Yaşam döngüsü boyunca maliyet fayda analizi ile entegre edilmiş sistem teorik süreç analizi yaklaşımı
Integrating system theoretical process analysis with life cyle cost benefit analysis
Yazar: GAMZE TANRISEVEN
Danışman: DOÇ. DR. ESRA BAŞ
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / Endüstri Mühendisliği Ana Bilim Dalı / Endüstri Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2023
Konu: Endüstri ve Endüstri Mühendisliği = Industrial and Industrial Engineering
Dizin: Fayda-maliyet analizi = Cost-benefit analysis ; Maliyet analizi = Cost analysis ; Süreç analizi = Process analysis ; Yaşam döngüsü = Life cycle

Özet:
STPA yöntemi, Nancy Leveson tarafından geliştirilen ve geleneksel yöntemlere nazaran daha yeni ve bir sistemin daha kapsamlı bir şekilde ele alınabilmesini sağlayan bir tehlike analizi yöntemidir. STPA yöntemi; sistemin genel özelliklerinin, sistem paydaşları açısından neler olduğuna karar verilmesine destek olan sistem kayıpları, sistem düzeyindeki tehlikelerin ve kısıtlamaların tespit edildiği birinci adım, toplanılan bilgilerin kullanılarak sisteme ait hiyerarşik kontrol yapısının oluşturulduğu ve bu yapıda yer alan emir-geri bildirim ilişkilerinin tespit edildiği ikinci adım, söz konusu ilişkilerden hangilerinin sistemin çalışması sırasında sistem akışını bozabilecek hale gelebileceğinin araştırıldığı üçüncü adım ve bu durumlara ait senaryoların türetildiği ve bu senaryolar üzerinden çözüm yollarının arandığı dördüncü adım olmak üzere toplam dört adımdan oluşmaktadır.
Bu tez çalışmasında yöntemin temel adımlarından olmayan bir "5. Aksiyon adımı" tanımlanmıştır. Bu adım, dördüncü adımda geliştirilen çözüm yollarından hangilerinin uygulanmak istendiği, bu uygulamaların ne zaman yapılmasının sistem açısından daha doğru olacağı başlıklarını içermektedir. Bu yüksek lisans tezi kapsamında yapılan literatür araştırmasında birçok makale incelenmiş ve konusunda STPA yöntemini merkeze alan otuz yedi makale özetlenmiştir. Bu çalışma sırasında, görece yeni olan bu yöntemin bazı uygulamalarda beklenen faydayı vermediği, geleneksel yöntemlerin daha çok tercih edildiği gözlemlenmiştir. Ayrıca literatürde STPA yöntemi ile Maliyet Fayda yaklaşımını entegre eden bir çalışmaya da rastlanılmamıştır. Dolayısıyla bu tez çalışmasında, STPA yönteminin dört adımının ve bu çalışmada eklenen beşinci aksiyon adımının sistem paydaşları tarafından belirlenecek yaşam döngüsü boyunca etkilerini daha iyi gözlemleyebilmek adına STPA yöntemi ile Yaşam Döngüsü Boyunca Maliyet Fayda Analizi (LCCBA) yönteminin entegre edilmesine karar verilmiştir. Bu amaçla ilk olarak STPA yönteminin adımları detaylı bir şekilde alt başlıklara ayrılmış ve her bir başlık için ortaya çıkabilecek fayda ve maliyetler teorik olarak tespit edilmiştir. Bu başlıklar, ilgili adımda yapılması gereken işlere veya alınması gereken kararlara yönelik konulara ilişkindir. Ayrıca STPA uygulamasına başlamadan önce, söz konusu uygulamanın firma çalışanları tarafından veya danışman bir firma tarafından gerçekleştirilmesi konusunda bir karar alınmalıdır. Çalışmada, bu kararın alınması için "Üret ya da Satın Al" yaklaşımından yararlanılması önerilmiştir.
