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1

QRA of accidental events initiated by leaks causing a fire in process industries

Briš R., Grunt O.

Eksploatacja i Niezawodność

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2015

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Vol. 17, no. 4

582--590

EN

Risk to safety of personnel in process industries is normally modelled by the application of Event Trees, where the risk is defined as a product of event frequency and its consequences. This method is steady state whilst the actual event is time dependent. For example, gas release is an event comprising the size of gas cloud being released, probabilities of ignition, fire or explosion, fatality, escalation to new releases and fire and/or explosion, and the probability of fatality, all varying with time. This paper brings new perspective, how the risk to safety of personnel could be evaluated in dynamic context. A new approach is presented whereby the time-dependent events and the time-dependent probability of fatality are modelled by means of the analytical computation method based on modeling of different accident scenarios by use of the directed acyclic graph (DAG) and Fault Tree Analysis (FTA) method. Using these methods the modeled scenarios change with relevant probabilities at defined times to configurations with appropriate probabilities of fatalities.The paper uses a realistic example from the offshore industry, where different sizes of leak have different probability characteristics. Specifically small, medium and large leaks are evaluated. Based on the dynamic evolution of the probability of fatality, it is concluded that the most dangerous leak is the large one. Probability of fatality caused by the leak increased very rapidly within first 5 minutes. At the end of 5th minute, there is approximately one order of magnitude difference in the probabilities of fatality associated with the respective leak sizes.

PL

Zagrożenie dla bezpieczeństwa pracowników w przemyśle przetwórczym jest zwykle modelowane za pomocą drzewa zdarzeń, gdzie ryzyko jest zdefiniowane jako iloczyn częstotliwości zdarzenia i jego skutków. Metoda ta dotyczy stanu stacjonarnego, podczas gdy rzeczywiste zdarzenie jest zależne od czasu. Na przykład, ulatnianie się gazu jest zdarzeniem, które wiąże się z wielkością obłoku uwalnianego gazu, prawdopodobieństwem zapłonu, pożaru lub wybuchu, śmiertelnością, eskalacją pod kątem dalszego wycieku i pożaru i/lub wybuchu, oraz prawdopodobieństwem ofiar śmiertelnych, w każdym przypadku zależnie od czasu. Niniejsza praca pokazuje nowe podejście do tego, jak zagrożenie dla bezpieczeństwa pracowników może być rozpatrywane w kontekście dynamicznym. Nowe metoda polega na tym, iż zdarzenia zależne od czasu i zależne od czasu prawdopodobieństwo śmiertelności są modelowane za pomocą analitycznej metody obliczeń opartej na modelowaniu różnych scenariuszy wypadków przez zastosowanie skierowanego grafu acyklicznego (DAG) i metody analizy drzewa błędów (FTA). Dzięki zastosowaniu niniejszych metod, modelowane scenariusze zmieniają się wraz z odpowiednimi prawdopodobieństwami w określonych czasach na konfiguracje z właściwymi prawdopodobieństwami śmiertelności. Artykuł wykorzystuje rzeczywisty przykład z branży morskiej, gdzie różne rozmiary wycieku wykazują różne parametry prawdopodobieństwa. Szczegółowo oceniane są małe, średnie i duże wycieki. W oparciu o dynamiczną ewolucję prawdopodobieństwa ofiar śmiertelnych, należy stwierdzić, że najbardziej niebezpieczny jest duży wyciek. Prawdopodobieństwo ofiar śmiertelnych spowodowanych wyciekiem gwałtownie wzrasta w ciągu pierwszych 5 minut. Na koniec 5. minuty, występuje różnica w przybliżeniu o jeden rząd wielkości w prawdopodobieństwie śmiertelności związanej z odpowiednimi wielkościami wycieku.

2

Survival analysis on data of different surgery techniques to evaluate risk of postoperative complications

Briš R., Praks P., Janurová K., Martínek L.

Journal of Polish Safety and Reliability Association

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2011

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Vol. 2, No. 1

33--38

EN

Medical survival censored data of about 850 patients are evaluated to compare two basic surgery techniques. Data comes from patients who underwent colectomy in the University Hospital of Ostrava. The data has been screened into three general groups: all patients (data from both rectum and colon operations), data from rectum operations, data from colon operations. Two basic surgery techniques are used for the colectomy: either classical (open) or laparoscopic operation. Basic question which arises at the colectomy operation is which type of operation to choose to guarantee longer overall survival time. Two methodological approaches have been used to answer this relevant question. First is the non-parametric approach which results from Kaplan- Meier estimates of the survival function. For each data group two survival curves are constructed, i.e. for both open and laparoscopic type of operation. Final survival curves are compared and evaluated using advanced methods of statistical inference (e.g. log-rank test). Second is parametric approach which results from modelling of survival time. It is based on Maximum Likelihood Estimation method to estimate parameters of probability distribution of overall survival time. Moreover, two Weibull models are used to compare the two surgery techniques. Mean survival times assigned to particular types of operation are compared

3

Direct unavailability calculations of highly reliable systems

Briš R.

Journal of Polish Safety and Reliability Association

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2009

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Vol. 1

67--76

EN

The paper presents a new analytical algorithm which is able to carry out direct and exact reliability quantification of highly reliable systems with maintenance (both preventive and corrective). A directed acyclic graph is used as a system representation. The algorithm allows take into account highly reliable and maintained input components. All considered models are implemented into the new algorithm. The algorithm is based on a special new procedure which permits only summarization between two or more non-negative numbers that can be very different. If the summarization of very small positive numbers transformed into the machine code is performed effectively no error is committed at the operation. Reliability quantification is demonstrated on a real system from practice and on its highly reliable modifications. The selected system is frequently used system - high pressure injection system occurring in many late references.

4

Stochastic ageing models : extensions of the classic renewal theory

Briš R.

Journal of Polish Safety and Reliability Association

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2007

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Vol. 1

29--38

EN

Exact knowledge of the reliability characteristics as the time dependent unavailability coefficient for example, under influence of different ageing processes as well as under different failure types is very useful to the practitioners who have to find the optimal maintenance policy for their equipment. In this paper found models and their solutions have potential to face the optimisation task under the conflicting issues of safety and economics. Most of the solved models take into account ageing processes. An increasing tendency lately exists to include aging effects into the risk assessment models to evaluate its contribution. We developed different renewal models taking into account different ageing distributions of failures (Weibull, Erlang, log-normal): models with negligible renewal time, models with periodical preventive maintenance, alternating renewal process with lognormal distribution of failure time, and with two types of failures.

5

Simulation approach for modeling of dynamic reliability using time dependent acyclic graph

Briš R., Praaks P.

Eksploatacja i Niezawodność

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2006

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nr 2

26-28

EN

The dynamic reliability approach takes into account changes (evolution) of the system structure (hardware). For instance, the dynamic reliability allows modeling a human operator (or an electronic control system) naturally. In these cases, the structure of the system is usually changed in order to keep the functionality and/or safety of the system. The main purpose of the paper is to illustrate, by means of a model example, the ability of acyclic oriented graph, terminal nodes of which are programmable components, to model simple dynamic system and to assess its performance via Monte-Carlo simulations. To demonstrate the availability of our framework a test case study with the deterministic evolution is presented. The here presented numerical results are in agreement with the exact analytical solution.