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Calculated time vs duration

6 replies [Last post]
floris gering
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Posts: 24

I have for example a safety valve which has to be replaced and calculated the disassembly and assembly time for example to be 2 hours.

My colleague scheduler says he always sets the duration for this activity to 10 hours (in this case the length of a shift), because he is only interested in the fact that the work on the safety valve is done on a specific day during this shift and not exactly in which 2 hours it is done.

Is this good practice ? Why ? (or why not)


Rafael Davila
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Too easy, why not?

  • The replacement of 15 valves with no logical ties between them.
  • The replacement of 5 of them done in one Monday shift, in batches of five by 1 worker, the 15 will take 3 Mondays.

Essentially same rule, to replace in batches of 5 but 15 valves, not a big deal, easy if using consumable resources leveling for the milestones. 


In the following schenario the second milestone skips one week in order to have the required five for a full batch.

More interesting is the following.


Good Luck 

Include in your schedule a milestone with the special calendar - can happen only on Monday before the start of the regular work day.

Set FS dependnecies from this milestone to all five valve replacement activities.

If you have only one worker that can do this work resource leveling will schedule all these activities on Monday one after another if other dependencies do not prevent this.

floris gering
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Joined: 30 Jul 2014
Posts: 24

Thanks for these insights.

I'm still confused:

Let me try to describe my problem with understanding this for most people "easy question":

Basic data:

Calender: Mon-Sat, 06:00-16:00 so 10 hours.

5 Safetyvalves to be replaced, each calculated within a normset to 2 hours, so budgeted units in P6 = 2. With 1 Budgeted Unit/Time we get a Original Duration of 2 hours per SV.

I'm not interested in a specific sequence of the replacement of these 5 valves, so there should be no logical ties between the 5 of them, correct ?

I AM interested in getting this replacement of all 5 of them done in one shift, lets say the Monday., by 1 worker.

What is the best approach to schedule this and why ?

Zoltan Palffy
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if the planning units is in hours then show the 2 hours because if you show 10 hours as others have stated it will over alloate the resource and it will also prevent follwo on work from starting sooner. 

bad practice

replace safety valve #1 - 10 hours-----> replace safery valves #2 - 10 hours = total 20 hours 2 valves replaced

good practice

replace safety valve #1 - 2 hours-----> replace safert valves #2 - 2 hours -------> replace safety valve #3 - 2 hours-----> 

replace safety valve #4 - 2 hours-----> replace safert valves #5 - 2 hours -------> = total 10 hours 5 valves replaced


now if your planning unit was days then the smallest unit of time that you can plan for is 1 day regarless if it takes 2 hours or 10 hours to do it is still 1 day. 


Steven Auld
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I agree with Vladimir, each activities duration should be based on how long it is estimated to take.

If you allocate 10 hours to 1 safety valve that takes only 2 hours to complete, then the Resource availability will no longer show as available for other activities.

If you have 5 safety valves to overhaul then by allocating only the required 2 hours per safety valve, then all could be completed within a 10 hour shift.

If you have allocated 10 hours then this would mean that you would not show sufficient resource availability & would drive the duration out to 5 shifts to complete this scope of work.

In the shutdown plans that I work on, resource availability is the single critical driver for the shutdown duration due to limited bed space availability. To incorrectly allocate durations would have a significant impact on the overall shutdown duration & would also reduce the amount of work that could be completed wihtin the Critical Path duration.



No, it is not best practice.

Best practice includes assigning resources to project activities and calculation of resource constrained schedule based on volumes of work to be done and assigned resource productivities. Activity duration shall be calculated and resources optimized.

Activity duration does not depend on assigned resources only for few activity types.

Duration based schedules may be used only if the job will be done by someone else and the schedule is used for contract execution analysis.

Shutdown projects are usually planned at least in hours (better in minutes) because delays cost a lot. Good schedule optimizes resource usage and minimizes project duration.