Uygulama bölümünde, online satış platformu üzerinden çanta satışı yapan bir firma ile ilgili STPA yöntemi uygulaması gerçekleştirilmiştir. Uygulamaya başlamadan önce yapılan analizle firmanın içinden bir çalışanın yöntemi uygulamasına karar verilmiştir. STPA uygulamasının tamamlanmasından sonra, bu süreçte ortaya çıkan fayda ve maliyetler yine adım adım tespit edilmiştir. Bu faydalardan, etkilerinin uygulama sırasında ortaya çıkacak olanları "anlık faydalar" ve etkilerinin yaşam döngüsü içerisinde ortaya çıkacak olanları "sonraki faydalar" olarak gruplandırılmıştır. Bu gruplandırma ile sayısallaştırma çalışmasında önceliğin belirlenmesi ve sayısallaştırılamayan faydaların daha kolay değerlendirilebilmesi amaçlanmıştır. Söz konusu tespitlerin etkilerini daha iyi görebilmek adına, 5 yıl olarak belirlenen yaşam döngüsü üzerinden mümkün olan fayda ve maliyetlerin kantitatifleşmesi için bir çalışma yapılmıştır. Yapılan bu çalışmanın farklı durumları da kapsayabilmesi adına üçfarklı senaryo geliştirilmiş ve her biri için ayrı ayrı nakit akışları oluşturularak Fayda/Maliyet oranları hesaplanmıştır. Her bir senaryo için de %10-%100 aralığında değişen yıllık faiz oranlarına göre Fayda/Maliyet oranı 1'den büyük çıkmıştır. Ayrıca tespit edilen on dokuz faydadan altı tanesi, on dört maliyetten on bir tanesi sayısallaştırılabilmiştir. Söz konusu on üç faydadan altı tanesi sonraki fayda, yedi tanesi ise anlık fayda niteliğindedir. Dolayısıyla bu faydaların hem uygulama sürecinde hem de yaşam döngüsü boyunca etkilerinin görülmesi beklenmektedir. Sayısallaştırılamayan on üç faydanın, üç maliyetten daha çok etkisinin olacağı da göz önüne alındığında uygulamanın yapıldığı firma açısından STPA yönteminin faydalarının, maliyetlerinden daha fazla olduğu sonucuna varılmıştır.
STPA ile LCCBA yöntemlerinin entegresinin daha sonra yapılacak STPA uygulamaları açısından; bu uygulamalara yönelik bir maliyet fayda analizi ile ortaya çıkan veya sonraki süreçlerde ortaya çıkacak fayda ve maliyetlerin tespit edilebilmesine ve bu tespitler aracılığıyla sistem hakkında alınacak kararlar konusunda yol gösterici olmasına katkıda bulunması amaçlanmaktadır.
Daha genel bir yaklaşımla bakıldığında bu tez çalışmasında, bir tehlike analizi yöntemine Maliyet Fayda Analizi'nin entegre edilmesi üzerine çalışılmış, gerçekleştirilen uygulamada da bu entegrenin mümkün ve faydalı olduğu görülmüştür. Dolayısıyla FMEA, FTA gibi diğer tehlike analizi yöntemlerinin de Maliyet Fayda Analizi ile entegresinin söz konusu süreçleri destekleyeceği düşünülmektedir.

Abstract:
It is inevitable to experience malfunctions in systems whose working principle is based on systemic structures. In order to minimize the damages that these malfunctions can cause to the system, many researches have been carried out from the past to the present, and various approaches and methods have been developed. One of these approaches is hazard analysis. Failure Mode and Effects Analysis(FMEA) and Fault Tree Analysis(FTA) are some of the hazard analysis methods that were developed during periods when systems were analyzed with an analytical approach, which focuses on the system units, and which is frequently used today. With the development of the System Theory approach, which focuses not only on the system units but also on the relations between the units, new hazard analysis methods have been introduced. One of the relatively newly developed methods is System Theory Accident Model and Processes(STAMP).This model has advantages over traditional models, such as being applicable to more complex systems thanks to its working principle, increasing the variety of inputs that may be causal factors for undesirable situations, and being suitable for developing new tools based on this model. System Theoretic Process Analysis (STPA) and Causal Analysis based on Systems Theory (CAST) are among the most preferred approaches developed based on the STAMP method. While STPA focuses on identifying all potential scenarios before losses occur, CAST analyzes the scenarios that have occurred. With the STPA method, unsafe control actions that may occur at every level of the control structure of the system are also studied and more comprehensive results can be obtained compared to the other hazard analysis methods mentioned. 
STPA method, developed by Nancy Leveson from MIT, consists of four steps. In the first step, an information gathering study is performed about the system on which the STPA method will be applied. In this step, concepts such as losses, system-level hazards and system-level constraints defined in the STPA method are emphasized and the meanings of these definitions for the system are defined. In the second step, a hierarchical control structure is created and the relationships in this structure are defined so that the problems that have arisen or may arise in the system can be detected more easily. In the third step, by examining the defined relationships, it is determined which relationships may cause system-level hazards in which situations. This results in determining unsafe control actions. In the last step, scenarios that lead to unsafe control actions are identified and solutions are sought for these scenarios.
In the literature research conducted within the scope of this master's thesis, the sources focusing on STPA were examined and a total of thirty-seven articles were summarized in order to determine the improvement potentials of the STPA method. During this study, it was observed that the STPA method has not given give the expected benefit in some applications. In addition, in many articles, different hazard techniques were applied together with the STPA method and the results were compared. As a result of evaluations, it was concluded that the STPA method could not develop a new situation as expected, and that it was possible to obtain more efficient results when integrated with a different method. In short, there have been some criticisms about the benefit-cost balance of the STPA method in the literature. In order to resolve these criticisms, no study has been found that integrated the STPA method and the Cost Benefit approach. Therefore, in this thesis, it was decided to integrate the STPA method and the Life Cycle Cost Benefit Analysis (LCCBA) in order to observe the effects of the four steps of the STPA method better. The fifth action step added in this study throughout the life cycle to be determined by the system stakeholders. There are many reasons why STPA method is integrated with the LCCBA method in this study. First of all, making an assessment that includes solutions suggested by STPA method, will provide more realistic results and will guide the stakeholders to make decisions more effectively.  In addition, by applying the LCCBA analysis for each of the proposed solutions, the solutions can be prioritized. Even if the solutions obtained according to STPA method are not applied, the benefits of the STPA method in terms of the next processes will be seen thanks to the Cost Benefit Analysis throughout the Life Cycle. It is thought that this situation will also contribute to the decisions to be made by the system stakeholders.
