Engineering Activities, man - hour estimates ______________
by Andrew C. Whyte, BSc, MSc, CEng, M.I.Mech.E.
© Copyright February 2001, January 2021
Web site: https://acwhyte.droppages.com
This paper provides guidance for young engineers, on completing man-hour estimate sheets.
These sheets are necessary when a breakdown of the engineering activities of a project is required.
The text is written from
an engineering design viewpoint and is based on my pressure vessel experience, but the principles are general
and should be capable of being applied to any engineering activity.
As part of the normal engineering design procedures
it is often necessary to provide an estimate of the man-hours required to carry out
design activities such as: Design Calculations or Stress Analysis.
The cost of a particular design task is directly related to the
man-hours spent on the project. Underestimates of the man-hours run the risk of loss of profit
to the department, and hence the company, while overestimates run the risk of being uncompetitive
and loss of the contract. It is therefore essential to make an accurate estimate of the work content,
in terms of man-hours, as possible.
Unfortunately it requires some experience to make an accurate estimate of
engineering design activities. Design engineers are often asked (pressurised) into making "off the cuff"
estimates by personnel, outwith the design department, who do not have sufficient knowledge of the design process
and so inaccurate estimates are to be expected in these circumstances.
The following guidelines based on SIX simple steps and
the examples presented should help design engineers and others, come to terms
with the scope of engineering activities involved in a design project.
3. Preparation of Engineering Activity Sheets.
The man-hour estimate
is best made by completing an activity sheet(s). A typical sheet is shown in Figure 1 below.
The essential part of the sheet are the columns for: Task No., Engineering Activities and Man-Hours.
Figure 1 Typical man-hour estimate sheet
You can download a copy of Figure 1 as a pdf file [24kB].
The estimate is best carried out in 6 stages or steps as follows:
These are now discussed in detail.
- Step 1 Basic Information
- Step 2 Preliminary Activities
- Step 3 Design Activities
- Step 4 Communication Activities
- Step 5 Covering Memo. or Note
- Step 6 Get Feedback
3.1 Basic Information.
When first asked
to complete a man-hour estimate you will require some basic information on which to
start your estimate. For example if the job is for a potential order for a pressure vessel,
the proposal engineer responsible for the job should supply, as a minimum, the following information:
- The project number and name.
- The clients specification, specification number and revision.
- The clients drawing or design sketch of the vessel and internals if these are to be included.
- The pressure vessel design code and any construction category or division.
- The design pressure and temperature and details of any additional loading.
- The specified material of the vessel shell (and internals if applicable) including the details of any cladding or liner.
- Where the vessel is to be manufactured.
- The final destination of the vessel and if there is likely to be any shipping restrictions.
- Any special hydro testing requirements.
- Any research or development testing requirements.
- When is the contract likely to be placed and are there tight time scales to be met. Has the client imposed a late penalty clause.
Summary of Basic Information Requirements
If the design work is likely to involve any stress, thermal, fatigue or fracture analysis then it is essential
that you have some idea of the final geometry to be analysed and the nature of the loading being applied.
If the geometry or loading is such that "design by rule", to the agreed pressure vessel rules, is not
possible then "design by analysis" might be required. This "design by analysis" could range from some additional checks using simple theory
all the way through to a full 2D or 3D finite element analysis with assessment to the agreed pressure vessel assessment rules.
If a finite element analysis is required this can be a time consuming exercise. It might range from a relatively
simple two dimensional axisymmetric model with pressure loading only and liner-elastic material properties, taking perhaps two or three weeks to analyse
(for an experienced stress analyst). On the other hand a complex
three dimensional model with asymmetric loading and non-liner material properties might be required, taking months of work to analyse.
On these "design by analysis" contracts the design engineer should communicate direct with the client so as to make
sure that sufficient information is obtained, from the client, on the geometry and loading history throughout the service life, so that
a reasonable man-hour estimate can be made.
Once you have got sufficient basic information you are in a position to make a start on completing
the engineering activity sheets. A typical example is shown in Figure 1 above.
