Event Tree Construction Research Articles

WOVOdat – An online, growing library of worldwide volcanic unrest

The World Organization of Volcano Observatories (WOVO), with major support from the Earth Observatory of Singapore, is developing a web-accessible database of seismic, geodetic, gas, hydrologic, and other unrest from volcanoes around the world. This database, WOVOdat, is intended for reference during volcanic crises, comparative studies, basic research on pre-eruption processes, teaching, and outreach. Data are already processed to have physical meaning, e.g. earthquake hypocenters rather than voltages or arrival times, and are historical rather than real-time, ranging in age from a few days to several decades. Data from >900 episodes of unrest covering >75 volcanoes are already accessible. Users can visualize and compare changes from one episode of unrest or from one volcano to the next. As the database grows more complete, users will be able to analyze patterns of unrest in the same way that epidemiologists study the spatial and temporal patterns and associations among diseases. WOVOdat was opened for station and data visualization in August 2013, and now includes utilities for data downloads and Boolean searches. Many more data sets are being added, as well as utilities interfacing to new applications, e.g., the construction of event trees. For more details, please see www.wovodat.org.

A blended hazard identification methodology to support process diagnosis

A novel hazard identification methodology applied to process systems is presented in this paper. This blended hazard identification (BLHAZID) methodology blends two different types of HAZID methods: the function-driven and component-driven approach. The BLHAZID method is based on a conceptual framework called the Functional Systems Framework, which describes structure–function–goal relationships in process systems.The goals of the BLHAZID methodology are to generate outcomes that contain a high coverage of hazards, describe detailed failure causality in process systems and express this knowledge in a structured form for effective reused in subsequent applications, such as fault diagnosis, operator training, design reviews, fault and event tree construction and hazard updates to satisfy major hazard facility requirements.Both the BLHAZID methodology and the Functional Systems Framework were developed with involvement and advice from two major industrial partners. An industrial case study of a benzene saturation unit is presented to illustrate how the BLHAZID methodology operates in practice.

A long-term volcanic hazard event tree for Teide-Pico Viejo stratovolcanoes (Tenerife, Canary Islands)

We propose a long-term volcanic hazards event tree for Teide-Pico Viejo stratovolcanoes, two complex alkaline composite volcanoes that have erupted 1.8–3 km 3 of mafic and felsic magmas from different vent sites during the last 35 ka. This is the maximum period that can be investigated from surface geology and also represents an upper time limit for the appearance of the first phonolites on that volcano. The whole process of the event tree construction was divided into three stages. The first stage included the determination of the spatial probability of vent opening for basaltic and phonolitic eruptions, based on the available geological and geophysical data. The second, involved the analysis of the different eruption types that have characterised the volcanic activity from Teide during this period. The third stage focussed on the generation of the event tree from the information obtained in the two previous steps and from the application of a probabilistic analysis on the occurrence of each possible eruption type. As for other volcanoes, the structure of the Teide-Pico Viejo Event Tree was subdivided into several steps of eruptive progression from general to more specific events. The precursory phase was assumed as an unrest episode of any geologic origin (magmatic, hydrothermal or tectonic), which could be responsible for a clear increase of volcanic activity revealed by geophysical and geochemical monitoring. According to the present characteristics of Teide-Pico Viejo and their past history, we started by considering whether the unrest episode would lead to a sector collapse or not. If the sector collapse does not occur but an eruption is expected, this could be either from the central vents or from any of the volcanoes' flanks. In any of these cases, there are several possibilities according to what has been observed in the period considered in our study. In the case that a sector collapse occurs and is followed by an eruption we considered it as a flank eruption. We conducted an experts elicitation judgement to assign probabilities to the different possibilities indicated in the event tree. We assumed long term estimations based on existing geological and historical data for the last 35 Ka, which gave us a minimum estimate as the geological record for such a long period is incomplete. However, to estimate probabilities for a short term forecast, for example during an unrest episode, we would need to include in the event tree additional information from the monitoring networks, as any possible precursors that may be identified could tell us in which direction the system will evolve. Therefore, we propose to develop future versions of the event tree to include also the precursors that might be expected on each path during the initial stages of a new eruptive event.

