CPM/PERT ile proje planlama ve kontrol
Project planning and controlling by CPM/PERT
- Tez No: 39907
- Danışmanlar: PROF.DR. AYHAN TORAMAN
- Tez Türü: Yüksek Lisans
- Konular: Mühendislik Bilimleri, İşletme, Engineering Sciences, Business Administration
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1993
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 112
Özet
ÖZET Başarıya ulasmaraxz için öncelikle hedeflerimizi tanımlamamız ge reklidir. Sonra bu hedefler doğrultusunda belirlediğimiz, birbirleriy le ilişkili bir dizi faaliyetlerin art arda gerçekleşmesini sağlayarak ileriye doğru adımlar, atabiliriz. İşti bu sebeplerle her düşündüğümüz proje için öncelikle bir planlama, programlama ve daha sonra projenin planlandığı gibi yürülmesini sağlamak için bir kontrol mekanizması ku rulmalıdır. Proje planlama, programlama ve kontrolü için bir çok teknik geliş tirilmiştir. Bunlardan en çok kullanılan Kritik Yol Metodu (CPM) ve Proje Değerleme ve Yeniden Gözden Geçirme (PERT) metodu bu kez çalışma sında ele alınmıştır. Kritik yörünge metodunda olaylar deterministik yaklaşımla değerlen dirilerek, projeyi meydana getiren faaliyetler kritik ve kritik olmayan olarak ayrılmış ve kritik olan faaliyetler üzerinde çalışılarak projede süre, maliyet ve kaynak dengelemenin optimize edilmesi anlatılmıştır. Aynı değerlendirme ile PERT metodunun projeler üzerinde nasıl uy gulandığı gösterilerek, belirsizlikler altında proje süresi, maliyeti nin nasıl etkilediği üzerinde durulmuştur. Daha sonra CPM ve PERT ile planlanan ve programlanan projenin kont rol edilmesinin gerekliliği ve nasıl kontrol edildiği anlatılmıştır. xıı
Özet (Çeviri)
SUMMARY PROJECT PLANNING AND CONTROLLING BY CPM/PERT A project defines a combination of interrelated activities that must be executed in acertain order before the entire task can be completed. The activities are interrelated, in a logical sequence in the sense that some activities cannot start until others are completed An activity in a pro ject is usually viewed as ajob requiring time and resources for its comp letion. In general, a project is a one time effort; that is the same se quence of activities may not be in the future. The growing complexities of today's projects have caused to be crea ted sistematic and effective planning techniques with the objective of optimizing the efficiency of executing the project. The most developed techniques for project planning, scheduling and control are Critical Path Method (CPM) and Program Evaluation Techniques (PERT). Fundamentally, PERT and CPM are techniques of project management use ful in the basic managerial functions of planning, scheduling and control The planning phase of any venture involves a listing of tasks or jobs that must be performed to bring about the venture's completion. Gross require ments for material, equipment and manpower are also determined in this phase, and estimates of costs and durations for the various jobs are made. Scheduling on the other hand, is the laying out of the actual jobs of the project in the time order which they have to be performed. Manpo wer and material requirements needed at each stage are calculated, along with the expected completion time of each of the jobs. Control begins wi th reviewing the difference between the schedule and actual performance once the project has begun. The analysis and correction of this differen ce forms the basic aspect of control. Tthe critical path technique had its origin from 1956 to 1958 in to parallel but different problems of planning and control in projects in the United States. In one case, U.S. Navy was concerned with the control of cotraccts for its Polaris Missile program. Theese contracts research and development work as well as the manufacture of component parts not previously made. Hence neither cost nor time could be accuately estima ted, and completion times therefore had to be based upon pr'öba&iiliity^ Contractors were asked to estimate their operational time requirements on three bases: optimistic, pessimistic and most likely dates. These es timates were then mathematically assessed to determine the probable comp letion date for each contract, and this procedure was referred to as Program Evaluation And Review Technique (PERT). PERT systems involve a“probability approach”to the problems of planning and control of project and are best suited to reporting on works in which major uncertain ties exist. In the other case the E.I. Du Pont DeNemours Company was constructing major chemical plants in America. These projects required that both time and cost be accuratelly estimated. The method of planning and control that 3C3.X1was developed was originally called Project Planning and Scheduling (PPS) and covered the design, construction and maintenance work required for several large and complex jobs. PPS requires realistic estimates of cost and time and is thus a more definitive approach than PERT. It is this approach that has since been, developed into the critical path method(CPM) which is finding increasing use in construction industry. Altough there are some uncertainities in any construction project, the cost and time required for each operation involved can be reasonably estimated and all operations may then be reviewed by CPM in accordance with the anticipated conditions and hazards that may be encountered on site. The first step for CPM and PERT is to prepare the list of activities in a project. After list of activities has been prepared the arrow diag ram that represents the inter dependencies and precedence relationships among the activities of the project is drawn. The application of PERT - CPM should yield a schedule specifying the start and completion dates of each activity. The arrow diagram rep resents the first step toward achieving the goal. Because of the inter action among the different activities, the determination of the start and completion times requires special computations. These calculations are performed directly on