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Ahmadi, Alireza
Publications (10 of 63) Show all publications
Khajehei, H., Ahmadi, A., Soleimanmeigouni, I. & Nissen, A. (2019). Allocation of effective maintenance limit for railway track geometry. Structure and Infrastructure Engineering
Open this publication in new window or tab >>Allocation of effective maintenance limit for railway track geometry
2019 (English)In: Structure and Infrastructure Engineering, ISSN 1573-2479, E-ISSN 1744-8980Article in journal (Refereed) Epub ahead of print
Abstract [en]

The objective of this study has been to develop an approach to the allocation of an effective maintenancelimit for track geometry maintenance that leads to a minimisation of the total annual maintenancecost. A cost model was developed by considering the cost associated with inspection, preventivemaintenance, normal corrective maintenance and emergency corrective maintenance. The standarddeviation and extreme values of isolated defects of the longitudinal level were used as quality indicatorsfor preventive and corrective maintenance activities. The Monte Carlo technique was used tosimulate the track geometry behaviour under different maintenance limit scenarios and the effectivelimit was determined which minimises the total maintenance cost. The applicability of the model wastested in a case study on the Main Western Line in Sweden. Finally, a sensitivity analysis was carriedout on the inspection intervals, the emergency corrective maintenance cost and the maintenanceresponse time. The results show that there is an optimal region for selecting an effective limit.However, by considering the safety aspects in track geometry maintenance planning, it is suggestedthat the lower bound of the optimal region should be selected.

Place, publisher, year, edition, pages
Taylor & Francis, 2019
Keywords
Maintenance limit, track geometry, maintenance, degradation, Monte Carlo simulation, ordinal logistic regression
National Category
Infrastructure Engineering Other Civil Engineering
Research subject
Operation and Maintenance
Identifiers
urn:nbn:se:ltu:diva-75414 (URN)10.1080/15732479.2019.1629464 (DOI)000475267100001 ()2-s2.0-85068115609 (Scopus ID)
Available from: 2019-08-06 Created: 2019-08-06 Last updated: 2019-08-06
Block, J., Ahmadi, A., Xun, X. & Kumar, U. (2019). Spares Provisioning Strategy for Periodically Replaced Units within the Fleet Retirement Period. International Journal of Systems Assurance Engineering and Management, 10(3), 299-315
Open this publication in new window or tab >>Spares Provisioning Strategy for Periodically Replaced Units within the Fleet Retirement Period
2019 (English)In: International Journal of Systems Assurance Engineering and Management, ISSN 0975-6809, E-ISSN 0976-4348, Vol. 10, no 3, p. 299-315Article in journal (Refereed) Published
Abstract [en]

Within aviation enterprises, the process of dismantling an aircraft at the end of its life is referred to as parting-out. Obviously, the asset value of the units and materials parted out from the retired airframes can be considerable. The benchmarked best practice within the aviation industry is to dismantle the retired aircraft and use the parted-out spares to support the remaining fleet or to offer them on the surplus market. Part-out-based spares provisioning (PBSP) has been a major focus of attention for aviation companies. The PBSP approach is a complex task that requires a multidisciplinary and integrated decision-making process. In order to control the stock level and fulfil the decision criteria within PBSP, it is necessary to make decisions on the termination, at specific times, of both the parting-out process and the maintenance and repair actions performed on the units.

This paper considers repairable units and introduces a computational model to identify the applicable alternatives for repair termination times that will minimize the number of remaining spares at the end of the retirement period, while fulfilling the availability requirement for spares during the PBSP period, at the lowest possible cost.  The feasible alternatives are compared with regard to their respective costs, and the most cost-effective solution is selected. The cost model uses estimates of future maintenance requirements, the turn-around times, the cost of the various maintenance tasks, the future spares consumption, and the estimated salvage of spares from retired aircraft. The output of the model is a set of applicable alternatives which satisfy the availability requirements for spares for the active fleet. The method is illustrated using a case study performed on the Saab-105 training aircraft. 

The results show that the proposed PBSP approach and computational model provide added value from a sustainability point of view, since the use of existing resources is maximized during the retirement process, through the process of reclaiming units and the applicable maintenance termination alternatives. The implementation of the proposed computational model in a PBSP programme provides a detailed and situation-based overview of the stock level dynamics, and contributes to the spares provisioning process by providing solutions to issues such as obsolescence, last-time buys and cannibalization.

