assessment of structural systems, aimed at taking into account the whole uncertainties of the input parameters within hazard and fragility models. In the proposed approach, probability of failure becomes in turn a random variable and therefore new indexes are introduced, namely Expected Failure Rate, Failure Rate Dispersion, Characteristic Failure Rate, Center of Seismic Reliability and Characteristic Seismic Reliability. Lastly, such approach is applied to a case study, where seismic reliability of an existing open-spandrel reinforced concrete arch bridge is analyzed, and results are discussed highlighting some relevant issues

Injecting Intermittent Faults for the Dependability Assessment of a Fault-Tolerant Microcomputer System

© 2016 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.As scaling is more and more aggressive, intermittent faults are increasing their importance in current deep submicron complementary metal-oxide-semiconductor (CMOS) technologies. This work shows the dependability assessment of a fault-tol- erant computer system against intermittent faults. The applied methodology lies in VHDL-based fault injection, which allows the assessment in early design phases, together with a high level of observability and controllability. The evaluated system is a duplex microcontroller system with cold stand-by sparing. A wide set of intermittent fault models have been injected, and from the simulation traces, coverages and latencies have been measured. Markov models for this system have been generated and some dependability functions, such as reliability and safety, have been calculated. From these results, some enhancements of detection and recovery mechanisms have been suggested. The methodology presented is general to any fault-tolerant computer system.This work was supported in part by the Universitat Politecnica de Valencia under the Research Project SP20120806, and in part by the Spanish Government under the Research Project TIN2012-38308-C02-01. Associate Editor: J. Shortle.Gil Tomás, DA.; Gracia Morán, J.; Baraza Calvo, JC.; Saiz Adalid, LJ.; Gil Vicente, PJ. (2016). Injecting Intermittent Faults for the Dependability Assessment of a Fault-Tolerant Microcomputer System. IEEE Transactions on Reliability. 65(2):648-661. https://doi.org/10.1109/TR.2015.2484058S64866165

Analysis and modeling of human performance in nuclear power plants

This work investigates two important areas in human reliability engineering: models for human performance analysis, and reliable data for these models. In the first area, the author addresses the two modeling techniques THERP and SAINT showing that the flexibility and nuclear relevance of the second give it the merit to be used in modeling and analyzing human tasks in complex man-machine systems such as nuclear systems. A case study is investigated and simulated by the SAINT to explore its dynamic capabilities;In the second area, the author evaluates all the available human data sources, develops a new description format for human related events with all the information matrices needed for human reliability analysis. A complete operations data bank for sixty-five commercial power plants in the U.S.A. over a period of ten years is constructed;Data analysis, with qualitative and quantitative evaluations using the information matrices provided in the operations data bank, is presented. Operator error probability computations are made for some operational tasks in selected nuclear engineered safety feature systems. Moreover, the assessment of the role of the performance shaping factors is discussed