the arrow diagram using simple arithmetic functions. The result is to classify the activities of the project as critical or noncritical. An activity is said to be critical if a delay in its start will cause a delay in the completion date of. thefi: entire project. A noncritical activitiy is such that the time between its ear liest start and its latest completion dates (as allowed by the project) is longer than duration. In this case the noncritical activity is said to have a slack or float time. A critical path defines a chain of critical activities that connects the start and end events of the arrow diagram. In other words, the criti cal activities of the project. The critical path calculations include two phases. The first phase is called the“forward pass”where calculations begin from the start node and move to the end node. At each node a num ber is computed representing the earliest occurrence time corresponding event. The second phase, called“ che”backward pass“ begins calculations from the end node and moves to the start node. The number computed...at each node represents the latest occurence time of the corresponding event. Let's an activity Let ' s an activity is said to be critical If delay in its sart will ca use a delay in the completion date of the entire project. A noncritical i.. activity is such that time between its earliest start and its latest corn- let ion dates (as allowed by the project) is longer than its actual dura tion. In this case the normal noncritical activity is said to have a slack or float time. The end project of network calculations is the constructions of the time chart (or schedule), As important factor in scheduling is the clas sification of activities into two groups, continous or intermittent. When a continous operations activity is start it must be worked without interruption until finished; an intermitted operation activity can be xivproceeded piecemed in isolated sections at irregular periods of- time. This division becomes vital when a choise is to be made whether it is ad vantegous to do part of an activity at a particular time and balance later. To schedule the project we must know the floats. So after the cri tical path is calculated floats of the activities has to be determined. Naturally a critical activity must have zero float. In this is the main reason it is critical. There two types of floats: total float arid free float. The free float must be zero when the total float is zero..? Scheduling must be within the limitations of available recourees, since it may not be possible to execute cocurrent activities because of personnel and equipment limitations. The total floats for noncritical ac tivities become useful for this point. By- shifting a noncritical activi ty between its maximum allowable limits, one may be able to lower the maximum resource requirements. In any case, even the absence of limited resources, it is common practice to use total floats to level resources over the duration of the entire project. In essence, this.would mean a more steady work force compared to the case where the work force would yary astically from one day to the next. The cost aspect included in a project scheduling by defining the cost duration relationship for each activity in the project. Costs are defined direct elements only. Indirect costs cannot be included. Their effect will be included in the final analsis. To find the time-cost re lationship we have to define two points, these points are normal point (if the activity is executed under normal conditions) and crash point (if the normal point is compressed by increasing the allocated resour ces and hence by increasing the direct costs, there would be a limit.) The straight- line relationship is used mainly convinience, since it can be determined for each activity from the knowledge of the normal and crash points only., A nonlinear relationship complicate the calcu - _ lations. There is one exeptional case, however, where nonlinear rela tionship can be approximated by a piecewise linear curve. Under such conditions, the activity can be broken down into a number of subactivi- tieseach corresponding to one of the line segments. After defining the cost-time relationship, the activities of the project are assigned their, normal durations. The corresponding critical path is then computed associated (direct) costs are recorded. The next step is to consider reducing the duration of the project. Since such a reduction can be affected only if the duration of a critical activity is reduced, attention must be paid to such activities alone.' To- achieve a reduction in the duration at least possible cost, one must compress as much as possible the critical activity having the smallest cost time slo pe. At the end of the compressing there will be a new schedule, perhaps with a new critical path. The cost of the new schedule must ' be greater than preceding one. The new schedule must be now considered for compres sion by selecting the critical activity with the least slope. The proce dure is repeated until all critical activities are at their crash times. The final result of these calculations is a cost-time curve for the vari- xvous schedules and their corresponding costs. We can assume that as the duration of the project increases, the in direct cost must also be increase. The sura of these two costs ( direct +' indirect ) gives the total cost of the project. The optimum schedulecor- responds to the minimum total cost. Planning and scheduling ”with GPM require a reasonable accurate knowledge of time and cost for each activity, for the CPM network model is essentially deterministic. In many situations, the duration of an ac tivity cannot be accurately forecast, any time estimate being sub jectto dubt. If such an activity lies on a noncritical path the usual GPM cal culations remain valid, but there will be a local uncertainity in resour ce leveling and in the scheduling of men and materials, equipment and finance. In those situations we have to use PERT. PERT introduces un certainity into the time estimates for activity and project durations.lt is therefore well suited for those situations where there is either un- sufficient background information to specify accurately utility data or where project activities require research and development. Probability considerations are incorporated in project.scheduling by assuming that the time estimate for each activity based on three dif ferent values : optimistic time, pessimistic time, most likely time.