A.J.B. van Roermund
Improving and measuring warehouse efficiency.
Report 2002.LT.5595, Transport Technology, Logistic Engineering.
Running a successful warehouse today is a great challenge. The
ever-increasing requirements have placed a tremendous emphasis on the
ability to establish smooth and efficient logistic operations. Order
picking, the activity by which a number of goods are retrieved from a
warehousing system to satisfy a number of customer orders, is an essential
link in the supply chain and is the major cost component of warehousing.
The critical issue is to simultaneously reduce the cost and increase the
speed of the order picking activity. This essay's main objective is to examine
the possibilities of optimising the order picking system. This can be achieved
by choosing the optimal storage strategy, by choosing the optimal routing
procedure, or by optimising the warehouse layout. Since performance
measurement is an important aid to making decisions, methods for measuring
warehouse performance are discussed in this essay.
A storage strategy assigns a stock-keeping unit to a specific location in
the warehouse. Roughly, random and flow-based storage strategies can be
distinguished. Random strategy appears especially useful when the range of
products is too large for the number of product locations. The
disadvantage is the possibility of large travel times from having to
traverse the entire warehouse. For flow-based storage, fast moving
products are stored at pick height and at the front end of the warehouse
closest to the depot to minimize picker travel. However, aisle congestion and
an unbalanced utilization of the warehouse can be the result. In practice
however, many warehouses use random storage.
Routing policies determine the sequence in which the items are picked on
an order picking tour. Roughly, routing heuristics and optimal routing
procedures can be distinguished. Optimal routing procedures generate
routes that are as short as possible; heuristics create a route with an
easy-to-understand structure. Experiments show that the solution gap
between routing heuristics and optimal routing is highly dependent on the
storage policy and on the number of picks. Overall the savings of an
often-confusing optimal routing procedure are not as large as one might expect,
especially not for larger pick list sizes. Managers must analyse the trade-off
between the efficiency of the optimal routine and the ease of
implementation and use of heuristic procedure.
For optimising the warehouse layout, a choice has to be made between a
one-block, and a multiple-block warehouse layout. Experiments, testing the
routing policies in these two layout alternatives, show that for all pick
list sizes the layout with a middle aisle gives considerable lower average
travel times for the order pickers. As an alternative for these
experiments, a formula can be obtained to compute the average travel
distance in a multiple-block warehouse. The results from the formula
follow the results of the simulation very closely, therefore the formula can be
used to determine the average route length, instead of developing a simulation
model for the order pick area. The location of the depot also influences
the average travel time. From results of simulation, its clear that a
depot in the middle of the front cross aisle is the best option.
Performance measurement is an important aid to making judgements and to
making decisions. In order to compare and measure the level of efficiency
in production systems, the so-called "zero-based analysis" can
be used. The basic idea of this analysis is to divide the resource
consumption into three parts: the resource consumption in an ideal
production system without waste of any kind, loss inefficiencies and
system costs. The benefits of such an analysis are that value-adding
activities are focused on and losses can be directly compared. Another measuring
technique is time study. Time study is the direct observation of work with a
time standard for the work being derived by converting the observed time
through performance rating of the worker carrying out the work.
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