BNSF Tests Risk-Based Ultrasonic Detection
Date
2001-02
Journal Title
Journal ISSN
Volume Title
Publisher
Railway Track & Structures Magazine
Abstract
As the railroad industry continues its focus on increased safety, rail defects and resulting rail caused derailments, have become an important area of interest. Analysis of
FRA statistics on reportable mainline derailments, attributed to rail defects, shows an overall
industry increase in reported derailments per billion gross ton mile (BGTM) of 4% in the
period 1997 to 1999 [1,2]. This trend is illustrated in Figure 1 together with the
corresponding increase in the average cost of rail related derailments for that same time
period of more than 40%.
Examining the FRA derailment data further, Figure 2 shows the distribution of
derailments by rail defect type along with the average cost of derailment for that defect type.
This figure clearly shows that the most predominant cause of rail related derailments is the
transverse defect or TD class of defects, with the Detail Fracture (DF) representing the
second most common cause of rail related derailments. For the entire range of rail defects,
the average derailment cost varied from $200,000 to $1,400,000 depending on defect type,
with an overall average of the order of $400,000. Note, this is FRA reportable cost only, the
actual cost of the derailment, which could include loss of lading, train delays, or train
rerouting, can be double that amount.
This increasing trend in rail related derailments suggests that there is a need for
improved rail maintenance and/or inspection practices to prevent the occurrence of these
defects or to find the defects before they cause these expensive derailments. These
improvements can take several forms, to include more aggressive rail replacement or
maintenance practices or improved rail testing equipment. However, the focus of this article
is on a easier to implement approach, one that can be applied almost immediately with a
relatively modest impact on a railroad’s maintenance of way budget, specifically the
improvement in the scheduling of conventional rail test equipment.
As rail accumulates tonnage, it tends to develop more internal fatigue defects, based
on various factors such as metallurgy of the rail, traffic (to include such factors as axle
loading and speed), track support conditions, etc. This behavior is illustrated in Figure 3. As
defects occur more frequently, it becomes important to test more frequently in order to insure that internal defects can be located and replaced before they have the opportunity to
propagate to failure, and possibly result in a derailment. Earlier studies have indicated that
approximately 1.3 derailments occur per thousand defects (detected plus service), thus
highlighting the importance of matching test frequency to the rate of defect occurrence [3].
Simplistic rail test scheduling approaches, such as those based on annual tonnage
levels, which do not account for aging rails and corresponding increased defects, do not give
the railroad the flexibility to adjust test frequency to the actual rail conditions encountered.
Likewise, simplified “rules of thumb” for scheduling ultrasonic testing, while often
accounting for such factors as age of rail (usually in cumulative MGT) annual traffic density,
class of track, type of traffic, defect counts, etc., do not do so in a manner that is directly tied
into the “risk” of a derailment occurring. Rather, it is necessary to have a risk based
scheduling methodology which makes use of site specific and directly measurable
performance parameters that, in turn, can be related to a defined level of risk. Such a
methodology was developed by US Department of Transportation Center Volpe National
Transportation Systems Center [4], and further enhanced by ZETA-TECH Associates, Inc.
[5].
Description
Keywords
Rail defect, Derailment
Citation
Palese, J.W., & Zarembski, A.M., “BNSF Tests Risk-Based Ultrasonic Detection”, published Railway Track & Structures Magazine, February 2001.