Critical mass duration for process outages
Avoid this surprise when determining how long your next process outage will take.
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Continuous processing or manufacturing facilities will occasionally require planned outages to primarily address reliability or maintenance issues. Due to the high cost of these outages (opportunity and real), careful planning and scheduling practices are used to come up with detailed execution plans to reduce the planned duration as much as possible. While outage duration is typically determined using the critical path methodology, large quantities of smaller but similar in nature jobs can lead to unexpected schedule extension. A planning team recently applied this lesson-learned by calculating the schedule impact of such critical mass work.
Importance of Determining a Realistic Duration
Almost all continuous processing or manufacturing industries will require some form of planned production outage at a regular interval. These are typically required to perform inspection and maintenance work but can also be for equipment upgrades or process changes. This work is more often driven by safety, environmental (regulatory), equipment reliability or production quality considerations, among others.
The basic economics behind these industries result in the outages to adversely affect the ‘bottom-line’. In some cases, the opportunity cost (such as, buy vs. make decisions) can be in the millions of dollars per day. Add to this the actual cost of doing the work during the outage – can also be millions of dollars per day – and you end up with the strongest case for not only reducing the outage’s duration to its minimum but also ensuring that it is a realistic one.
In most cases, lost production due to the outage must be made up by either pre-building inventories, securing purchases from other locations or third-parties or reducing shipments to customers.
While never experienced by this author, the same reasoning could be applied to outages of continuous services. Examples that come to mind include major software changes, relocation of offices or medical services, and so on.
A few years ago, a planning team put in place a simple methodology to ‘test’ an outage’s duration against a large amount of similar work, none of which individually determined the overall duration. This article will describe how this was successfully done.
Planning vs Scheduling
Many papers and industry documents are available to describe outage planning and scheduling best-practices. We will not attempt to duplicate these here but the planning team rigorously applied the following:
• All activities were planned. This included all production slowdown and shutdown, decommissioning, quality control, recommissioning and start-up steps
• Simply put, the planning work covered who (how many) is going to do what and how, for how long and with what resources, e.g., tools
Once this was largely completed, the team then proceeded to logically connect the plans, identifying predecessor, successor and parallel steps. This was deliberately done starting when the production ended to return to service “on spec”, including any necessary production ramp-up.
Using a specialised software, the team could then evaluate the work and determine its minimum duration
Critical Path Duration
Here again, much has been written on determining critical path (CP) duration of a logically-connected series of steps. The planning team, as a first pass, used this technique to calculate the outage duration.A quick recap: CP is the sequence of dependent tasks that form the longest duration, allowing you to determine the most efficient timeline possible to complete a project. In the simple example shown in Figure 1, we can quickly determine that:
• The duration for the series of activities following the top path is 3 days,
• The middle one takes only 1 day,
• The bottom path (with red arrows) adds up to 4 days
Therefore, the critical path duration is 4 days.
The team’s next step was to check the sequence of steps for its integrity and for any inherent execution risks.
As before, the team made use of some specialised software to accomplish this. They were specifically looking for
• Open, incomplete or missing logic
• Work sequence conflicts such as circular logic
They were also able to build-in any uncertainty to individual activity’s duration. This allowed them to come up with a probability curve for the overall event’s duration – expressed as the probability of completing the outage in ‘x’ days
In addition to determining the CP sequence of steps, the team identified all the near-CP work, in this case, all work sequences that are expected to be completed within 10% of the CP duration.
Throughout this important work, several optimisation undertakings were completed, using SMED or equivalent techniques.
Outcome of Planning and Scheduling Work
In the end a final schedule is produced, showing the optimal sequence of all activities, as well as the definitive expected outage duration.
Using the CP methodology, the expectation is that there is a single series of logically connected activities that dominantly determines the overall duration.
As in our case, readers that are familiar with the petroleum refining or petrochemical industries will recognise that the CP duration normally ‘travels’ through large process fixed equipment (distillation towers, reactors, separator drums, etc.) or major machinery trains, e.g. large compressors.
In this particular situation, the planning team suspected there could be more …
In addition to the significant and dominant work during the outage, there is typically a fairly large quantity of work of similar equipment type using the same worker trade. We called these bulk-type work.
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