- And these values help us to find an activity time called expected mean time. Pert uses expected mean time for each activity together with an associa ted measure of the uncertainity may be expressed either as the standart deviation or variance of the duration. The expected mean time is intend ed to be atime estimate having approximately a.. %50 chance that the ac tual duration realized will be less, and a %50 chance that the actual du ration will exceeded it. From this point, it is clear that the formal de termination of such activity data necessties using a probability distri bution curve for the activity completion times. Since no information ac- tuallyexists regarding the probability distribution of the activity com- letion times and since its determination is not feasible because of the activity durations are subject random distrubances and delays, it is ne cessary to assume a probability distribution curve. PERT can be used to good advantage in planning and control of any project involving uncertainity such as in research programs or - unusual design and construction projects. And it can also be used to evaluate the alternative plans. In many situations when a network model is being prepared for a construction project it is not known beforehand which of several alternative plans will be adopted for the completion of specific portions of the project. Each of these alternative plans, willi require specific and possibly different durations, resources, and so on and the arbitrary selection of one for inclusion in the network model mayt'be mis leading. It may be desirable therefore to build into the network model the uncertainity associated with the selection of the alternative plans. It is possible to introduce the uncertainity associated with alternative plans into the network model and the associated calculations for schedu led durations and resources, using probabilistic concepts. If, as is u- xvisual, we associate a probability of 1.0 for certainty on a scale ranging from zero to unity, then we can consture the zero probability as repre - senting total rejection of an alternative plan and unit probability as representing the total acceptance or decision to implement a particular alternative plan. After project planning and project scheduling has been completed we have to think that the arrow diagram can be discarded as soon as the ti me schedule is developed. This is not so. In fact, an important use of the arrow diagram occurs during the execution phase of the project.lt seldom happens that the planning phase will develop a time' schedule that can be followed exactly during the execution phase. Quite often some of the jobs are delayed or expedited, which naturally dpends on actual work conditions... As soon as such disturbances occur in the original plan, it becomes ne cessary to develop a new time schedule for the remaining portion of the project. This section outlines a procedure for monitoring and controlling the project during the execution phase. It is important to follow the progress of the projecton the arrow di agram rather than solely on the time schedule. The time schedule is used principally to check if each activity is on time. The effect of a delay in a certain activity on the remaining portion of the project can best be traced on the arrow diagram. Suppose that asthe project progresses over time, it is discovered that delay in some activities necessitates developing a completely new schedule. How can the new schedule be obtained using the present arrow diagram? The immidiate requirement is to update the arrow diagram by ase signing zero values to the durations of the completed activities. Partial ly completed activities are assigned times equivalent to their unfinished portions. Changes in the arrow diagram such as addition or deletion of any future activities must also be made. By repeating the usual computa tions on the arrow diagram with its new time elements, we can determine the new time schedule and possible changes in the duration of the project Such information is used until it is agin necessary : to update -the time schedule. In real situations, many revisions of the time schedule are ut- sually required at the early stages of the execution phase. A stable pe riod follows in which little revision of the current schedule may be required. Toreview the current procedures and forecast the future requirements of the job, so that the works may be successfully completed, is the pri mary purpose of project control. To work effectively, there must be some means of determining rapidly sound solutions to the day-to-day problems, so that the essential requirements for remedial measures may be promptly initiated. The more accurate and logical the planning, the easier it will be for the work to be performed in accordance with the program. Highly de tailed planning, however, takes time and costs money. Consequently pro- xviiject planning at the bidding stage may not be proceed far enough to provide all the neccessary particulars for project control. For this reason it is essential that before beginning operations on the site, the project schedule be rewieved for special details. Although this reviewing procedure is actually the final phase of the detailed planning, it is also the first step toward the actual site control of the project and should be done with considerable care. XVX1X
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