Place, publisher, year, edition, pages
Springer, 2019
Keywords
Provisioning, Spare parts, End-of-Life, Maintenance, Retirement, Parting-out, Repairable units, Stock level, Dismantling.
National Category
Civil Engineering Other Civil Engineering
Research subject
Operation and Maintenance
Identifiers
urn:nbn:se:ltu:diva-66222 (URN)10.1007/s13198-019-00791-z (DOI)000474561200001 ()2-s2.0-85066897265 (Scopus ID)
Note

Validerad;2019;Nivå 2;2019-07-12 (johcin)

Available from: 2017-10-23 Created: 2017-10-23 Last updated: 2019-08-16Bibliographically approved
Soleimanmeigouni, I., Ahmadi, A., Arasteh Khouy, I. & Letot, C. (2018). Evaluation of the effect of tamping on the track geometry condition: a case study (ed.). Proceedings of the Institution of mechanical engineers. Part F, journal of rail and rapid transit, 232(2), 408-420
Open this publication in new window or tab >>Evaluation of the effect of tamping on the track geometry condition: a case study
2018 (English)In: Proceedings of the Institution of mechanical engineers. Part F, journal of rail and rapid transit, ISSN 0954-4097, E-ISSN 2041-3017, Vol. 232, no 2, p. 408-420Article in journal (Refereed) Published
Abstract [en]

Tamping is one of the major activities applied by railway maintenance managers to recover track geometry condition. Modelling the tamping effectiveness along with track geometry degradation is essential for long term prediction of track geometry behaviour. The aim of this study is to analyse theeffect of tamping on the different track geometry measures, i.e. longitudinal level, alignment and cant; on the basis of inspection car records from a part of Main Western Line in Sweden. To model recovery after tamping a probabilistic approach is applied. The track geometry condition before tamping was considered as the dominant factor for modelling the model parameters. Correlation analysis has performed to measure the linear relation between the recoveries of the different geometry measures. The results show a moderate correlation between the recovery of the longitudinal level and that of the cant, and a weak correlation between the recovery of the longitudinal level and that of the alignment. Linear regression and Wiener process are also applied to model track geometry degradation and to obtain degradation rates. Theeffect of tamping on degradation rate is analysed. It is observed that the degradation rate was increased after tamping intervention.  

Place, publisher, year, edition, pages
Sage Publications, 2018
National Category
Other Civil Engineering
Research subject
Operation and Maintenance
Identifiers
urn:nbn:se:ltu:diva-15883 (URN)10.1177/0954409716671548 (DOI)000424780200006 ()f7391512-a026-4e7c-a4d0-96223a5957dd (Local ID)f7391512-a026-4e7c-a4d0-96223a5957dd (Archive number)f7391512-a026-4e7c-a4d0-96223a5957dd (OAI)
Note

Validerad;2018;Nivå 2;2018-02-12 (svasva)

Available from: 2016-09-29 Created: 2016-09-29 Last updated: 2019-08-21Bibliographically approved
Soleimanmeigouni, I., Xiao, X., Ahmadi, A., Xie, M., Nissen, A. & Kumar, U. (2018). Modelling the evolution of ballasted railway track geometry by a two-level piecewise model. Structure and Infrastructure Engineering, 14(1), 33-45
Open this publication in new window or tab >>Modelling the evolution of ballasted railway track geometry by a two-level piecewise model
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2018 (English)In: Structure and Infrastructure Engineering, ISSN 1573-2479, E-ISSN 1744-8980, Vol. 14, no 1, p. 33-45Article in journal (Refereed) Published
Abstract [en]

Accurate prediction and efficient simulation of the evolution of track geometry condition is a prerequisite for planning effective railway track maintenance. In this regard, the degradation and tamping effect should be equipped with proper and efficient probabilistic models. The possible correlation induced by the spatial structure also needs to be taken into account when modelling the track geometry degradation. To address these issues, a two-level piecewise linear model is proposed to model the degradation path. At the first level, the degradation characteristic of each track section is modelled by a piecewise linear model with known break points at the tamping times. At the second level, Autoregressive Moving Average models are used to capture the spatial dependences between the parameters of the regression lines indexed by their locations. To illustrate the model, a comprehensive case study is presented using data from the Main Western Line in Sweden

Place, publisher, year, edition, pages
Taylor & Francis, 2018
National Category
Other Civil Engineering
Research subject
Operation and Maintenance
Identifiers
urn:nbn:se:ltu:diva-63815 (URN)10.1080/15732479.2017.1326946 (DOI)000415674800003 ()2-s2.0-85019663898 (Scopus ID)
Note

