Romero-Silva, Rodrigo
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Romero-Silva, Rodrigo
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Romero Silva, Rodrigo
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rromeros
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Reducing the variability of inter-departure times of a single-server queueing system : the effects of skewness

2019 , Romero-Silva, Rodrigo , Marsillac, Erika , Shaaban, Sabry , Hurtado-Hernandez, Margarita

A critical performance measure in serial production lines is the variability of inter-departure times of a single-server queue. Increasing upstream inter-departure time variability generates greater downstream variability, diminishing overall line performance. Theory suggests that the variability of inter-departure times of a single-server queue is reduced by decreasing the variance of inter-arrival and service times. This study investigates the effects of the skewness of inter-arrival and service time distributions on the variability of inter-departure times. Contrary to previous results suggesting that mean waiting times of a GI/G/1 queue can be reduced by increasing inter-arrival time skewness, this experimental study of a GI/G/1 queue with triangular inter-arrival and service times shows that the inter-departure time coefficient of variation is reduced through a combination of negative inter-arrival time skewness and positive service time skewness. These results also suggest that the absolute value of the negative autocorrelation between consecutive departures is reduced by the same combination of negative inter-arrival time skewness and positive service time skewness for low values of server's utilization, while positive skewness for both inter-arrival and service times reduces this value for high values of server's utilization. Finally, it was found that queue capacity constraints increase the coefficient of variation of inter-departure times, as has been previously suggested, as well as the skewness and the absolute correlation values of the inter-departure time distribution. © 2019 Elsevier Ltd

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The impact of unequal processing time variability on reliable and unreliable merging line performance

2021 , Romero-Silva, Rodrigo , Shaaban, Sabry , Marsillac, Erika , Laarraf, Zouhair

Research on merging lines is expanding as their use grows significantly in the contexts of remanufacturing, reverse logistics and developing economies. This article is the first to study the behavior of unpaced, reliable, and unreliable merging assembly lines that are deliberately unbalanced with respect to their coefficients of variation (CV). Conducting a series of simulation runs with varying line lengths, buffer storage capacities and unbalanced CV patterns delivers intriguing results. For both reliable and unreliable lines, the best pattern for generating higher throughput is found to be a balanced configuration (equal CVs along both parallel lines), except for unreliable lines with a station buffer capacity of six. In that case, the highest throughput results from the descending configuration, i.e. concentrating the variable stations close to the beginning of both parallel lines and the steady stations towards the end of the line. Ordering from the least to most steady station also provides the best average buffer level. By exploring the experimental Pareto Frontier, this study shows the combined performance of unbalanced CV patterns for throughput and average buffer level. Study results suggest that caution should be exercised when assuming equivalent behavior from reliable and unreliable lines, or single serial lines and merging lines, since the relative throughput performance of some CV patterns changed between the different configurations.

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Influence of unbalanced operation time means and uneven buffer allocation on unreliable merging assembly line efficiency

2018 , Romero-Silva, Rodrigo , Shaaban, Sabry

Unbalanced, unreliable (UR), unpaced, merging assembly lines are simulated in this study with varying line lengths, buffer storage capacities, imbalance degrees and unequal mean operation time configurations and uneven buffer capacity (BC) allocation. This paper contributes to the literature by suggesting that, in many cases, imbalance can improve merging lines’ performance, as compared to a corresponding balanced merging line. It was found that an inverted bowl or descending patterns for mean operation times (MTs), and an inverted bowl (concentrating BC towards the centre of the line) or an ascending pattern for buffer allocation, result in higher throughput (TR). In terms of average buffer level (ABL), the best pattern is a monotone decreasing order regarding MTs and a monotone increasing order with respect to BC allocation. Additionally, it was found that when considering a profit function, the best performing patterns for UR lines tend to be the patterns that reduce ABL, even when considering very low inventory holding costs; contrary to the behaviour of the profit function in reliable lines, which suggests that either patterns that increase TR or reduce ABL can lead to a good performance, depending on the values of the unitary inventory holding costs.

