Researcher: Daniël van Dijk
Project Duration: May 2019 – October 2020
Project Partner: Arcadis
Research Summary:
The Dutch railway network is expected to transport over 2.2 million passenger trains and over 42 million tons of freight in 2019. To ensure exploitation of the railway, maintenance is needed. Bad track conditions result in lower machine efficiency and breakdowns result in unavailability and possible safety hazards. The requirements regarding the Reliability, Availability, Maintainability and Safety (RAMS) of the rail network are high. The asset manager, ProRail, optimizes the balance between performance (RAMS indicators), the risks and the costs over the whole life cycle. To mitigate the risk on a rail breach, the asset manager contracted the small-scale mainte-nance to a contractor via Performance Based Maintenance contracts and, with that transferred a part of the risks for the duration of the contract.
For ASSET Rail, a maintenance contractor, some factors that can cause defects (grinding regime, deferred maintenance of earlier contracts) lie outside their sphere of influence. Despite this being true they are consid-ered responsible for mitigating the risks on rail defects caused by these factors. ASSET Rail wants insights in the defects and their causes to optimize their own maintenance plan and to be able to make claims with the asset manager when defect causes lie outside their sphere of influence.
The main research question is “What are the risks of rail defects?”. Only the rail defects on a free track are considered, switches are not looked further into. The method that has been used to analyze the risks of rail defects is Fault Tree Analysis (FTA). A Fault Tree is constructed based on interviews with experts and literature review. The defects that are taken into consideration are squats and headchecks (over 90% of the ultrasonic notifications of ASSET Rail consist of squats and headchecks).
The main results of this research are:
- Comprehensive fault tree model of the main defects and their causes.
- Identification of challenges for using FTA as a method to improve inspection and maintenance practice.
- Distinction between technical cause and process/contract related cause.
The interviews and literature study revealed that squats are the result of an irregularity in the rail surface (or just beneath it) that induces high impact loading, which lead to crack development. Typical locations that exhibit these irregularities are transitions from a low- to high track modulus such as bridges, culverts and level crossings as well as electrically insulated joints and construction welds. Headchecks are cracks resulting from high shear stresses, often induced by a deviation from the design speed in a curve. Once the defects are initiated, a process and/or contract related risk arises. The contractor is responsible for defects that are detectable by an ultrasonic measurement device. This happens at a stage that in most cases only allows replacement as a repair action. This is very undesirable for the lifecycle of the rail and could have been prohibited by an earlier maintenance action. These are the defects and the most important technical causes of the defects that lead to a rail breach and therefore to the hazardous event of derailment.
It is recommended to make the defects, and their location of occurrence, predictable. This is possible through data analysis, ProRail already collects a lot of data but this data is not yet utilized to its full potential. The com-pleteness of this data is not guaranteed. A benefit of an adequate defect prediction is that the maintenance actions can be carried out only in situations where needed instead of on the whole rail network. This not only decreases expenditures on maintenance but also increases the life cycle of the rail. Furthermore, it is recom-mended to use extensions such as fuzzy numbers and Markov techniques to make the FT model for rail defects more accurate to enable evaluation of different inspection and maintenance strategies simultaneously.