Validerad;2017;Nivå 2;2017-11-01 (andbra)

Available from: 2017-06-09 Created: 2017-06-09 Last updated: 2019-08-21Bibliographically approved
Mahmood, Y. A., Garmabaki, A., Ahmadi, A. & Verma, A. K. (2018). Reliability model for frequency converter in electrified railway. International Journal of Electrical Power & Energy Systems, 94, 385-392
Open this publication in new window or tab >>Reliability model for frequency converter in electrified railway
2018 (English)In: International Journal of Electrical Power & Energy Systems, ISSN 0142-0615, E-ISSN 1879-3517, Vol. 94, p. 385-392Article in journal (Refereed) Published
Abstract [en]

Reliability analysis of frequency converters based on failures and outages reports constitute an important basis for asset performance and management. Two- and four-state reliability models that recognize the operating characteristics of base load units and peaking units are presented and compared in this study. In this study, a four-state model is modified to a three-state model by combining the ‘needed’ and ‘not-needed’ forced-out states. Moreover, the transitions in the three-state model for power frequency converter have been designed according to real operational data. An outage-reporting database modelled considering IEEE STD 762 is presented and compared with the existing failure-reporting database of the case considered here. Furthermore, a method to extract information missing in the failure-reporting database by electrical readings is proposed to meet the requirements of the outage-reporting database. The study found that the results of indexes based on the IEEE four-state model are not reasonable for the frequency converter given their differences with the gas-turbine results under operational conditions. The forced outage rates and availability factors of twelve actual traction frequency converters of Swedish railways network are presented to validate the modified model.

Place, publisher, year, edition, pages
Elsevier, 2018
Keywords
Outage-data reporting system, Unit state reliability model, IEEE STD 762, Frequency converter
National Category
Reliability and Maintenance Other Civil Engineering
Research subject
Operation and Maintenance
Identifiers
urn:nbn:se:ltu:diva-65173 (URN)10.1016/j.ijepes.2017.08.002 (DOI)000412610000035 ()2-s2.0-85027709160 (Scopus ID)
Note

Validerad;2017;Nivå 2;2017-08-17 (rokbeg)

Available from: 2017-08-17 Created: 2017-08-17 Last updated: 2018-05-07Bibliographically approved
Soleimanmeigouni, I., Ahmadi, A. & Kumar, U. (2018). Track geometry degradation and maintenance modelling: A review (ed.). Proceedings of the Institution of mechanical engineers. Part F, journal of rail and rapid transit, 232(1), 73-102
Open this publication in new window or tab >>Track geometry degradation and maintenance modelling: A review
2018 (English)In: Proceedings of the Institution of mechanical engineers. Part F, journal of rail and rapid transit, ISSN 0954-4097, E-ISSN 2041-3017, Vol. 232, no 1, p. 73-102Article in journal (Refereed) Published
Abstract [en]

Increased demand for railway transportation is creating a need for higher train speeds and axle loads. These, in turn,increase the likelihood of track degradation and failures. Modelling the degradation behaviour of track geometry anddevelopment of applicable and effective maintenance strategies has become a challenging concern for railway infrastructuremanagers. During the last three decades, a number of track geometry degradation and maintenance modellingapproaches have been developed to predict and improve the railway track geometry condition. In this paper, existingtrack geometry measures are identified and discussed. Available models for track geometry degradation are reviewedand classified. Tamping recovery models are also reviewed and discussed to identify the issues and challenges of differentavailable methodologies and models. Existing track geometry maintenance models are reviewed and critical observationson each contribution are provided. The most important track maintenance scheduling models are identified and discussed.Finally, the paper provides directions for further research.