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Performance of merging lines with uneven buffer capacity allocation: the effects of unreliability under different inventory-related costs

2020 , Shaaban, Sabry , Romero-Silva, Rodrigo

This simulation study investigates whether machine efficiency, mean time to failure (MTTF) and mean time to repair (MTTR) significantly affect the performance of uneven buffer capacity allocation patterns for merging lines. Also studied is the trade-off between increasing throughput via bigger buffers and their associated inventory-related costs, since previous studies have shown that higher overall buffer capacity and higher average inventory content result in higher throughput. Results suggest that an ascending buffer allocation pattern (concentrating buffer capacity towards the end of the line) produces higher throughput in shorter, more unreliable lines; whereas the balanced pattern shows better performance in longer, more reliable lines. Increasing average buffer capacity per station and/or having higher average buffer content was found to be more cost-effective in lines with lower machine inefficiency, shorter MTTF and MTTR, and longer lines. Results differed between reliable and unreliable lines since reliable lines were particularly penalised by buffer capacity investiment/maintenance costs due to a relatively low increase in throughput resulting from the addition of extra buffer capacity.

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Serial production line performance under random variation : dealing with the "law of variability"

2019 , Romero-Silva, Rodrigo , Marsillac, Erika , Shaaban, Sabry , Hurtado-Hernandez, Margarita

Many Queueing Theory and Production Management studies have investigated specific effects of variability on the performance of serial lines since variability has a significant impact on performance. To date, there has been no single summary source of the most relevant research results concerned with variability, particularly as they relate to the need to better understand the ‘Law of Variability’. This paper fills this gap and provides readers the foundational knowledge needed to develop intuition and insights on the complexities of stochastic simple serial lines, and serves as a guide to better understand and manage the effects of variability and design factors related to improving serial production line performance, i.e. throughput, inter-departure time and flow time, under random variation. © 2019 The Society of Manufacturing Engineers

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Studying the effects of the skewness of inter-arrival and service times on the probability distribution of waiting times

2020 , Romero-Silva, Rodrigo , Shaaban, Sabry , Marsillac, Erika , Hurtado-Hernandez, Margarita

Previous studies have shown that the mean queue length of a GI/G/1 system is significantly influenced by the skewness of inter-arrival times, but not by the skewness of service times. These results are limited because all the distributions considered in previous studies were positively skewed. To address this limitation, this paper investigates the effects of the skewness of inter-arrival and service times on the probability distribution of waiting times, when a negatively skewed distribution is used to model inter-arrival and service times. Subsequent to a series of experiments on a GI/G/1 queue using discrete-event simulation, results have shown that the lowest mean waiting time and the lowest variance of waiting times can be attained with a combination of positive inter-arrival skewness and negative service skewness. Results also show an interesting effect of the skewness of service times in the probability of no-delay in environments with a higher utilization factor. © 2020 Brazilian Operations Research Society.

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Exploiting the characteristics of serial queues to reduce the mean and variance of flow time using combined priority rules

2018 , Romero-Silva, Rodrigo , Shaaban, Sabry , Marsillac, Erika , Hurtado-Hernandez, Margarita

This paper addresses the trade-off challenge from reducing either the mean or variance of flow time when using simple sequencing rules in balanced, multi-class, serial queues. Study results show that instead of the expected zero sum situation, a balance between the two objectives can be achieved by assigning different priority rules to different queues. The order of priority rule assignments in different queues is shown to be relevant because variability along the line creates unbalanced queue lengths for each station, depending on the characteristics of the line. Thus, it was found that a simple heuristic for reducing both the mean and the variance of flow time in non-heavy traffic environments is to assign the first queue a priority rule that reduces its mean queue length while assigning the other queues a priority rule that reduces the variance of flow time. Conversely, for very-high traffic environments, performance improvements are shown from assigning the first queue a priority rule that reduces the variance of flow time while assigning the other queues a priority rule that reduces queue length. © 2017 Elsevier B.V.

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