Optimization of maintenances following proof tests for the final element of a safety-instrumented system

2019 The Authors Safety-instrumented systems (SISs) have been widely installed to prevent accidental events and mitigate their consequences. Mechanical final elements of SISs often become vulnerable with time due to degradations, but the particulars in SIS operations and assessment impede the adaption of state-of-art research results on maintenances into this domain. This paper models the degradation of SIS final element as a stochastic process. Based on the observed information during a proof test, it is essential to determine an optimal maintenance strategy by choosing a preventive maintenance (PM) or corrective maintenance (CM), as well deciding what degree of mitigation of degradation is enough in case of a PM. When the reasonable initiation situation of a PM and the optimal maintenance degree are identified, lifetime cost of the final element can be minimized while keeping satisfying the integrity level requirement for the SIS. A numerical example is introduced to illustrate how the presenting methods are used to examine the effects of maintenance strategies on cost and the average probability of failure on demands (PFDavg) of a SIS. Intervals of the upcoming tests thus can be updated to provide maintenance crews with more clues on cost-effective tests without weakening safety

Establishment of an isotope dilution LC-MS/MS method revealing kinetics and distribution of co-occurring mycotoxins in rats

An isotope dilution liquid chromatography-tandem mass spectrometry (LC-MS/MS) method with a fast sample preparation using homemade clean-up cartridges was developed for simultaneous determination of co-occurring mycotoxins exemplified with aflatoxin B1 (AFB1) and T-2 toxin (T-2) in representative biomatrices of rat plasma, heart, liver, kidney, spleen, lung and brain in a total run time of 7 min. The established approach using stable internal standards of [C-13(17)]-AFB1 and [C-13(24)]-T-2 was extensively validated by determining the specificity, linearity (R-2 >= 0.9990), sensitivity (lower limit of quantitation at 0.05 ng mL(-1)), accuracy (70.9-107.7%), precision (RSD = 70.8%). Based on this methodological advance, the subsequent kinetics and tissue distribution after oral administration of 0.5 mg kg(-1) b.w. of both AFB1 and T-2 in rats were thoroughly studied. As revealed, both AFB1 and T-2 were rapidly eliminated with the half-life time (t(1/2)) in plasma of 8.44 +/- 4.02 h and 8.12 +/- 4.05 h, respectively. Moreover, AFB1 accumulated in all organs where the highest concentration was observed in liver (1.34 mu g kg(-1)), followed by kidney (0.76 mu g kg(-1)). Notably, only low levels of T-2 were observed in spleen (0.70 mu g kg(-1)) and in liver (0.15 mu g kg(-1)). The achieved data as supporting evidence would substantially promote the practical application of the proposed LC-MS/MS method for in vivo toxicokinetics and toxicity studies of co-occurring mycotoxins imitating natural incidence in rat system

Prognostics and health management of safety-instrumented systems: approaches of degradation modeling and decision-making

Modern industries are developing towards a high-integrated direction with overwhelming complexities bringing benefits and potential risks with catastrophic consequences simultaneously. To reduce the occurrences of undesired events or mitigate their consequences, safety-instrumented systems (SISs), as a type of technical safety barrier, have been widely installed in different applications with the aim to protect people, the environment, and other valued assets. Examples of SISs can be emergency shutdown systems in oil & gas production, airbags in cars, fire sprinkler systems in buildings, etc. Many SISs operate in a demanded mode, meaning that they are only activated to perform safety functions while the unexpected occurs. For such systems mainly dormant in normal operation, it is important to conduct proof tests for checking system states and following-up maintenance in case of failures, to keep SISs highly available so as to ensure safety. In current studies, these activities are assumed following a predefined scheme with fixed intervals, independent from the actual system state. However, when more SIS state information can be collected by sensors and in manual tests, the prognostics and health management (PHM) strategy is expected to be more reasonable and cost-efficient. This PhD project thus aims to explore a new approach to evolve the SISs management from time-based to performance-based taking the technological advancement in data collection. This primary objective is then divided into five sub-objectives from the modeling approach and decision-making aspects that are addressed in the form of four journal articles and two conference papers. This PhD thesis bridges SISs performance assessment and degradation process through addressing different influence factors in the operational phase, including aging, and impact of demands, etc, for the decision-making in PHM by proposing: 1.A stochastic process-based degradation model with a specific threshold to describe the time-dependent system performance deviations with the target performance requirement. This model releases the as-good-as-new assumption even though the system is verified as being functional in tests. The proposed stochastic process-based degradation model provides an advantage of calculating the conditional system performance based on the collected information in tests. 2.An approach to quantify the side-effect of operational history on system degradation by introducing abrupt Gamma-distributed increments following a homogeneous Poisson process with arrival rate λde. Impacts of random demands are thus considered in performance evaluation. 3.A maintenance strategy with multiple follow-up actions to adapt the manifested system state in tests. The role of preventive maintenance on SIS management is emphasized in the operational phase. Relying on effective collected information contributes, such a strategy helps to keep an SIS at the required safety level while reducing the frequency of corrective maintenance. 4.A new decision-making support tool on updating testing and maintenance activities with coordinating the system unavailability and life cycle cost. The conditional system unavailability in the required safety integrity level will be the priority principle for updating test intervals, accompanying lower estimation intervention cost in the life cycle. The practical utility of the thesis resides in the provision of a comprehensive consideration of the time- and event-dependencies of SIS performance, as well as safety and economic meanings of testing and maintenance activities. In particular, the first is to provide hints of system deterioration and relevant health management to reliability analysts when they evaluate SIS design. The second is for operational managers of SISs as the decision-makers, to help them to update testing and maintenance plans and identify the optimal intervention opportunities. To conclude, this thesis will contribute to the implementation of PHM on SISs and other systems with similar operational characteristics. The research results on degradation assessment and predictive maintenance optimization can be generalized to more applications where production and maintenance need to be in synergy in consideration of safety and economics. Further research is, however, necessary for testing and validating the proposed methods with practical cases