Functional block diagrams and automated construction of event trees

Functional block diagrams are introduced as graphical representations of the function of a system. A functional block provides the output of a system as the outcome of a joint event defined by the inputs to the system and its various states. Functional blocks corresponding to different subsystems are combined together to form a functional block diagram representing the functional characteristics of the combined system. Conversely, a complex system represented by a single functional block is decomposed to constituent components with a corresponding functional block diagram. It is demonstrated that a functional block diagram is an alternative representation of an event-tree corresponding to the outcome space of the underlying joint event. The concept of output space partition is introduced and an algorithm is developed for generating the most compact form of the event-tree consistent with a given partition. This algorithm forms the basis of a technique for the automated construction of an event-tree starting from a developed functional block diagram. This technique is amenable to computerization. Finally, as a demonstration of the technique, the event-trees of a boiling water reactor (BWR) are developed.

A Methodology for Knowledge Acquisition and Reasoning About Faults in Conditions of Incomplete Information

In this paper the problem of modelling knowledge about fault propagation and utilizing this model for reasoning in conditions of incomplete information is addressed. The term incomplete information is used to indicate the situations when available amount of information isn’t enough to build well known analytical or statistical models for problem of failure analysis. A solution that merges digraph and event tree failure analysis with three types of reasoning is presented. The methodology is divided into two phases. The first phase is structural modelling phase and the second phase is the model utilization phase for cause-consequence rule derivation. The results of these phases are the topological knowledge base and the deep knowledge rule base, respectively. The two knowledge bases are combined to form a novel hierarchical knowledge representation scheme. Upper levels of this hierarchy are made up of the structural models visualized by digraphs. Each level represents the system from the point of view of morphological and functional structure, respectively. The lowest level consists of a set of event trees. The two knowledge bases support three kinds of reasoning, namely, reasoning in logic, diagnostic and predictive reasoning. Formal methods for model building, event tree construction and derivation of cause-consequence rules are discussed.

Living PRAs made easier with IRRAS

Abstract The Integrated Reliability and Risk Analysis System (IRRAS) is an integrated PRA software tool that gives the user the ability to create and analyze fault trees and accident sequences using an IBM-compatible microcomputer. This program provides functions that range from graphical fault tree and event tree construction to cut set generation and quantification. IRRAS contains all the capabilities and functions required to create, modify, reduce and analyze event tree and fault tree models used in the analysis of complex systems and processes. IRRAS uses advanced graphic and analytical techniques to achieve the greatest possible realization of the potential of the microcomputer. When the needs of the user exceed this potential, IRRAS can call upon the power of the mainframe computer. The role of the Idaho National Engineering Laboratory in the IRRAS program is that of software developer and interface to the user community. Version 1.0 of the IRRAS program was released in February 1987 to prove the concept of performing this kind of analysis on microcomputers. This version contained many of the basic features needed for fault tree analysis and was received very well by the PRA community. Since the release of Version 1.0, many user comments and enhancements have been incorporated into the program providing a much more powerful and user-friendly system. This version is designated ‘IRRAS 2.0’. Version 3.0 will contain all of the features required for efficient event tree and fault tree construction and analysis.

Knowledge-based support for event tree construction

Abstract A prototype system called EventMAP has been developed to provide knowledge-based support for event tree construction. It comprehends an event tree model of a physical system, which represents the possible event sequences following an initial incident. This model can be used to identify potentially hazardous accident or failure scenarios, to help in assessing the possible outcomes of an observed incident, and to support decisions about appropriate actions. It can also be used during modification to provide insights into the needs for specific safety features. EventMAP incorporates explicit knowledge about safety engineering and good event tree construction practice. This assures that model building will be more systematic, and will produce more complete and correct event trees. Assumptions underlying the model are tracked explicitly, allowing ‘what-if’ exploration. Moreover, the safety-related design rationale will be preserved despite personnel changes over time. EventMAP incorporates knowledge about: plant structure in terms of subsystems, components, and interconnections; cause/effect relationships and other interactions among subsystems; and failure modes, including human errors. The role of EventMAP is to act as an ‘intelligent assistant’ for the event tree developer; it monitors the evolving model and issues suggestions and warnings to the user as appropriate.