Compressing a Substation Energization Schedule
You are the project controls engineer for a 33/11 kV substation upgrade. The transformer supplier has confirmed a 10-week delivery, and the client wants the original energization date restored after a six-week delay. The approved schedule has these activities and durations: A permits/outages, 3 weeks, no predecessor; B civil foundations, 6 weeks, after A; C transformer procurement/delivery, 10 weeks, after A; D switchgear installation, 5 weeks, after B and C; E protection and control wiring, 4 weeks, after D; F HV cable termination, 3 weeks, after D; G commissioning, 2 weeks, after E and F. The contractor has priced acceleration as follows: C can be reduced by up to 2 weeks for $70,000 for the first week and $80,000 for the second; D by up to 2 weeks at $45,000 per week; E by up to 2 weeks at $35,000 per week; F by 1 week at $25,000; B by up to 3 weeks at $20,000 per week but has no effect on the current critical path. All acceleration requires a written change order before work starts.
1. Determine the critical path, project duration, and total float of activities B and F, showing the forward pass.
Read the worked answer
Using early start/early finish: A: ES 0, EF 3. B: ES 3, EF 9. C: ES 3, EF 13. D: ES = max(B EF 9, C EF 13) = 13, EF 18. E: ES 18, EF 22. F: ES 18, EF 21. G: ES = max(E EF 22, F EF 21) = 22, EF 24.
Project duration = 24 weeks. The critical path is A–C–D–E–G, because it has zero total float.
Activity B: it must finish by D’s start at week 13; it finishes at week 9, so total float = 13 − 9 = 4 weeks. Activity F: it must finish by G’s start at week 22; it finishes at week 21, so total float = 1 week. These floats are why accelerating B does not shorten the project unless the critical path changes and B becomes critical. F has only one week of float, so it can become critical quickly once D or E is compressed.
2. Find the least-cost plan to recover six weeks, showing the sequence and the resulting critical path. Should B be accelerated?
Read the worked answer
Work from the critical path and re-check after each compression. The cheapest critical activity initially is E at $35k/week, but E cannot be compressed indefinitely without making F critical because F is 3 weeks against E’s 4 weeks.
A least-cost six-week plan:
- Compress E by 2 weeks ($70k). This reduces the E path by 2 weeks but, with F still 3 weeks, F becomes critical after the first week; the second E week is only fully effective once F is also compressed.
- Compress F by 1 week ($25k). With E at 2 weeks and F at 2 weeks, both paths through D are balanced.
- Compress D by 2 weeks ($90k). D is common to both the E and F paths, so this reduces both.
- Compress C by 2 weeks ($150k). C is common to all paths and reduces the remaining duration.
Total acceleration cost = 70 + 25 + 90 + 150 = $335,000. Resulting durations: A 3 + C 8 = 11; D 3 starts at 11 and finishes at 14; E 2 and F 2 both finish at 16; G 2 finishes at 18. The project is recovered from 24 to 18 weeks. The new critical paths are A–C–D–E–G and A–C–D–F–G, both at 18 weeks.
B should not be accelerated. It still finishes at week 9 against D’s start at week 11, leaving 2 weeks of float. Spending $20k/week on B would add cost without reducing the project duration. Keep B’s float as schedule contingency rather than consuming it.
3. What contractual and risk-management steps should precede authorizing this acceleration?
Read the worked answer
First establish entitlement and authority. The client’s request for a six-week recovery is not automatically a compensable instruction; check the contract for acceleration, variation, and time-bar provisions. Issue a written request for proposal setting out the required date, the proposed measures, the $335k cost, the revised schedule, and the effect on quality and commissioning. Do not start acceleration before a signed change order or written direction, or the contractor risks a constructive-acceleration claim.
Second, preserve the schedule basis. Update the critical path, resource histogram, and procurement plan; confirm that the transformer supplier can actually deliver in 8 weeks rather than merely quoting it. Compressing C may increase factory-testing risk; require witness testing and hold points. Compressing D and E may require additional crews and overtime, which raises quality and safety risks; check supervision, permits, and fatigue management.
Third, protect commissioning. G remains 2 weeks and should not be compressed without the utility’s agreement and a documented test plan. Notify the utility of the revised energization date and confirm outage windows. Record the basis of the acceleration, the client’s acceptance of residual risk, and the cost/time impact in the change register. If the six-week target cannot be met without unacceptable quality risk, say so in writing rather than accepting an undeliverable plan.