Reliability Analysis of Continuous Structural Systems

This study mainly deals with developing another approximate method for system reliability analysis and its applications to the continuous structures such as an assembly of stiffened cylindrical and rectangular sections used in Tension Leg Platform (TLP). Various methods developed for the structural system reliability analysis are reviewed The developed system reliability method, called herein the "Extended Incremental Load Method", is an extended approach of the conventional incremental load method. It has been developed in order to extend its applicability to the system reliability analysis of a structure under multiple loadings. It directly uses existing component strength formulae in the system analysis and more realistically takes account of the post-ultimate (post-failure) behaviour of a failed component when assessing the system reliability and ultimate strength. This is an important merit of the method over other methods. The method allows for load re-distribution during the development of elasto-plastic moments in large cross-sections under the action of axial and bending forces and in the presence of lateral hydrostatic and hydrodynamic pressure. The effects of shearing actions are ignored. A search is made for the most important failure modes to give the lowest system safety index. In the method the modified safety margin equation, which has been proposed to use existing strength formulae for principle components of a floating offshore structure, is employed in which the strength modelling parameter is treated as a basic random variable in system reliability analysis as well as component reliability analysis and the concept of the first-order second moment method is adopted to obtain the resistance coefficients and the loading coefficients in the safety margin equation. Details about deriving the safety margin equation by the proposed reliability method, calculation of the total load factor, the procedure of identifying the most important failure modes and flow vectors of principle component are described in the Appendices. Applications to discrete structures are demonstrated to show the validity of the proposed method. The method has been applied to the Hutton TLP and two variants, TLP-A and TLP-B, which are modified models of the Hutton TLP and of the design using TLP Rule Case Committee type loading and improved strength models, under the design environmental loading conditions. Components and systems safety indices of the models, Hutton TLP, TLP-A and TLP-B, are illustrated with three dimensional collapse mechanisms figures. Reserve and reserve strength characteristics are derived for the design as built and for more economical and efficient variations of the design. The TLP form is shown to possess high redundancy and systems safety. Sensitivity studies to changes in stochastic parameters of resistance and loading variables have been carried out. For this purpose the strength modelling parameter, yield stress and certain member sizes are selected as resistance variables, and effects of their mean values and/or coefficients of variation on the system, as well as on the component reliability index, have been investigated. The effects of mean bias and coefficient of variation of load effects, namely, static, quasi-static and dynamic component, on the the system as well as on the component reliability index have also been investigated. The results are discussed with regard to effects of various parameters on safety, with illustrating figures, from which the relative importance of random variables can be seen. As an another important resistance variable, the post-ultimate behaviour of failed components has been taken account of in system reliability assessment, which should be the most important resistance variable affecting the system reliability and the effective residual strength of a structure. Some case studies have been carried out with the simplified non-linear model which has a form of piecewise multi-state (more than two states) and is characterised by the post-ultimate slope and the residual strength. The results are illustrated in figures and tables and discussion made about its effects on the system reliability level. (Abstract shortened by ProQuest.)

Software reliability and dependability: a roadmap

Shifting the focus from software reliability to user-centred measures of dependability in complete software-based systems. Influencing design practice to facilitate dependability assessment. Propagating awareness of dependability issues and the use of existing, useful methods. Injecting some rigour in the use of process-related evidence for dependability assessment. Better understanding issues of diversity and variation as drivers of dependability. Bev Littlewood is founder-Director of the Centre for Software Reliability, and Professor of Software Engineering at City University, London. Prof Littlewood has worked for many years on problems associated with the modelling and evaluation of the dependability of software-based systems; he has published many papers in international journals and conference proceedings and has edited several books. Much of this work has been carried out in collaborative projects, including the successful EC-funded projects SHIP, PDCS, PDCS2, DeVa. He has been employed as a consultant t

Expert Elicitation for Reliable System Design

This paper reviews the role of expert judgement to support reliability assessments within the systems engineering design process. Generic design processes are described to give the context and a discussion is given about the nature of the reliability assessments required in the different systems engineering phases. It is argued that, as far as meeting reliability requirements is concerned, the whole design process is more akin to a statistical control process than to a straightforward statistical problem of assessing an unknown distribution. This leads to features of the expert judgement problem in the design context which are substantially different from those seen, for example, in risk assessment. In particular, the role of experts in problem structuring and in developing failure mitigation options is much more prominent, and there is a need to take into account the reliability potential for future mitigation measures downstream in the system life cycle. An overview is given of the stakeholders typically involved in large scale systems engineering design projects, and this is used to argue the need for methods that expose potential judgemental biases in order to generate analyses that can be said to provide rational consensus about uncertainties. Finally, a number of key points are developed with the aim of moving toward a framework that provides a holistic method for tracking reliability assessment through the design process.Comment: This paper commented in: [arXiv:0708.0285], [arXiv:0708.0287], [arXiv:0708.0288]. Rejoinder in [arXiv:0708.0293]. Published at http://dx.doi.org/10.1214/088342306000000510 in the Statistical Science (http://www.imstat.org/sts/) by the Institute of Mathematical Statistics (http://www.imstat.org

The problems of assessing software reliability ...When you really need to depend on it

This paper looks at the ways in which the reliability of software can be assessed and predicted. It shows that the levels of reliability that can be claimed with scientific justification are relatively modest