Harita projelerinin CPM-PERT yöntemleri ile programlanması

Abstract

Bir projenin, önceden belirlenen bir zamanda ve sınır lı kaynaklarla amacına ulaşması, planlı, programlı ve kontrollü bir çalışmayla mümkündür. Deneyimli veya gerekli eğitimden geçirilmiş personel ve yeterli donanımın bulunmadığı bir projenin başarı yüzdesinin düşük olması doğal karşılanabilir. Ancak, personel ve diğer kaynaklar bakımından yeterli olan ekiplerle yapılan çalışmanın çoğunun da, normal yapım süresinde tamamlanabildiği ve sağlıklı ürünler elde edildiği söylenemez. Olumsuz hava ve arazi koşulları, beklenmedik olaylar projeyi olumsuz etkileyen faktörlerdir. Fakat esas sorun, proje sorumlusunun süre ve maliyet açısından etkin bir program ve denetim yapamamasıdır. Bu çalışmada, çağdaş proje yönetimi hakkında genel olarak bilgi verildikten sonra, birer yöneylem araştırma sı tekniği olan ve projelerin programlanmasında kullanılan CPM (Critical Path Method) ve PERT (Program Evaluation and Review Technique) yöntemleri ayrıntılı olarak ele alınmıştır. Uygulama kısmında, İller Bankası Genel Müdürlüğü ' nce (1992 I. yarıyıl birim fiatlarına göre) 1.560.485.342 TL. tutarındaki 21 ay süreli bir halihazır harita alım proje si, üzerinde çalışacak proje olarak seçilmiştir. Söz ko nusu projeye CPM-PERT yöntemleri uygulanmış, aynı projenin (İ.T.ü. ölçme Tekniği Anabilim Dalı'nca hazırlatılan 1992 I. yarıyıl birim fiatlarına göre) 971.353.621 TL. bedelle ve yaklaşık 16,5 ayda tamamlanabileceği görülmüştür. Sonuç ve öneriler kısmında ise, projenin CPM-PERT yöntemleriyle programlanarak İller Bankası 'nca belirlenen süreden önce ve daha ucuza tamamlanabilmesi; işveren kurum, birim fiatları belirleyen kurum ve yüklenici kuruluşlar açısından incelenmiştir. Bu çalışmanın amacı, endüstri projelerinin bir çoğunda başarıyla uygulanan "Modern Proje Programlama" yön temlerinin, harita projelerinde de uygulanabileceğinin, etkin bir planlama ve denetim ile aynı kaynaklarla daha kısa sürede daha verimli çalışmalar elde edilebileceğinin gösterilmesidir.