Optimization of maintenances following proof tests for the final element of a safety-instrumented system

2019 The Authors Safety-instrumented systems (SISs) have been widely installed to prevent accidental events and mitigate their consequences. Mechanical final elements of SISs often become vulnerable with time due to degradations, but the particulars in SIS operations and assessment impede the adaption of state-of-art research results on maintenances into this domain. This paper models the degradation of SIS final element as a stochastic process. Based on the observed information during a proof test, it is essential to determine an optimal maintenance strategy by choosing a preventive maintenance (PM) or corrective maintenance (CM), as well deciding what degree of mitigation of degradation is enough in case of a PM. When the reasonable initiation situation of a PM and the optimal maintenance degree are identified, lifetime cost of the final element can be minimized while keeping satisfying the integrity level requirement for the SIS. A numerical example is introduced to illustrate how the presenting methods are used to examine the effects of maintenance strategies on cost and the average probability of failure on demands (PFDavg) of a SIS. Intervals of the upcoming tests thus can be updated to provide maintenance crews with more clues on cost-effective tests without weakening safety

Performance analysis of redundant safety-instrumented systems subject to degradation and external demands

Safety-instrumented systems (SISs) play a vital role in preventing hazardous events in the offshore facilities. Many of existing performance analysis of SISs are based on the constant failure rate assumption, which is however doubtful when it is applied to actuator sub-systems or mechanical final elements of a SIS. These mechanical SIS components can become vulnerable with time and with upcoming demands given the past exposures to shocks/demands. In this paper, we analyze SIS reliability and unavailability by considering that a failure occurs when total degradation of a SIS component, including continuous degradation and increments caused by random demands, exceeds to a predefined critical threshold. The dependency of two components in a redundant structure of mechanical actuators caused by random demands is also taken into account in the analysis. Approximation formulas for reliability and unavailability of the redundant SIS sub-system under a degradation process are developed. Finally, a numerical example is conducted to illustrate effects of degradation parameters on SIS performance

Prognostic and health management for safety barriers in infrastructures: opportunities and challenges

Different types of safety barriers are deployed in many infrastructures to reduce the occurrences of hazards, and protect people, environment and other assets in case the unexpected events have occurred and the capacity of these barriers against hazards can be weakened by degradations or the failures related to changes over time. It is natural to adapt the approaches of Prognostic and Health Management (PHM) to monitor the conditions and measurable parameters of safety barriers, and predict their future performance by assessing the extent of degradations. This study aims to identify the uniqueness and possible challenges when implementing PHM on safety barriers. Definitions and classifications of safety barriers will be discussed with considering their installation environment in infrastructures, in order to reveal what kind of characteristics of barriers can lead to higher demand on prognosis and heath monitoring. Another objective of this paper is to review the qualitative and quantitative measures for the capacity and performance of safety barriers, and to explore the possible methods and research gaps in the assessments for different PHM strategies, taking account their effects on safety barriers, and effects on the infrastructures being protected by the barriers

A degrading element of safety-instrumented systems with combined maintenance strategy

International audienceSafety-instrumented systems (SISs) are widely used to prevent hazardous events. The mechanical actuator subsystem in a SIS can become more vulnerable with time due to progressive degradation mechanisms, such as erosion, corrosion and wear-out etc. Such kind of phenomenon challenges the assumption of constant failure rates or exponentially distributed lifetime that the existing reliability analysis depends on. This study aims to assess the performance of the actuator of a SIS subject to a continuous degradation, which will be modeled by homogeneous gamma process. Periodic tests with the interval τ are executed to check the subsystem state. A combining maintenance strategy including corrective maintenances (CMs) and imperfect preventive maintenances (PMs) will be adapted according to the state, which can be evaluated by actual degradation level. Given that maintenances are triggered only at inspection dates, the actuator can experience downtime in cases of failures. The expected downtime in each test interval will be used to estimate the average unavailability of the SIS. A numerical example is shown that the average unavailability of such a SIS sub-system is changing with time rather than keeping as a constant value