Start by deciding what will not stop
A refurbishment plan starts not with the list of works but with how long each part of production can be down. Some lines cannot stop at all; others can stop between shifts or at the weekend. A programme drawn without that map does not survive contact with the site.
For the parts that cannot stop you build a temporary line, a by-pass or a standby supply. These temporary arrangements are the invisible half of the job, the most labour-intensive part, and they need their own duration in the programme.
As-built drawings are rarely up to date
Additions made over the years may never have reached the drawings. Starting a refurbishment with a physical survey closes off most of the later surprises: valve positions, line diameters, and where each line actually goes get verified on site.
The updated schematic produced during that survey is an output as valuable as the refurbishment itself: it is the first document anyone will reach for at the next intervention.
Recommissioning runs in reverse of dismantling
When the shutdown window is tight, the most time is lost at the moment the work ends and the system is being brought back. If the order of energising equipment and opening valves is not written down beforehand, this stage stretches.
A written recommissioning sequence is also a safety document: it records who must not be near a line that is about to be pressurised.
Hot work and confined space permits
Welding in a running plant, or entering a tank, requires a permit. A hot work permit covers clearing combustible material from the area, posting a fire watch, and keeping the area under observation for a period after the work ends.
Confined space entry requires measurement: oxygen level, flammable gas and toxic gas. A tank entered without those readings is a greater risk than the whole refurbishment. These permits are issued by the operator and they take time in the programme.