A project reaches its aim with a defined time and sources if a planned, programmed and controlled working exist. A project may not be succeed if there are not enperienced personnel and enough equipment. This is normal. But, most of the work can not be completed in normal time despite having experienced personnel and enough equipment. Bad meteorological and land conditions or other reasons affect project negatively. But, beyond this, main cause of delays is the fact that project menager can not make effective planning and control in terms of time and cost. In this study, the necessity of an effective planning and control in surveying projects, as in all projects, is considered. Of all operation research techniques, CPM (Critical Path Method) and PERT (Program Evaluation and Review Technique) methods are selected since they are more proper for the philosophy of surveying projects. In application part of the study, these techniques are applied to a surveying project, performed before without using such project planning techniques, and the results are compared with the ones, taken in the real project, with respect to cost and time. The main purpose of this study is to demonstrate that the "Modern Project Programming Methods", applied succesfully in many industrial projects, can also be applied in the surveying projects and that, by means of an effective planning and control, a more productive work can be performed. 1- THE TERMINOLOGY OF NETWORKS A graph consists of a set of points called nodes. Certain pars of the nodes being joined by lines are called branches. VI (ET)j =max. { (ETJi+Dij} (1) Dj_ j : Activity time If there are orbits more than one and if different results are found at the end of the additional operation, the greatest one is taken as "the earliest time" of the j event. The latest time for an event is the estimated last time. The event can occur without delaying the completion of the project beyond its earliest time. The latest times are obtained successively for the events by making a backward pass through the network, starting with the final events and working backward in time toward the initial events. The latest time of final events is equal to the earliest time of final event. Required formula is given below for calculation of the earliest time; (LT)i =min. (LT)j-Dij (2) If there are orbits more than one and if different results are found at the end of the operation, the smallest one is taken as "the latest time" of i. The slack for an event is the difference between its latest and earliest time. The slack represents how much delay in reaching the event can be tolerated without delaying the project completion. A critical path for a project is a path through the network such that the events on this path have zero slack. If an activity has these three conditions, it is called critical activity. 1- The earliest time must be equal to the latest time of event of i. (ET)i=(LT)i (3) 2- The earliest time must be equal to the latest time of event of j. (ET)j=(LT)j (4) 3- Activity time must be equal to difference between the earliest time (the latest time) of event of j and the earliest time (the latest time) of event of i. IX The original application of PERT systems was for evaluating the schedule for a research and development program* it is also being used to measure and control progress on numerous types of special projects. Examples of these project types include construction programs, programming of computers, maintenance planning and the installation of computer systems. A PERT-type system is designed to aid in planning and control, so it may not involve much direct optimization. Sometimes öne of the primary objectives is to determine the probability of junction specified deadlines. it also indentifies the activities that are most likely to be bottlenecks. A third objective is to evaluate the effect of changes in the program. Por example, it will evaluate the effect of a contemplated shift of resources from the less criti- cal activities to the activities identified as probable bottlenecks. Ali PERT ör CPM type systems use a network to graphi- cally portray the interrelationships among the elements of a project. This network representation of the project plan shows ali the precedence relationships regarding the order in which tasks must be performed. There are some terminologies about CPM-PERT. Activity is each branch of the network. This is öne of the tasks reguired by the project. Each node of graph represents an event. it is usually defined as the point when ali activities leading into that node are completed. The node toward which ali activities lead is the event corresponding to the completion of the currently planned project. Dummy activities are dashed-line arrows. They show precedence relationships only. They do not represent real activities. Dummy activities can also be used to avoid violating this rule when there are two ör more concurrent activities. Dummy activities never spend time and coşt anyway. After developing the network for a project, the next step is to estimate the time. it is required for each of the activities. These times are used to calculate two basic guantities for each event. Öne of them is earliest time (ET) and the other is the latest time(LT). The earliest time for an event is the estimated time. The event will occur if the preceding activities are started as early as possible. The earliest times are obtained for the events by making a forward pass through the network. it starts with initial events and continues until final events. The earliest time of initial event is taken zero. Reguired formula is given below for calculation of the earliest time; lw'i^I^ ^^Ç)(LT)3 (ET)j =max. { (ETJi+Dij} (1) Dj_ j : Activity time If there are orbits more than one and if different results are found at the end of the additional operation, the greatest one is taken as "the earliest time" of the j event. The latest time for an event is the estimated last time. The event can occur without delaying the completion of the project beyond its earliest time. The latest times are obtained successively for the events by making a backward pass through the network, starting with the final events and working backward in time toward the initial events. The latest time of final events is equal to the earliest time of final event. Required formula is given below for calculation of the earliest time; (LT)i =min. (LT)j-Dij (2) If there are orbits more than one and if different results are found at the end of the operation, the smallest one is taken as "the latest time" of i. The slack for an event is the difference between its latest and earliest time. The slack represents how much delay in reaching the event can be tolerated without delaying the project completion. A critical path for a project is a path through the network such that the events on this path have zero slack. If an activity has these three conditions, it is called critical activity. 1- The earliest time must be equal to the latest time of event of i. (ET)i=(LT)i (3) 2- The earliest time must be equal to the latest time of event of j. (ET)j=(LT)j (4) 3- Activity time must be equal to difference between the earliest time (the latest time) of event of j and the earliest time (the latest time) of event of i. IX (ET) j-(ET)i=(LT) j-(LT)i=Dij (5) This information on earliest and latest times, slack and the critical path is invaluable för the project manager. The differences betveen CPM and FERT were pointed out in former chapter. Thus far we have assumed that use is raade of jüst öne estimate of the time required för each activity of the project. However, there usually is considerable uncertainty about what the time will be, in actuality it is a random variable having some probability distribution. in recagnition of this, the original version of PERT prescribed using three different types of estimates of the activity time to obtain basic information about its probability distrubition. in guestion three estimates are "a. most likely estimate", "an optimistle estimate" and "a pessimistic estimate". 1- The Most Likely Estimate is intended to be the most realistic estimate of the time. it is denoted by "m" and obtained by experienced personnel ör previous working. 2- The Optimistle Estimate is intended to be the unlikely but possible time if everything goes well. it is denoted by "a", 3- The Pessimistic Estimate is intended to be the unlikely but possible time if everything goes badly. it is denoted by "b". O a m b Fiğ. 2 Model Of Probability Distribution For The PERT Three-Estimate Approach The standard deviation (sguare root of the variance), 02 = [_1_ (b-a)]2 (6) and, the expected value of the distribution (denoted by te in the PERT literatüre) is approximately; te = -|- (a+4m+b) (7) 3- CHOOSING BETWEEN CPM AND PERT The choice between the PERT three-estimate approach and the CPM method of time-cost trade offs depends primarily upon the type of project and managerial objectives. PERT is particularly appropriate when there is considerable uncertainty in predicting activity times and it is important to effectively control the project schedule. CPM is particularly appropriate when activity times can be predicted well, but these times can be adjusted readily.