Change in the maintenance strategy as a method of improving the efficiency of the process of operation of railway means of transport

. The paper presents a description of methods of changing the maintenance strategy concerning measures of intervals between maintenance levels based on the example of a selected railway vehicle type. The process starts with monitoring the vehicle after it is put into operation and the operational database is built, through the statistical and RAMS analyses, to the approval of changes in the maintenance system documentation by qualified railway safety bodies. Following the applicable formal and legal requirements, one of the key problems involved in the introduction of changes in the railway vehicle maintenance system is an assessment of the proposed changes for safety.


Introduction
Projects intended to improve the efficiency of the process of operation (use and maintenance) of means of transport are amongst the fundamental areas of the strategies pursued by rail transport companies. In the group of maintenance projects, particularly noteworthy is the change in the maintenance strategy concerning the maintenance plan structure and the change in the measures of intervals between checks and scheduled repairs (so-called maintenance levels). Reduction of preventive maintenance costs and implementation of modern maintenance methods, such as Condition-Based Maintenance (CBM) is the subject of many scientific publications devoted to this issue [1][2][3]. The research and development work, which were done on the change in the maintenance strategy [4][5][6][7][8][9][10], prove that the costs of preventive maintenance of railway means of transport form up to 30,5% of the Life Cycle Costs (LCC) and are the second dominating cost category after the costs of power or fuel consumption. Currently, such projects are very often applied by Polish railway carriers. Following the model of leading foreign companies, in their strategic ventures, Polish carriers intend to introduce dynamic maintenance plans in which the criterion for referring a vehicle to a check /repair are measurements the vehicle actual operational work.
On the basis of an analysis of the writings and the experience gathered in conducting research and development work for Polish railway carriers, e.g.: [11][12][13], Table 1 presents a description of the praxeological sequence illustrating the change in the maintenance strategy concerning the measurements of intervals between checks and scheduled repairs with the fulfilment of the following criteria: • ensuring a high level of safety in railway transport, • ensuring effective outlays on maintenance.
The process starts with monitoring the vehicle after it is put into operation (purchased or modernised) and the development of an operational database, through a statistical and reliability analysis of the data, risk analysis of the proposed changes, to the approval of the changes in the maintenance system by the authorised railway safety bodies [14].

Example of method application
The method for changing the maintenance strategy is universal in its nature and may be applied to various types of rail vehicles (diesel and electric traction vehicles, cargo and passenger carriages). This paper presents selected aspects based on the example of a modernised diesel 6Dg type locomotive (Fig. 1).  Table 1. Description of the method of change of the maintenance strategy concerning the measurements of intervals between checks and scheduled repairs [14]. Analysis of the applicable law concerning the maintenance system and changes in rail vehicle maintenance systems.

No. Name Description of stage
3 Statistical analysis of the operational process Analysis of statistical data on kilometrages and time of operation between maintenance levels and comparison with the assumptions in the maintenance system documentation. The analysis was the subject of a series of research and development papers [15][16][17]. Identification of the LCC cost structure of the 6Dg diesel locomotive demonstrated that scheduled checks done at the P2 level (P2/1 and P2/2 totalling 38,8%) and scheduled repairs done at the P4 maintenance level -28,7% (Fig. 2) form the highest proportion of preventive maintenance costs. The cost verification, done in the operation stage, initiated the measures undertaken by the railway carrier (PKP Cargo S.A.) in cooperation with the research body (Institute of Rail Vehicles, Cracow University of Technology) intended to improve the efficiency of the locomotive maintenance process by changing the measurements of intervals between the maintenance levels and the scope of maintenance activities. It was assumed that the changes should correspond to the actual operating work and include advanced level of diagnostics and registration of the operating condition of modernised locomotives.

Development of operational database
During the period from the start of operation and the first repair at the P4 level, done after 24 480 mth or after 5 years, the modernised locomotives were monitored during their operation and operational data were gathered therein, concerning: • their kilometrages in km/year and in km/day; • time of operation in h/year and in h/day; • time to failure; • causes for the failure; • duration of maintenance repairs; • costs of maintenance repairs, checks and scheduled repairs. The electronic information, which was gathered and processed, was formally recorded on a preliminary basis in Microsoft Excel. Universal failure codes were used comprising 7 locomotive systems and 30 subassemblies.

Formal requirements related to the maintenance system change
An assessment of the possibility to change the maintenance strategy should take account of formal and legal requirements under the law which applies in Poland, including: The analysis of legal conditions has shown that the current regulations do not allow for performance of inspections or periodic repairs of railway vehicles according to a strategy based on the actual state of vehicle assemblies and subassemblies (CBM technique). The structure of the maintenance plan for railway vehicles must be defined with fixed distance measurements expressed in kilometers and/or units of time.

Statistical analysis of the process of operation
On the basis of the operational data gathered by the railway carrier for 2009÷2015, i.e. from the beginning of operation of a series of 121 vehicles, Table 2

Concept of changes in the locomotive maintenance plan
The concept of changes in the maintenance plan for the 6Dg type locomotive required, above all, the collection of practical knowledge from specialists engaged in the maintenance of rolling stock about the possibility to change the intervals between the maintenance levels and modify the scopes of activities in the Maintenance System Documentation (DSU). Additionally, the results of the theoretical analyses were used to identify the locomotive weakest elements from the viewpoint of the impact on the vehicle technical availability. The analysis used the Fault Tree Analysis (FTA) with Monte Carlo simulation which is presented in papers [14,18,19]. One of the indices serving the purpose of selecting the most relevant elements was the ReliaSoft's Downing Event Criticality Index (RS DECI) expressing the percentage of downtimes caused by a particular locomotive assembly or subassembly in the total downtime figure [20]: where: C NSDE -number of downtimes caused by the particular locomotive assembly, subassembly; N ALLdown -number of all locomotive downtime.  On the basis of the analysis, changes in the current plan for the maintenance of the SM42 series 6Dg type locomotives were proposed (Table 3).