3.2 Preliminary Activities.
Get some engineering activity sheets and proceed to fill in the
engineering activity column. You can add comments to the activity column as required. You can allocate a task number or identifier if required.
The task numbering can be arbitrary as long as each task is uniquely identified. Do NOT at this stage allocate any man-hours but get all the activities down first.
The important thing is not to miss out any important activities.
Before you proceed to the design activities there will be some preliminary activities to consider. These are likely to be:
- Client documents. The client will normally provide an outline drawing or design sketch and a specification or other documents relating to the contract. You must allocate a task to allow
these documents to be studied so that a full understanding of the project is obtained. Drawings and specifications are important documents. The specifications are
often large documents and can take many hours to scrutinise. There will usually be at least one meeting with client plus normal communication with the client so that there is
full understanding of the scope of work to be carried out.
- Quality procedures. As part of your company's Quality Management System (possibly to ISO 9001) the departmental procedures will be required to be followed. Tasks will be required for activities such as:
- setting up a contract file system
- maintaining the file system throughout the contract (there may even be an audit)
- preparation and approval of a design quality plan
- preparation and review of design data sheets
- final archiving of the file to store or micro film
Start-up meetings. There will usually be a start-up meeting especially on contracts with a manufacturing content
and there will be normal communication between departments just to get the job underway.
Summary of Preliminary Activities
Once you have got the preliminary activities complete you can add hours to the activities
(or wait to the end before allocating hours). Now you can proceed to the design activities as discussed in section 3.3 below.
3.3 Design Activities.
The engineering activities for the design or analysis
work to be undertaken are now listed. As before do NOT at this stage allocate any man-hours but get all the activities down first.
The important thing is not to miss out any important activities. Every contract is different, the examples given in Appendix A for
a pressure vessel design and Appendix B for a finite element analysis are presented as a guide and the hours shown are indicative.
It should be appreciated
that design activities are often iterative and it is unlikely that a final design will be arrived at, at the first attempt.
This is especially true if there is anything new, novel or "design by committee" is involved.
3.3.1 Design Contract for a Pressure Vessel.
For a pressure vessel design contract
the following design activities, as a minimum, are likely to be required:
- Vessel shell and nozzle scantlings. Design calculations on the main vessel shell and nozzle (branch) reinforcement is required
to meet the applicable code rules for design pressure and temperature.
The amount of work content depends on how many nozzles there are and how close they are to other nozzles or critical areas of the vessel
shell or supports. Some vessels can have interacting nozzles resulting in heavy reinforcement. Other nozzles may be too
close to vessel ends or vessel supports and require repositioning. It is not unusual for the client to demand changes in
the size and/or position of nozzles. Repeat calculations are normal but the use of a computer program greatly reduces the
amount of hours involved. If the vessel is subjected to external pressure additional stiffening checks will be required and out of roundness
of the vessel shell becomes important.
- Local stresses due to nozzle loads. Design calculations are required for the local stresses in the vessel shell due to
nozzle loads from the connected pipework. The client usually
supplies the loads in the form of a table of forces and moments. These can be significant and there have been cases where the vessel shell
has had to be thickened or the loads limited in order to meet design code stress limits. Allow extra hours for the iterative nature of this work.
Tasks are also required for calculating and assessing the stresses in the nozzle necks and equivalent pressure on bolted flanges.
Computer programs are available to calculate and assess the stresses.
- Local stresses due to vessel supports. All vessels have to be supported in some way. The two common ways are: twin saddle supports for horizontal vessels or a skirt support for vertical vessel.
Tasks are requires to calculate and assess the stresses in the vessel.
Computer programs are available to calculate and assess twin saddle stresses. For skirt supports a discontinuity program such as AJAP-1 or finite element programs are available.
- Vessel support structural strength. The structural strength of the vessel supports requires to be checked. The client usually provides a sketch of the supports and anchor bolt positions and sizes.
If this is not the case then allow extra hours to design the support and anchors. If the support requirements are in any way special then it is best to do a separate man-hour estimate.