Place, publisher, year, edition, pages
Sage Publications, 2018
National Category
Other Civil Engineering
Research subject
Operation and Maintenance
Identifiers
urn:nbn:se:ltu:diva-14797 (URN)10.1177/0954409716657849 (DOI)000419833100006 ()2-s2.0-85040348348 (Scopus ID)e3794ef4-cb25-49e0-b55c-a579ff2ea6f5 (Local ID)e3794ef4-cb25-49e0-b55c-a579ff2ea6f5 (Archive number)e3794ef4-cb25-49e0-b55c-a579ff2ea6f5 (OAI)
Note

Validerad;2018;Nivå 2;2018-01-18 (svasva)

Available from: 2016-09-29 Created: 2016-09-29 Last updated: 2019-08-21Bibliographically approved
Pourgol-Mohammad, M., Hejaz, A., Ghasemi, P., Ahmadi, A. & Jalali-Vahid, D. (2017). Design for reliability of automotive systems: case study of dry friction clutch. International Journal of Systems Assurance Engineering and Management, 8(3), 572-583
Open this publication in new window or tab >>Design for reliability of automotive systems: case study of dry friction clutch
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2017 (English)In: International Journal of Systems Assurance Engineering and Management, ISSN 0975-6809, E-ISSN 0976-4348, Vol. 8, no 3, p. 572-583Article in journal (Refereed) Published
Abstract [en]

Design and production of highly reliable and safer automotive systems with longer life has been a challenge. The pressure is outcome of high competitive market and recent safety issues of reputable car manufacturers. In this paper, an integrated methodology is proposed based on design for reliability of automotive systems and considering its reliability/safety critical sub-systems. In the proposed approach, the FMEA results are used in the process of failure mode/mechanism identification. The basic failure data, mostly obtained from generic databases, are adjusted by multiplicative corrective factors to account for the design and environment impacts on system failure characteristics. The system is modeled by reliability block diagram method, simulated by Monte Carlo technique. The results of FMEA and reliability evaluation are used for system improvement by reducing the components’ failure rates and potential change of system configuration. The components’ reliability is improved by increasing the quality of components by utilization of high quality materials and modern manufacturing techniques. Modification of system configuration, e.g., adding redundancy, is an alternative for system reliability improvement in some cases. The results show that the friction lining component is the most critical elements in terms of reliability importance. After completion of this phase, an assessment is done for system reliability by comparing the system reliability targets. As a case study, dry friction clutch is studied for assessment of the proposed method. In this study, the life test requirement is researched for each component using a reliability testing techniques. Finally, the uncertainties are computed associated with the failure data and final reliability estimations and the results were presented with a confidence interval.

Place, publisher, year, edition, pages
Springer, 2017
National Category
Other Civil Engineering
Research subject
Operation and Maintenance
Identifiers
urn:nbn:se:ltu:diva-65581 (URN)10.1007/s13198-017-0644-2 (DOI)000410268700006 ()2-s2.0-85029228563 (Scopus ID)
Note

Validerad;2017;Nivå 2;2017-09-11 (andbra)

Available from: 2017-09-11 Created: 2017-09-11 Last updated: 2018-09-14Bibliographically approved
Ahmadi, A., Soleimanmeigouni, I., Letot, C. & Block, J. (2017). Inspection Optimization under imperfect maintenance performance. In: Diego Galar, Dammika Seneviratne (Ed.), Proceedings of MPMM 2016: 6th International Conference on Maintenance Performance Measurement and Management, 28 November 2016, Luleå, Sweden. Paper presented at Maintenance Performance and Measurement and Management 2016(MPMM 2016). November 28, Luleå, Sweden (pp. 139-). Luleå: Luleå tekniska universitet
Open this publication in new window or tab >>Inspection Optimization under imperfect maintenance performance
2017 (English)In: Proceedings of MPMM 2016: 6th International Conference on Maintenance Performance Measurement and Management, 28 November 2016, Luleå, Sweden / [ed] Diego Galar, Dammika Seneviratne, Luleå: Luleå tekniska universitet, 2017, p. 139-Conference paper, Oral presentation with published abstract (Refereed)
Abstract [en]

Scheduled maintenance and inspection development is one of the main requirements for emergency equipment and safety devices. These types of devices have hidden functions which are used intermittently or infrequently, so their failure will not be evident to the operating crew. The analytical model presented in this paper deals with the periodically tested units with overhauls (preventive maintenance) after certain number of inspections and a renewal after a series of overhauls. The cost based optimization method presented in this paper identifies the optimum interval and frequency of Failure Finding Inspection (FFI) and restoration. In the proposed model, repair due to failures found by inspection makes the unit As Bad As Old, and restoration/overhaul action rejuvenates the unit to any condition between As Good As New and As Bad As Old. As Good As New effectiveness also is considered for renewal action. It considers inspection and repair times, and takes into account the costs associated with inspection, repair, restoration, and also the cost of accidents due to the occurrence of multiple failure. The results show that when the unit is not under aging process, the optimal alternative for each inspection interval is the one with highest possible number of inspection without restoration. Finally, it is observed that when the cost of accident is quite high it is needed to perform inspections at smaller intervals to control the risk of accident.