RAMS analysis in the area of safety
The RAMS analysis in the area of safety was based on the results of the operational tests of a selected sample of 75 locomotives of the 6Dg type, operated by a railway carrier over a period of 15 months. This enabled the observation of the course of operation of the locomotives in different conditions thus providing creditable data for the reliability analysis. The reliability data were gathered in the IT system supporting the management of the means of transport, enabling precise registration of the Time to Failure (TTF) and the Time to Restore (TTR). The RAMS indices relating to safety were determined for relevant elements, i.e. assemblies and sub-assemblies of locomotives, whose failure results in hazards to the reliability and safety of railway operations (so-called hazardous failure). On the basis of the analysis of operation data for the locomotive sample tested over the 15-month period, a total of 151 failures were recorded which were qualified as hazardous ones. This is approximately 30,6% of the total number of failures. The structure of the hazardous failures is as follows [14] The definitions of the above ratios are available in specialist writings on reliability, availability, maintainability and safety, e.g.: [14,21,22] and in the applicable standards: • PN-EN 50126:2002 Railway applications. The specification and demonstration of reliability, availability, maintainability and safety, • PN-EN 61703:2016-12 Mathematical expressions for reliability, availability, maintainability and maintenance support terms.

Analysis of the risk involved in the proposed changes
The principles of the Common Safety Method (CSM) concerning risk analysis are described in Commission Implementing Regulation (EU) No. 402/2013 [23]. The detailed algorithm of the risk management process is presented in the Appendix to the Regulation entitled "Risk management process and independent assessment". The procedure of risk qualification where changes are made in the maintenance system requires: • analysis of the relevance of the proposed changes, • hazard identification and development of a register of hazards, • assessment of risk acceptability.
In accordance with the Regulation, the acceptance of the risk concerning the assessed system is tested using one or several of the following risk acceptance principles: • use of codes of practice, • use of a reference system, • explicit risk estimation.
In the analysed case, following the procedure for identification of hazards and risk assessment of the Safety Management System applied by the railway carrier operating 6Dg type locomotives, the risk was qualified using the (Failure Mode and Effects (FMEA) Analysis. The FMEA method is a quantitative one in which the risk of hazard occurrence is estimated using the Risk Priority Number (RPN) [22].
The analysis demonstrated a possibility to make changes in the plan for maintenance of 6Dg type locomotives without breaching the accepted safety level and introducing entries in accordance with Table 2. Nonetheless, the changes in the maintenance plan required the development of additional safety measures concerning the assemblies and subassemblies of major importance for rail traffic safety: wheel sets, brake system, bogie support and frame.

Assessment of the effectiveness of the changes in the maintenance process
The effectiveness of the changes in the 6Dg locomotive maintenance plan was assessed on the basis of a comparison of the costs of preventive maintenance (PMC t ), which was expressed through the formulae (2) and (3) where PMC i is costs of preventive maintenance in the year t for the maintenance level "i" in EUR: where: PMCL i -the costs of labour for maintenance level "i"; PMCM i -the costs of materials and spare parts for maintenance level "i", EUR; NPMA i (t) -number of preventive maintenance activities at maintenance level "i" in a year of operation; MMH i -mean labour intensity of preventive maintenance at maintenance level "i", man-hours; CPH P -cost of man-hour in preventive maintenance: CPH P = 13,71 EUR/man-hour; ACM i -average cost of consumption of materials in preventive maintenance at maintenance level "i", EUR. It was concluded on the basis of the calculations that as a result of the change in the maintenance strategy the costs of maintaining the modernised 6Dg type locomotive calculated in the full maintenance plan were reduced by 24,7% or EUR 232300,0 (Fig. 4a). For the entire series of 121 modernised vehicles owned by the carrier, the maintenance cost savings are EUR 28,1 million. In comparison with the unmodernised SM42 6D locomotive, the maintenance costs were reduced by 53,0% or EUR 799100,0 per vehicle (Fig. 4b).
a) b) Fig. 4. a) Comparison of the costs of preventive maintenance before and after the change in the maintenance plan, b) Comparison of the costs of preventive maintenance of a nonmodernised 6D locomotive and a 6Dg modernised one after the maintenance plan change [14].

Conclusion
The costs of maintenance are the second, following the costs of power or fuel consumption, dominating cost category in the Life Cycle Costs LCC of railway means of transport. Hence, projects undertaken by rail carriers to enhance the efficiency of the process of vehicle operation by changing the maintenance strategy are fully justified. The methods presented in the paper are based on many years of experience from research and development work related to the change in the strategy of maintenance of traction vehicles and rail carriages. They indicate the important role of the RAMS analysis in the area of safety thus enabling proper classification of hazards, quantification of the frequency of their occurrence and adoption of appropriate criteria for risk assessment.
The considerations presented in the paper are of general relevance. They may be applied not only to rail vehicles but to other technical objects, which are subject to a complex maintenance system.