Page 311 - Orlicky's Material Requirements Planning

290 PART 3 Managing with the MRP System

questions for this environment are, “How is my project doing?” and “What is the item
that is extending the critical path, and if it is reduced, what is the next item?” These ques-
tions are very difficult to answer with standard MRP functionality. Use of a different
approach to project scheduling, critical-chain scheduling, for a project provides improved
visibility of lead time and other resource buffers. Tasks are scheduled with realistic
expected times, and the buffers are moved to the appropriate spot in the critical chain.
If traditional project planning is used, with slack-time potential at each step except
those on the critical path, then decisions must be made for planning material. The usual
strategy is to order the material at lead time prior to the early start date. The advantage is
that if the preceding task is completed early, the materials will be ready, so the project can
continue. The disadvantage is that critical cash resources may be expended before they real-
ly need to be. Also, when materials have to sit around for a long time, they tend to get lost.

CAPACITY DEPLOYMENT

Capacity deployment in the project manufacturing environment also can be a challenge.
The normal response is to attempt to have all projects in work simultaneously. This allows
personnel to easily move from one project to another when a problem arises. The down-
side of this approach is that typically projects will take much longer, and the company
may develop another constraint, such as space or cash. To demonstrate this point, Hal
Mather presents a wonderful riddle on how to deploy cranes in a ship unloading exercise.
Assume that you have six ships. They will arrive in port at exactly the same time at six dif-
ferent piers. You own six movable cranes that are each capable of unloading a single ship
in six days. If you put two cranes on one ship, then that ship can be unloaded in three days
while the other ship waits. If you are in the shipping business and you make money by
having ships on the sea and not in port, how should you deploy your cranes? Assume no
interference between the cranes. The traditional approach to project management is to
have all six ships in work at the same time. All projects would have some resource sprin-
kled on it. This is rather like trying to play poker with 52 people. Each person gets one
card, and nobody can get a winning hand. The enterprise as a whole loses. Back to the
ships, assuming that you make money by having ships on the water, your desire is to min-
imize the amount of time the ships are in port. Here are the options available:

Turn Days for Each Ship
Average
Cranes/ship Ship 1 Ship 2 Ship 3 Ship 4 Ship 5 Ship 6 Days in port
1 6 6 6 6 6 6 6
2 3 6 3 6 3 6 4.5
3 2 4 6 2 4 6 4
4 1.5 3 4.5 6 3 6 4
5 1.2 2.4 3.6 4.8 6 6 4
6 1 2 3 4 5 6 3.5