Project MBTC 2017

Final Report

Dr. Richard Cassady

Dr. Darin Nutter

Chet-Tuck Wong

January 2002

University of Arkansas

EXECUTIVE SUMMARY

The distribution of fresh and frozen foods requires the use of refrigerated trailers. In addition, Reliability and Maintainability (RAM) is an important issue in the operation of refrigerated trailer fleets. Often, as trailers age, their reliability decreases. This study explores the optimization of refrigerated trailer retirement and job assignment under consideration of container aging and usage. By achieving this objective, Tyson and other organizations that operate similar refrigerated transportation systems know when to retire the trailers and how to assign trailer duty. Also, a better understanding of RAM performance of refrigerated trailer fleets is obtained.

We began by collecting maintenance history for 195 trailers. The data covers the period January 1, 1994 to March 2, 2001. We categorized the trailers as a series system comprised of five major subsystems: refrigeration, engine, tire, wheel assembly, and structure. Next, from the maintenance history data, time between failure data for each subsystem for each trailer was collected.

Given the time between failure data, we used the Weibull ++ software package and maximum likelihood estimation to model the reliability performance of refrigerated trailers. Where appropriate consecutive failure numbers (1^st failure, 2^nd failure, …) were combined into a single probability distribution model. For each set of failure numbers, either a Weibull or exponential probability distribution was fitted to the data.

Finally, a discrete-event simulation model was developed and used to evaluate Tyson’s trailer retirement policy and trailer duty. The trailer retirement policy analysis was based on total maintenance costs, salvage value, and purchase costs for a trailer. Results show that the total annual cost is minimized if the trailer is retired after 7 years of service. Retirement policies 8 years and beyond were not considered in this research because the probability distributions used to model trailer reliability was limited to the 7-year data collection period. In the trailer duty analysis, analysis tables were created to be used as a guideline for the fleet manager to compare trailers of any age based on total maintenance costs and the total number of failures. Also, using the raw data, the actual number of trailers in each percentile was created. The two analysis tables and the actual number of trailer in each percentile table are provided as shown below.

Expected Life-to-Date Total Number of Failures for Given Percentile

Year	Percentile
Year	1%	5%	10%	25%	50%	75%	90%	95%