- Lifting and transpotation requirements. Vessels require to be lifted and transported. Include a task to design the lifting lugs. The client sometimes provides a sketch of the lifting lug requirements. Vessels that require
special transportation and heavy lifts will require additional hours for design of lifting beams, tackle and discussions with shipping and construction personnel.
- Pressure testing requirements. Pressure vessels require to be pressure tested, usually by hydraulic testing. If a pneumatic test is necessary then additional safety precautions will be required.
The facilities required to test the vessel will require blind flanges, gaskets and bolting. Computer programs are available to calculate and assess
the flange stresses. If the testing requirements are in any way special then it is best to do a separate man-hour estimate.
- Additional loads. Allow tasks for any additional loading on the vessel and supports, e.g. wind or earthquake loads.
If any "design by analysis" is required then do a separate man-hour estimate.
- Checking and approval. Calculations will require to be checked and approved as part of the quality procedures.
Summary of Design Activities
3.3.2 Analysis Contract.
Design Calculations for a Pressure Vessel Contract
For a stress analysis contract the extent of the design work is very dependent on the
complexity of the geometry and loading especially if there is any: cyclic loading (fatigue), thermal transients, elevated temperatures (creep) or
instability (buckling) effects involved.
The following activities are likely to be the minimum required for a linear-elastic finite element analysis. It should be
appreciated that it is essential that checking is carried out throughout the process as an error discovered "late in the day" will probably invalidate the previous work.
- Geometry data. The geometry data requires to be collated and checked. It is essential that the geometry to be analysed is available before starting
a finite element analysis. It very time consuming to start changing the geometry once the analysis has started. The type of elements
has to be decided on and locations of mesh refinement.
Pre-processors are available to aid the build of the geometry mesh. Geometry checking is an essential requirement.
- Material data. The material data requires to be collated and checked. The data required such as: Young's modulus, Poisson's ratio,
density and if a thermal analysis is being carried out, thermal conductivity and coefficients of expansion. Most of this material data
can be obtained from published tables of material properties.
- Loading data. The loading data requires to be collated and checked. The loading and its direction and the boundary conditions for each load run requires to be decided on.
The clients specification and design data sheets should provide all the necessary information. Some editing of the
data for input to the analysis is often necessary.
- Design conditions. The finite element model is loaded and run for the required number of mechanical load cases.
The results require checking for deflections and stresses with the aid of a post processor and other checks.
A log of the loading runs should be kept and their status.
- Assessment of design conditions. The mechanical load cases are assessed for design conditions to the applicable code "design by analysis" rules.
Post processors are available to aid the assessment and provide stress linearisation of the results. A check of the
assessment results is required.
- Heat transfer coefficients. If a thermal analysis is necessary then surface heat transfer coefficients are required. If the client does not supply
the coefficients then they need to be calculated. This can be a time consuming exercise especially if the geometry
is complex and a transient is to be investigated.
- Thermal analysis model. The model for the thermal analysis is often the same as that for the mechanical analysis, however in some cases
surface heating elements must be applied to the geometry mesh. The mesh requires checking as will the material thermal properties.
- Operating condition temperatures. The model is loaded with heat transfer coefficients and applied boundary conditions. Run for steady state temperatures.
If a thermal transient is being investigated there will be several runs with sets of temperature results.
The amount of output will be considerable. Check the temperature results with the aid of a post processor.
A log of the temperature runs should be kept and their status.
- Temperature and combined stresses. The temperature results are loaded and run to obtain thermal stresses. Combine the mechanical and thermal load cases to obtain the necessary operating (service) conditions.
The results require checking for deflections and stresses with the aid of a post processor and other checks.
A log of the loading runs should be kept and their status.
- Assessment of operating conditions. The combined load cases are assessed for operating conditions to the applicable code "design by analysis" rules.
Post processors are available to aid the assessment and provide stress linearisation of the results. A check of the assessment
results is required.
Summary of Design Activities
Once you have got the design activities complete you can add hours to the activities
(or wait to the end before allocating hours). Now you can proceed to the communication activities discussed in section 3.4 below.
