- Estimating Man hour based on Stress Packages: Normal practice is to estimate the number of stress packages from available P&ID, then multiply the same by some constant value (decided based on experience). Few organisation divide stress packages in three groups, highly critical, moderately critical and lower critical. Lets for example if for a project 250 stress packages are estimated out of which 20 are highly critical, 100 are moderately critical and rest 130 are lower critical then manhour estimates will be =1.2 X (20×50+100×40+130×30)=10680 considering 50 man hour for a highly critical package, 40 man hour for a moderately critical stress package and 30 man hour for lower critical stress package. The total value is increased by 20% as data is not firm.
- Estimating man hour based on Stress Critical Lines: In this method the total number of stress critical lines are estimated from existing P&ID. Then that value is multiplied by a factor (Normally 12 to 18 depending on experience of existing organization). As for example is the project mentioned above contains 4 stress critical lines (you have to calculate critical lines from P&ID) on an average the total number of stress critical lines are 250×4=1000. So total man hour estimated would be approximately 1000×12=12000.
- Estimating man hour based on Critical equipment: This method is somewhat similar to the method mentioned in point no 1. In this method total number of equipments are calculated from P&ID. All equipments are then categorized in three groups-Highly critical (Turbines, Compressors, High temperature Reactors, Furnaces etc), Moderately critical (Air Fin Cooler, Pumps, High temperature Columns and Heat exchangers etc) and Lower Critical (Low temperature Columns, Heat exchangers etc). Then the number of equipments are multiplied by same predecided (based on experience) factors to get the total man hour.
In any of the above situation the following input documents are required at a minimum:
- P&ID (If not available then man hour estimation can be done based on preliminary P&ID or FEED P&ID)
- Line List if available
- Equipment list if available
However in my opinion, one should look into the exact scope of working (activity wise) by the stress department. Then one should consider reasonable man hour (which normally takes based on experience) for each activity. And finally add all the man hours of each activity together to get the final man hour. This way one could get the actual man hour close to actual. The first step will be to list out all the activities in stress scope. As for example from initial start of any project broadly following stress activities has to be considered:
1. Study of Project Specifications and bid documents
2. Work Instruction and Specification preparation from bid documents if required
3. Rack Loading (Three or Four Stages)
4. Stress System definition from P&ID
5. Master Critical Line list preparation (Will be updated 3 to 4 times during project tenure)
6. Preliminary stress analysis before receipt of vendor equipment data (without proper documentation)
7. Final stress analysis after receipt of vendor equipment data (With proper documentation)
8. Spring datasheet and Spring Index preparation
9. Special support design
10. Datasheet preparation for miscellaneous items like expansion joint, sway brace, strut etc
11. Trunnion calculation
12. Review of Stress systems
13. Vendor communication
14. Interdepartmental communication with Civil, Mechanical and Process department
15. TBE of Spring and Special items
16. Job close out report preparation
17. Support Checking of critical lines
18. Support checking of Non critical line
19. Isometric review and issue
20. Other miscellaneous activities which are not listed above
After listing all such activities one need to decide actual time factors for each activity based on experience and then finally combine all those to get the final man hour. Request you to share your views or any other methods which will add value to the above article.