Summary Progress Curves
Progress curves are used extensively throughout the industry as an indicator to detect if the rate of progress is satisfactory to achieve the desired end date. However, there are many different methods for generating progress curves. Which one yields the most satisfactory results for managing turnarounds?
Early Start Curve ("Scheduled Start")
An early start curve is calculated by totaling the manhours scheduled from every task’s early start (ES) to early finish (EF) for every reporting period (or shift in the case of a turnaround). Fixing progress targets upon a baseline schedule is problematic because the scope for a turnaround will fluctuate. The progress targets will become meaningless as repair work is added to the scope. A more dynamic approach is to calculate progress targets based upon the remaining schedule, but this requires using completed work as the starting point (and therefore does not offer any valid means to measure past performance against meaningful targets). Managers need to maintain a history of these reports to analyze past performance with scope fluctuations in mind. This is less than ideal for quick, objective progress analysis.
Where the scope is fairly static, early start curves will tend to be aggressive (or front loaded). Actual progress will almost always appear to be insufficient, because it plots below this curve. It is not possible to know for sure if progress is really insufficient or satisfactory.
Late Start Curve
A late start curve is calculated by totaling the manhours scheduled from every task’s late start (LS) to late finish (LF) for every reporting period (or shift in the case of a turnaround). This method produces curves that are back end loaded. Actual progress will almost always appear to be satisfactory early in the turnaround/project, but quickly fall behind at the end (when the curve becomes aggressive). The problems with scope fluctuations will not be reflected on the curve (when calculated in a dynamic fashion as outlined above) until late in the turnaround.
Resource Leveled Curve
If an early start curve were calculated on a resource leveled schedule, the curve should be almost flat (or linear). However, historical studies show that progress is never linear with time - especially in turnarounds where fatigue is a very real factor. A leveled curve will be too relaxed during the first half of a turnaround (when productivity is greatest) and too aggressive at the end (when productivity is the worst). Problems with scope fluctuations are minimized with a resource leveled schedule, as additional work should not affect progress targets unless the span of time (critical path) changes. On average, the leveled resource curve should produce better results than the straight early start and late start curves, but it is still unsatisfactory for measuring progress in a turnaround.
Statistical (Minimum Attainment) Curve
A statistical (minimum attainment) curve is calculated from analyzing the progress attainment patterns of many successful turnarounds. A least squares curve fitting method can be used to obtain a time based formula for generating an attainment curve for turnarounds spanning any time frame. Statistical (minimum attainment) curves account for both the fluctuations in scope and the effect of fatigue. They offer the best results in allowing management to detect whether the rate of progress is sufficient to achieve the desired completion date.
In 1982, InterPlan Systems calculated the statistical (minimum attainment) progress curve used in ATC Professional based upon the analysis of more than five years worth of successful major turnarounds. The ATC Professional summary progress curve has proven itself as a very effective tool for monitoring overall progress for turnarounds ever since (for over 20 years now).
Authored by Bernard Ertl, Vice President, InterPlan Systems
Bernard Ertl has a bachelor’s degree in Computer Science and extensive field experience planning and managing turnarounds in the oil refining and petrochemical process industries.