Finite Element Analysis Contract
3.4 Communication Activities.
The completion of the engineering activities is not the completion of all the activities
that comprise a project. It is essential to communicate design information to interested parties both within the company and to the client. This communication is
part of the normal design process and can have significant man-hours.
The omission of the tasks associated with these communication activities is one of the reasons why man-hours are often underestimated.
- Information to the drawing office. It is essential to communicate design information to the drawing office. Allow tasks for transmitting the information, for the
checking of drawings and for meetings/discussions with the drawing office personnel. On some contracts these man-hours can be very significant
especially on contracts with a large number of detailed drawings.
- Information to the project engineer. On projects with a manufacturing contract allow tasks for communicating with the project engineer and other specialists e.g. production engineers, welding engineers, metallurgists.
Allow hours for transmitting information and discussions.
Experience has shown that on contracts with lots of machining or difficult weld access or unfamiliar manufacturing methods
then increased numbers of non-conformance reports (N.C.R.s) are likely to occur. Although it is impractical to allow for these hours beforehand, it is important to appreciate that
N.C.R.s are one of the reasons for unexpected escalation in man-hours on a contract.
- Special instructions. Some contracts may require specific assembly or test procedures.
Design department may be asked to contribute to or approve the technical specification for these procedures.
- Design report. Some contracts require a design or stress report to be issued. The detailed content of the report can be difficult to predict.
In some cases the client will accept a summary text with full detailed calculations in an appendix. If the design calculations are well prepared then this type of report can be
quickly put together. In the case of reports on stress analysis work the report is often extensive with many plots of mesh geometry, vector loading, boundary conditions, stress
and temperature contours. There have been cases where client will not pay for a full report and in these cases the report only contains a summary of the results.
The quality procedures will require the report to be numerically checked, reviewed and approved. Extra hours should be allowed for large reports
or reports that contain novel designs or methods of analysis.
Allow hours for the issue of the report. It can take some hours to collate a large report, get it photocopied and mailed to all recipients.
Allow hours for clients comments on the report. If the client is not familiar with design or stress analysis you could find that you are having
to explain details that you have taken for granted. In other cases the client can be very "nit picking" in his comments.
- Communication with the client. Allow hours for correspondence and additional meetings with the client. On lengthy contracts there will usually be regular meetings throughout the contract. If anything goes wrong, like an overstressed
component or an N.C.R. requiring a component to be scraped, then you can expect additional meetings and correspondence with the client.
Summary of Communication Activities
Once you have got the communication activities complete you can add the hours to the activities
and total the hours. Do not hand over the man-hour estimate without a covering note or memorandum, refer to section 3.5 below.
3.5 Covering Note.
The completed man-hour estimate should be sent to your
department manager. Always include a covering note or memorandum with the estimate. Your estimate could well end up as
part of a project managers Gantt chart so it is essential that the scope and limitations of the estimate are communicated to the
The note should briefly summarise the man-hour total, highlight any assumptions made and list any limitations
in the use of the estimate. The following are some of the items that may be necessary to include in a covering note:
- Summarise the man-hours
- Is the estimate based on limited data or was a full specification available. Did you have sufficient time to read the
- Is the client asking for a tight time scale on all or part of the work.
- Are there any special or particular computer hardware or software required for all or part of the time scale,
e.g. a particular finite element package with appropriate pre and post processing capability.
- Are there special skills required, e.g. stress analysts with a knowledge of finite element analysis.
- Is there any part of the work new or unfamiliar to the department. If so, has any allowance been included
for training or learning curves.
Summary of Cover Note Items to Consider
With the cover note issued this completes the preparation of the
estimate. The department manager will usually discuss the estimate with you and revise your estimated
hours, usually downwards - managers tend to be optimists.
3.6 Get Feedback.
With a contract underway get feedback on the progress of the project.
Keeping track of progress on individual tasks is one of the ways you can gain experience of how well you made your estimate (a Gantt
chart like that used in Microsoft Project is one of the ways you can keep track of a project). By identifying under
or overestimates of the man-hours and the reason for these differences is knowledge worth having for future estimates and
also to help explain to project managers why the project is not running to plan.