Natalia Trapani

MatCarloRe: An integrated FT and Monte Carlo Simulink tool for the reliability assessment of dynamic fault tree

With the aim of a more effective representation of reliability assessment for real industry, in the last years concepts like dynamic fault trees (DFT) have gained the interest of many researchers and engineers (dealing with problems concerning safety management, design and development of new products, decision analysis and project management, maintenance of industrial plant, etc.). With the increased computational power of modern calculators is possible to achieve results with low modeling efforts and calculating time. Supported by the strong mathematical basis of state space models, the DFT technique has increased its popularity. Nevertheless, DFT analysis of real application has been more likely based on a specific case to case resolution procedure that often requires a great effort in terms of modeling by the human operator. Moreover, limitations like the state space explosion for increasing number of components, the constrain of using exponential distribution for all kind of basic events constituting any analyzed system and the ineffectiveness of modularization for DFT which exhibit dynamic gates at top levels without incurring in calculation and methodological errors are faces of these methodologies. In this paper we present a high level modeling framework that exceeds all these limitations, based on Monte Carlo simulation. It makes use of traditional DFT systemic modeling procedure and by replicating the true casual nature of the system can produce relevant results with low effort in term of modeling and computational time. A Simulink library that integrates Monte Carlo and FT methodologies for the calculation of DFT reliability has been developed, revealing new insights about the meaning of spare gates.

Dynamic fault trees resolution: A conscious trade-off between analytical and simulative approaches

Abstract Safety assessment in industrial plants with ‘major hazards’ requires a rigorous combination of both qualitative and quantitative techniques of RAMS. Quantitative assessment can be executed by static or dynamic tools of dependability but, while the former are not sufficient to model exhaustively time-dependent activities, the latter are still too complex to be used with success by the operators of the industrial field. In this paper we present a review of the procedures that can be used to solve quite general dynamic fault trees (DFT) that present a combination of the following characteristics: time dependencies, repeated events and generalized probability failure. Theoretical foundations of the DFT theory are discussed and the limits of the most known DFT tools are presented. Introducing the concept of weak and strong hierarchy, the well-known modular approach is adapted to study a more generic class of DFT. In order to quantify the approximations introduced, an ad-hoc simulative environment is used as benchmark. In the end, a DFT of an accidental scenario is analyzed with both analytical and simulative approaches. Final results are in good agreement and prove how it is possible to implement a suitable Monte Carlo simulation with the features of a spreadsheet environment, able to overcome the limits of the analytical tools, thus encouraging further researches along this direction.

A Weibull-based compositional approach for hierarchical dynamic fault trees

The solution of a dynamic fault tree (DFT) for the reliability assessment can be achieved using a wide variety of techniques. These techniques have a strong theoretical foundation as both the analytical and the simulation methods have been extensively developed. Nevertheless, they all present the same limits that appear with the increasing of the size of the fault trees (i.e., state space explosion, time-consuming simulations), compromising the resolution.

Conception of Repairable Dynamic Fault Trees and resolution by the use of RAATSS, a Matlab® toolbox based on the ATS formalism

Dynamic Fault Tree (DFT) is a well-known stochastic technique for conducting reliability studies of complex systems. At the state of the art, existing tools (both academic and commercial) do not fully support DFT with repairable components and repeated events, lowering the penetration of this powerful technique in real industrial applications (e.g., industrial processes and plants, computer, electronic and network applications). One of the main reasons limiting the attractiveness of DFT is that, originally, DFTs were conceived without repairable components; only recently few related works have started to deal with a formal semantic, which would avoid undefined behavior and misinterpretation of DFT. Other researchers have tackled the problem by introducing extensions of the original Fault Trees (FTs) technique like Boolean Driven Markov Processes (BDMPs) and Generalized Fault Trees (GFTs). However, despite they consider repairable systems and repeated events, we have found that the introduction of a different formalism with more complex features has again limited the penetration of these powerful methods in real applications. The target of this work is the original DFT technique. Starting from the state of the art, a set of standardized rules that frame the behaviors of dynamic gates are designed and a well-defined semantic for repairable-DFT is drawn through the application of a novel formalism, the Adaptive Transitions System (ATS). The proposed theoretical framework is afterward used to code a software tool, RAATSS, for the resolution of extended, repairable-DFT. Moreover, this work introduces some novel concepts regarding the modeling of a system by a DFT and provides a basic hint of the ATS capabilities to describe interdependencies in complex system.

