Critical Mass is a term that refers to the many activities which, even though not on the Critical Path, have a potential to extend beyond the target completion date.

Critical Path activities typically seldom exceed ten percent (10%) of the estimated manhours for a turnaround. Even though critical path activities may be on schedule, insufficient manpower could force the bulk of other activities to be scheduled late in the turnaround, where they eventually become critical path and cause the turnaround to extend.

The jobs involved in critical mass are generally smaller, low priority, miscellaneous work. One interesting difference between activities on the Critical Path and those comprising a Critical Mass is that Critical Path activities must be executed (completed) as scheduled, in order to achieve the target completion date. Critical Mass activities, on the other hand, generally have plenty of float (slack time) and can be executed just about any time (in any order). Typical examples of potential Critical Mass activities are:

• piping installation
• valve replacement
• required insulation repair/installation
• etc.

It doesn’t matter which particular spool, valve or insulation is being installed as long as reasonable progress is achieved every day so that the available manpower will be able to finish all of the activities before they run out of float (slack time). Critical mass develops when the rate of progress is insufficient to allow existing manpower to complete the non-critical path work before the critical path is complete.

There are many possible reasons for an insufficient the rate of progress:

• insufficient manpower staffing
• increased scope (add-on work)
• delays caused by missing tools or materials
• equipment failure
• bad weather
• etc.

Rescheduling (reassigning) manpower, increasing manpower staffing or cutting the scope can prevent or solve a critical mass problem from extending the turnaround.