hi mohamed

the duration of the project become the sum durations of the critical path if the relations between themare fs only because there is overlab

and if you want to calculate the duration estimate the overlab and it will be o.k

best regards 

mahmoud el shamy

Mehdi

Looking at one of your posts on the activities and the durations, and also related to the critical path problem, you can consider that gobally, the  criticality of pipelines is mainly in resource, and not largely due to physical (activityrelationships). Subject to the budget/commercial constraints, If  there are more crews/teams, more work fronts can be openned up simultaneously.

Also to avoid SS relationships and long durations, split the pipeline lengths into smaller work sections, use natural boundaries, changes in terrains, road crossings, location of change in pipe class and diameters as your guide.

(As an illustration, in a  cross country pipeline project, our largest section was 12km, and the shortest was less than 3000m. We use the P&I Diagrams, Hydrotest Diagrams, and topography sections as guide)

Then, globally, within sections, create  resource sequence logic; example after Sec 01 (kM 000+100 ) (Welding Group Team A) -> move to Sec 05 (Km 090+500); Pipelaying Crew A will work at Sec 03 (KM 012+000) -> move to Sec 04 (Km 021+000). As a took, explore the use of excel to create a time space diagram.

After the above, then for each section, you can now create your detailed programme with relationship of the activities as you have mentioned below.

1– Topography
2 - Construction of the active runway.
3 - Excavation of the trench
4- cladding tubes along the trench

5 - Welding tubes (143 days) - in the example, this activity will commence only after the Team assigned to it as completed previous Section work.

6 - Laying in trench (145 days)
7 – Backfill (128 days)

etc.

I never worked on a truly linear job to see in action the theory on the diagram lines and slope interpretation but because the linear diagram can be a bi-product of a CPM there can be some work-around as to see on your Gantt where the adjustments are needed.

For a couple of activities with SS and FF relationship and short duration use of time lag this can be an acceptable compromise in the absence of volume of volume lag functionality. Even when my software has volume lag, sometimes I use time lags for feeding activities. 

A linear job can be managed without the use of linear diagrams as long as you make the adjustments to reflect the productivity trends.

For when you have many overlapping feeding activities I suggest avoiding the FF/SS link between any two activities by splitting the activities in two. As soon as you see a gap in between the activities then you can revise your planning by adjusting your production rates so the work flows smoothly without many interruptions in the use of resources.  I do not mean filling the gap by always reducing production rates or it would be better to keep the interruptions, you can close the gap by increasing production rates as to keep the job on track.

If you model the above using multiple activities that leapfrog can you imagine the mess for adjusting your plan?

Linear diagrams as well as CPM and resource planning are to be managed, they need constant monitoring and adjustments, it is not about keeping the plan static and observe it did not happened as planned.

I have a preference for Critical Path Methods and most probably in a Rail Metro job I would have thousands of activities that belong to a CPM while maybe a few dozens that belong to a line diagram. In such case I can set for the line diagram to include only those that belong to it and leave out of this diagram the other thousands, to be analyzed within the CPM.

Best regards,

Rafael

If a company has no time space diagram software, then what is the alternative, for horizontal projects?  P6 has limitations with respect to this type of projects. Will you insist to your management to purchase a time space software? Surely, even without time space software, a planner can plan with the tools readily available, like excel and maybe MS project or P6.

I am of knowledge of a metro (rail) project in hongkong, there was no time space diagram software; it was successfully planned and monitored using excel and P6;

Also Mehdi, for pipeline projects (horizontal/distance projects), it is recommended to plan thru a time space diagram programme (TILOS) than P6.

Alternatively, create time space diagram using excel spreadsheet for planning the sequence of activities and crews, PLUS  P6 for monitoring/analysis slippage.

Mehdi,

You cannot finish the construction of the runway without finishing the predecessor activity so a FF link is missing.

The successor activity depend on the actual physical progress of the predecessor not on how much time has passed. This is a volume of work relationship and modeling of lag using time lag will yield wrong modeling of the relationship as time lag will be accrued independently of the volume of realized work. Say after 17 days of topography you have not realized the required volume, say half have been realized then construction of the runway cannot start yet but a model base on time lag will erroneously tell it can. As the work progress the volume lag is still active and therefore must still exist such dependency, time lags after the initial specified time elapses are no longer active. Volume lags will always be active and will finish after the volume of work reported finishes.

I believe it would be a better model if you use volume of work activities with lag driven by resource productivity a more complex relationship but closer to reality. Not modeled in this way to keep it simple, of course you can have duration type activities linked with volume lag if this is your real need.

If you cannot model this scenario using volume lags at least make it less far than reality by adding the required FF links using the limited functionality of time lag, not good enough but not as bad as missing these links.

It is a phallacy there are perfect models, some are better than others, closer to reality while others are very far. Take your best available option.

Best regards,

Rafael

Hi Mehdi

That is correct - you will only get a true critical path using FS links.

When you think about it a SS lead lag link is a FS link disguised because some part of the predecessor has to be finished before the succesor can start - it is false and lazy planning and should be banned.

Best regards

Mike Testro

Mehdi

That quote works for correctly placed tasks with FS links only.

SS links with lags is not a true critical path - particularly when there are no successor links.

Add up the lags and you will probably get 119 days.

Best regards

Mike Testro