An open-source application to model and solve dynamic fault tree of real industrial systems

In recent years, a new generation of modeling tools for the risk assessment have been developed. The concept of ¿dynamic¿ was exported also in the field of reliability and techniques like dynamic fault tree, dynamic reliability block diagrams, boolean logic driven Markov processes, etc., have become of use. But, despite the promises of researchers and the efforts of end-users, the dynamic paradox hangs: risk assessment procedures are not as straight as they were with the traditional static methods and, what is worse, it is difficult to assess the reliability of these results. Far from deny the importance of the scientific achievement, we have tested and cursed some of these dynamic tools realizing that none of them was appropriate to solve a real case. In this context, we decided to develop a new DFT reliability solver, based on the Monte Carlo simulative approach. The tool is greatly powerful because it is written with Matlab® code, hence is open-source and can be extended. In this first version, we have implemented the most used dynamic gates (PAND, SEQ, FDEP and SPARE), the existence of repeated events and the possibility to simulate different cumulative distribution function of failure (Weibull, negative exponential CDF and constant). The tool is provided with a snappy graphic user interface written in Java®, which allows an easy but efficient modeling of any fault tree schema. The tool has been tested with many literature cases of study and results encourage other developments.

Life cycle assessment of CRT lead recovery process

In recent years, the high rate of technological obsolescence and subsequent replacement of ICT devices has led to a significant increase in waste from electrical and electronic equipment (WEEE) and oriented institutional stakeholders to promote recycling and material recovery to prevent the dispersion of hazardous substances into the environment and wastage of valuable resources. For cathode ray tube (CRT) WEEE, the standard treatment of end-of-life CRTs generates leaded glass which is usually landfill-disposed after inertisation. An experimental treatment was designed and applied to a pilot plant to obtain valuable secondary raw materials (SRMs). The industrialisation phase of the treatment requires assessing its sustainability according to the life cycle assessment approach. The paper analyses this treatment and from the significant results suggests it is environmentally sustainable.

Designing an Optimal Shape Warehouse

The paper addresses the topic of designing the shape of a warehouse and shows a comparison between a standard storage-handling system, which is designed taking into account the minimization of the handling planar path, and the one which is designed trying to minimizing the overall handling energy consumption. This comparison leads to a discussion on the opportunities which result from the construction of a shallow warehouse in term of building construction costs, layout management and storage surface efficiency. This paper is the first step in the analysis of a more comprehensive research about the life cycle assessment of a warehouse, the manpower utilization and the balanced equilibrium between handling energy requirement and performances of most used handling systems.

Failure Prevention Through Performance Evaluation of Reliability Components in Working Condition

The reliability of a system during operation can be expressed quantitatively through different time functions. Mathematical procedures and statistical laws allow to assess the precise analytical relations between these functions. Referring to a generic system or component is a common experience that its duration in-service is not predictable in a deterministic way. This consideration identifies the lifetime (or time to failure) of the component as a continuous random variable, susceptible to a statistical description, whose estimate is crucial in the design phase, or in any case before the commissioning of equipment. In a second stage, the actual values of reliability must be compared with the forecast values arising from the theoretical statistical model used. This comparison allows assessment of the goodness-of-fit level and the confidence level of the prediction model, in order to validate it for any future equipment redesigns or for similar equipment. In this context, the present work is aimed at identifying the most appropriate statistical tools for the comparison above, and then to assess the reliability of the forecast data, compared to the real performance of a reliability system. For this purpose, a literature analysis was conducted, with a dual purpose: The identification of statistical models most commonly used to describe the reliability function of a system; to provide a choice of appropriate indicators and effective tests for assessing the confidence of the statistical models for reliability scopes. The models identified were then applied, as an example, to the real case of a catalytic cracking catalyst with the fluidized bed of a petrochemical plant. The results obtained from the case study, discussed in the final section of the work, offer many points of comparison between the various statistical models as well as a first overview of their reliability.

A Behaviour Model for Risk Assessment of Complex Systems Based on HAZOP and Coloured Petri Nets

To support the knowledge of specialists during a HAZOP brainstorming session, a support system, which is able to automatically generate a preliminary HAZOP report was developed. The support system, which is based on Coloured Petri Nets (CPNs), simulates the behaviour of the system when different abnormal scenarios occur. The research demonstrates that integration of CPNs and HAZOP is very effective to obtain a smart tool for risk assessment of complex systems, improving the HAZOP analysis procedures.

Reliability Driven Standardization of Mechanical Seals for Petrochemical Applications

Mechanical seals are the most diffuse solution for rotating shaft sealing because of their high safety performances and low maintenance costs. The only requirement to meet high service level is a care in the choice of a sealing system suitable for fluids treated and operating conditions. Mechanical seal manufacturers usually have very large catalogues with a wide mix of products for each use and also offers to their clients customized solutions and maintenance service. This could result in high inventory cost for manufacturers which want to guarantee high maintenance service performances. This study shows a reliability driven methodology to reduce the mix and maintain high performances.