May 26, 2023

External Early Start Date and External Late Finish Date Constraints


This is the first blog post in a four-part series about external early start date and external late finish date constraints. This post discusses the interdependencies between independent work schedules on multi-prime projects.

On large, multi-prime projects, it is common for independent work schedules to be linked to other independent work schedules. These schedule-to-schedule interrelationships typically represent the logical handover or completion of predecessor work that is needed to start or finish subsequent work scopes.

For example, a site development work schedule could start and finish independently from the start and finish of a parallel engineering, procurement, and construction (EPC) work schedule, and the timely start of EPC work activities may depend on the start or finish of site development work activities. In Figure 1, Project ABC represents an independent site development work schedule compared to Project JKL, which is a separate work schedule for EPC.

Figure 1: Independent Work Schedule Link between Site Development and EPC

The work activity B3020, “EPC Activity L,” within Project JKL, is linked by a driving predecessor finish-to-start logical link with a zero-day lag (FS 0) from the site development work activity A1010, “SD Activity B,” in Project ABC. Therefore, the EPC activity B3020’s forecasted start date, finish date, and criticality are driven by the site development work activity A1010.

In another example, an EPC work activity within the Project JKL work schedule has a driving successor logical connection to another separate and independent work schedule. Figure 2 illustrates the linkage from Project JKL to Project XYZ, involving commissioning and startup work performed by the independent contractor XYZ.

Figure 2: Independent Work Schedule Link between EPC and Commissioning and Startup

The EPC work activity B3020 has a driving successor finish-to-start logic link with a zero-day lag (FS 0) to the commissioning/startup work activity R1020, “C/S Activity T,” in Project XYZ. Activity R1020’s forecasted start date, finish date, and criticality are driven by the EPC work activity B3020.

However, when individually exporting the JKL work schedule into a backup P6 *.XER data file using Primavera P6 Professional 21 software,1 these schedule-to-schedule interrelationships between Project ABC and Project XYZ are lost. The Primavera P6 software converts these missing links into external early start constraint dates or external late finish constraint dates. Understanding and monitoring these interdependencies between independent work schedules is essential on multi-prime projects.

1    A *.XER file is an electronic data file created by the Primavera P6 software, or P6 program, that allows for the exchange of schedule data among users. *.XER files are used to export or import work schedule data into the P6 program.


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