In order to integrate the two methods, firstly, the four steps of the STPA method were divided into sub-tasks and the decisions to be taken during each step were determined. Then, the benefits and costs that may arise related to these tasks and decisions have been determined. In addition, before starting the method, it should be decided whether the STPA method will be carried out with a team to be formed within the system or with a consultant firm. Benefits and costs have been determined for both cases of this decision, and it has been suggested to make a decision with the support of the Make or Buy decision approach. However, it is not possible to quantitate every value in today's systems. Therefore, as stated in the literature mentioned in this approach, it was thought that it would be more accurate to reach a conclusion by considering qualitative evaluations along with quantitative evaluations.
Observing the effects of the solutions suggested by STPA method, the "Action step" has been defined as the fifth step. In this step, two basic decisions should be made. Since it is not mandatory to implement all of the suggested solutions, two different situations arise that involve the application of some or all of the solutions. Also, the proposed solutions can be implemented can be implemented as soon as possible or later. As a result, two different scenarios arise seperately in terms of the two decisions to be taken. In order to determine which of these scenarios would be more beneficial for the system, the benefits and cost of the scenarios should be determined in the methodology section. the development of this step, it is aimed to enable the practitioners of the STPA method to determine the cost-benefit balance more accurately by taking into account not only the four steps in the application, but also the solution proposals. Moreover it has been proposed to classify the benefits identified during the study as primary benefit or secondary benefit. The reason for this classification is that giving priority to primary benefits during the quantification phase provides ease of work and the qualities of non-quantified benefits can be determined more easily. 
In order to see the effect of the studies described in the methodology section, an application was made in a company that sells bags on an online sales platform. As a result of the evaluation made before starting the application, it was decided that the application would be carried out by the owner of the company. As stated, during this decision making, the "Make or buy decision" approach was used and both qualitative and quantitative evaluations were made. In the first step of the application, information about the company and the systems it is associated with was collected. Then the system losses, system-level hazards and system-level constraints were determined by the company stakeholders. Stakeholders decided to deal with the sub-systems related to the operations on the online platform by using STPA method. So, in the second step, the hierarchical control structure of the subsystems that concern the subject matter was created and a total of seventy-nine order-feedback relations in this structure were defined. In the third step, three relationships that are likely to turn into problems were selected from these defined relationships, and a total of ten unsafe control actions were defined by referring to the definitions of unsafe control actions types suggested by the method. In the fourth step, it was determined which of the system-level hazards identified in the first step were associated with each of ten unsafe control actions. Four out of ten unsafe control actions were found to be associated with a greater number of system-level hazards, thus scenarios were generated based on these four unsafe control actions. Only one solution was proposed as it was realized that all scenarios were based on a common point. In this thesis, the fifth step, which is based on the application of the solution, could not be applied due to time constraints.
After the STPA implementation, the benefits and costs that arise in each step or may arise later were determined by using the methodology section. During the application, since the 3rd and 4th steps are quite intertwined, these steps were evaluated together. As a result, 19 benefits and 14 costs were found. 6 of these benefits and 11 of the costs have been quantified. It was thought that creating three different scenarios based on the 1st benefit of the 1st step would allow a more realistic evaluation, and the benefit and cost values were calculated separately for these three scenarios. Benefit/Cost ratios of these values were calculated according to annual interest rates varying between 10% and 100%. It was observed that the ratio was always greater than one for all three scenarios. Therefore, it can be said that the positive aspects of the STPA implementation throughout its life cycle will be expected to be more. 
In consequence of the application; it has been seen that the effects of the method are determined in a more comparable way and the proposed methodology contributes to the decision-making policies after the implementation, thanks to integrated the Life-Cycle Cost Benefit Analysis and STPA method. Also there should be more studied in order to include the effects of the proposed fifth step in the Cost Benefit Analysis. Due to the time constraint, such a study could not be done, but it is thought that this new step can be a guide for future STPA applications.
As a result, the implementation of the STPA method proposed in this thesis study by integrating the Life cycle Cost Benefit Analysis. These study contributes to the expansion of the literature on the STPA method, to increase the efficiency to be obtained from the STPA method, to base the decision-making process on comparable data, and thus to positively affect the management process of the companies. In future studies, it is considered to make a cost-benefit evaluation, especially for the fifth step, which cannot be studied.