The productivity factor on live plants
Shutdown windows, permits and congestion. What a brownfield tie-in does to installation norms, and how to build the factor area by area.
Treat the shutdown as its own project
The work inside a turnaround window needs its own estimate, schedule and risk model, separate from the months of preparation around it.
As-builts you cannot trust
Every brownfield estimate leans on drawings of what is already there. What to do when they are wrong, and how to price not knowing.