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Critical Chain/Buffer Management: Inserting buffers in a resource-constrained schedule

The Critical Chain/Buffer Management (CC/BM) approach uses project and feeding buffers to add safety time to the project baseline schedule and to guarantee the timely completion of the project with a high probability. Inserting buffers into the project schedule creates a buffered project baseline schedule that can act as a tool to measure performance and provides dashboards (i.e. the buffers) that need to be monitored to trigger corrective actions. In this article, the insertion of buffers is discussed and potential problems caused by activity shifts due to the time buffers are highlighted.

Buffer insertion
The project and the feeding buffers are treated as extra activities without predefined resource requirements and must be placed in the schedule to act as a protection for potential delays. The buffers are placed at the end of a chain for which the buffer is created, leading to an increase in the project duration compared to the unbuffered project schedule.
Figure 1 shows the buffered project network (left) and a corresponding schedule (right) with the time buffer places indicated in the schedule based on the example discussed in “Critical Chain/Buffer Management: Adding buffers to a project schedule”. The numbers above each node in the project network are used to refer to the aggressive activity durations while the label below the node refers to a renewable resource that is required to perform the activity. The renewable resources A, B, C, D and F have an availability of one, while the renewable resource E availability is restricted to two units. The schedule contains three time buffers and one resource buffer. One project buffer is added to protect the critical chain S - 2 - 6 - 8 - E and two feeding buffers FB4-6 and FB7-8 are added to protect the feeding chains 1 - 4 and 3 - 5 - 7, respectively. The resource buffer RB is added to assure that resource B will be available on time to start with activity 6.
?Figure 1. A buffered project network and a schedule with the buffer places indicated
Each buffer needs to be sized according to a predefined sizing method (see “Critical Chain/Buffer Management: Sizing project and feeding buffers”) and will therefore consume time. In figure 2, the buffered schedule is displayed using the cut and paste method (see “Sizing CC/BM buffers: The cut and paste method”) to determine the size of the buffers. Obviously, the buffered schedule has a longer duration (18 time units) than the unbuffered schedule displayed in figure 1 (12 time units). Since the resource buffer is nothing more than a warning signal and does not consume time, it has not been displayed in the buffered schedule.
Figure 2. A resource feasible buffered project baseline schedule using the cut and paste method?
Since activity durations are set to aggressive values (see “Aggressive activity time estimates: protecting against activity delays”), these buffers act as a protection to changes in activity durations and should therefore be able to absorb potential delays.
Resource conflicts
Although the insertion of the buffers in the baseline schedule looks like a straightforward task, this insertion is often a cumbersome task which leads to problems that harm the general philosophy of critical chain/buffer management. It is indeed not always easy to insert buffers in the schedule due to the shifts in activities. These activity shifts can lead to resource overallocations, known as resource conflicts (see “The critical path or the critical chain?: The difference caused by resources”) as illustrated in figure 3. In this figure, buffers are sized according to the adaptive procedure with density (see ”Sizing CC/BM buffers: The adaptive procedure with density”) resulting in the following buffer sizes:
  • Buffer size PB = 3.10
  • Buffer size FB4-6 = 1.07
  • Buffer size FB7-8 = 2.03
The project duration of the buffered schedule is equal to 15.1 time units (e.g. days) but shows a resource conflict for resource B. Since the availability of this resource is equal to 1 and the resource is used by both activities 5 and 6, there is a time period where resource B is used twice. This resource conflict caused by the shift of activity 5 leads to a buffered schedule which is no longer resource feasible.
?Figure 3. A buffered project baseline schedule using the adaptive procedure with density causing resource conflicts
Currently, there is no standardized approach available to cope with these resource conflicts generated by the insertion of the buffers. In ”Inserting buffers in a schedule: the problem with resource conflicts”), various approaches to solve these resource conflicts are discussed.

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