Place, publisher, year, edition, pages
Luleå: Luleå tekniska universitet, 2017
National Category
Other Civil Engineering
Research subject
Operation and Maintenance
Identifiers
urn:nbn:se:ltu:diva-63893 (URN)978-91-7583-841-0 (ISBN)
Conference
Maintenance Performance and Measurement and Management 2016(MPMM 2016). November 28, Luleå, Sweden
Available from: 2017-06-12 Created: 2017-06-12 Last updated: 2018-06-28Bibliographically approved
Lopes, J. C. .., Scarpel, R., Abrahão, F. T. .., Galar, D. & Ahmadi, A. (2017). Optmization In Performance-Based Logistics Contracts. In: Diego Galar, Dammika Seneviratne (Ed.), Proceedings of MPMM 2016: 6th International Conference on Maintenance Performance Measurement and Management, 28 November 2016, Luleå, Sweden. Paper presented at Maintenance Performance and Measurement and Management 2016(MPMM 2016). November 28, Luleå, Sweden (pp. 133-138). Luleå: Luleå tekniska universitet
Open this publication in new window or tab >>Optmization In Performance-Based Logistics Contracts
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2017 (English)In: Proceedings of MPMM 2016: 6th International Conference on Maintenance Performance Measurement and Management, 28 November 2016, Luleå, Sweden / [ed] Diego Galar, Dammika Seneviratne, Luleå: Luleå tekniska universitet, 2017, p. 133-138Conference paper, Published paper (Refereed)
Abstract [en]

Performance-based    Logistics    (PBL)    contracts    require  metrics  and  methodologies  set  in  a  systemic  way  to  provide  readiness  for  the  warfighter  at  reasonable  costs.  There  must be a clever and verifiable connection of metrics in order to access  and  analyze  data and  to  deliver  sound  and  consistent  inputs  for  the  entire  support  system  to  accomplish  its  goals.    A  system   of   incentives   and   penalties   usually   takes   place   and   suggests  functions  to  be  maximized  and/or  minimized  in  a  multi-criterion  and  highly  integrated  environment.  This  study  deals  with  the  optimization  of  the  entire  setup  of  Performance-Based Logistics  contracts  and  is  limited  to  a  preliminary  study  in  a  simple  scenario  that  has  showed  a  promises  results.    A  mixed  method, using both the Ɛ-Constraint  and    Goal  Programming  is  proposed to model the case. Results indicate that metrics used for reliability,    maintainability,    availability    and    supportability    (RAMS)  and  costs  can  be  optimized  simultaneously  for  clever  contracts.

Place, publisher, year, edition, pages
Luleå: Luleå tekniska universitet, 2017
National Category
Other Civil Engineering
Research subject
Operation and Maintenance
Identifiers
urn:nbn:se:ltu:diva-63875 (URN)978-91-7583-841-0 (ISBN)
Conference
Maintenance Performance and Measurement and Management 2016(MPMM 2016). November 28, Luleå, Sweden
Available from: 2017-06-12 Created: 2017-06-12 Last updated: 2017-11-24Bibliographically approved
Pourgol-Mohammad, M. & Ahmadi, A. (2017). Special issue of IREC2016 conference selected papers. Paper presented at 4th International Reliability Engineering Conference (IREC2016) held in Tabriz, Iran, on May 2–4, 2016. International Journal of Systems Assurance Engineering and Management, 8(3), 529-531
Open this publication in new window or tab >>Special issue of IREC2016 conference selected papers
2017 (English)In: International Journal of Systems Assurance Engineering and Management, ISSN 0975-6809, E-ISSN 0976-4348, Vol. 8, no 3, p. 529-531Article in journal, Editorial material (Other academic) Published
Place, publisher, year, edition, pages
Springer, 2017
National Category
Other Civil Engineering
Research subject
Operation and Maintenance
Identifiers
urn:nbn:se:ltu:diva-65421 (URN)10.1007/s13198-017-0662-0 (DOI)000410268700001 ()2-s2.0-85029211751 (Scopus ID)
Conference
4th International Reliability Engineering Conference (IREC2016) held in Tabriz, Iran, on May 2–4, 2016
Available from: 2017-08-30 Created: 2017-08-30 Last updated: 2018-07-10Bibliographically approved
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