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AKTİN, AYŞE TÜLİN

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    An analytical approach for analysing the impact of risks on production planning: Case of Öztiryakiler
    (2020) Telatar, E.; Bekeç, T.; Başaran, A.; Balıkçı, N.; Bilgin, B.; İlbay, E.; AKTİN, AYŞE TÜLİN
    Accurate and applicable production plans are a must for manufacturing companies. Although companies tend to prepare ideal production plans, some exogenous factors can affect their validity. These risks, which occur unexpectedly, will have a negative influence on the plan. This study aims to determine the exogenous factors affecting the success of production planning of square and rectangular food containers manufactured by Öztiryakiler, and analyse their impacts on the plans. The risk factors are evaluated using Failure Mode and Effects Analysis, and their risk priority numbers are calculated. A mixed-integer linear programming model with the objective of total cost minimisation is developed to obtain the production plan of containers. Initially, an ideal data set is used as input; hence, this model’s output displays a risk-free plan. Similarly, for each of the risk factor scenarios, mathematical models are solved with risk-related data. GAMS software and CPLEX solver is utilised in the solution of all models. Finally, for each of the selected high risk alternative, the expected total costs are calculated. This is achieved by multiplying the normalized risk priority number obtained from the Failure Mode and Effects Analysis with the corresponding optimal total cost of the risky plan. This analysis highlights the most critical risks, and comparison with the risk-free plan helps in proposing system improvements. © 2020, Springer Nature Switzerland AG.
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    THY Teknik’te Uçak Motorlarının Yurt Dışına Tamire Gönderim Sürecinin Yalın Üretim Araçlarıyla Analizi
    (2018) Gülen, Büşra; Asıl, Bekir Sıddık; Demirbozan, Burak; Özdamar, Melike; AKTİN, AYŞE TÜLİN; 109203
    Sahip olduğu ulusal ve uluslararası yetkiler kapsamında, THY ve diğer havacılık şirketlerine bütünleşik olarak uçak bakım-onarım-revizyon hizmeti veren THY Teknik, bulunduğu bölgedeki lider BOR kuruluşudur. Bu çalışmada, THY Teknik’in tamir kabiliyeti dışında olan uçak motorlarının yurt dışı tamire gönderim süreci, yalın üretim araçlarıyla ele alınacaktır. Bu süreç, Motor Söküm Plam’mn oluşturulmasıyla başlamakta, motor ve gerekli belgelerin kargo için hazırlanmasıyla devam ederek, motorun tedarikçiye (yani, ilgili motoru üreten firmaya) gönderilmesi için kargo uçağına yüklenmesiyle son bulmaktadır. Söküm öncesinde ve sırasında gerekli ekipmanlara zamanında ulaşılamaması, sürece dâhil olan departmanlar arasındaki iletişim aksaklıkları, gerekli belgelerin tedarikçiye eksiksiz olarak iletilememesi gibi sorunlar, gecikmelere neden olmakta ve para, zaman, iş gücü kaybına yol açmaktadır. Sürecin departmanlararası niteliği, bu departmanların içinde bulunduğu diğer süreçlerin de etkilenmesine ve verim kaybı yaşamasına sebep olabilmektedir. Bahsedilen sorunlardaki iyileştirmelerin şirket açısından çok yönlü bir fayda sağlayarak, motorların tamire gönderim süreci başta olmak üzere, diğer süreçlerdeki verimi de artırması beklenmektedir. Hedeflenen bu çıktılar, çalışmanın motivasyonunu oluşturmaktadır. Çalışmanın ilk aşamasında, yalın üretim araçlarından Değer Akışı Haritalama (DAH) yöntemiyle mevcut durum gözlemlenecek; sürece değer katan, değer katmayan adımlar ve israfların belirlenmesinin ardından, değer katmayan adımlarla israfları hedef alan Kaizen’ler çıkartılacaktır. Sonrasında, bu Kaizen’lerin “öncelik sıralaması” ve “Kaizen’lerin oluşturulduğu sorunlara yönelik karşılaşılma sıklığı”nın temel alındığı bir Pareto Analizi yapılarak, süreç için darboğaz oluşturan kısımlar belirlenecektir. Bu işlem sonucunda, ilgili kısımlar için İki-Aşamalı DAH yöntemi kullanılarak yeni Kaizen’lerin eklenmesi ve bunlar ışığında; Kök-Neden Analizi, 5S ve Poke-Yoke gibi yalın üretim araçları kullanılarak, darboğaz nokta için iyileştirmelere gidilmesi planlanmaktadır. Bütün bu düzenlemelerden sonra, Gelecek Durum ve İdeal Durum Değer Akışı Haritalarının çizilmesiyle birlikte, motorların yurtdışı tamire gönderim sürecinin standart, sürdürülebilir ve geliştirilebilir bir hâle getirilmesi hedeflenmektedir. İdeal durumun oluşturulmasında Nesnelerin İnterneti’nden de yararlanılmasının, bu bağlamda süreçte yapılacak değişikliklerin maliyetleriyle uçağın yerde beklemesinin maliyeti kıyaslandığında, uzun vadede avantaj sağlayacağı düşünülmektedir.
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    A Mathematical Programming Approach to Multi-Way Kidney Transplantation
    (2015-07) Kutlu, Fatma; Girgin, S.; YÜKSEKTEPE, FADİME ÜNEY; AKTİN, AYŞE TÜLİN; 273471; 108243
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    Joint Decision Making for Production and Marketing
    (1995) Ulusoy, Gündüz; AKTİN, AYŞE TÜLİN; 109203
    The prosperity of a firm depends highly on the functional integration of its various departments, and in particular the cooperation between the production and the marketing departments appears to have a large impact on the well-being of a firm. The aim of this study is to model an aggregate production planning problem that considers the production and marketing functions in a simultaneous manner. The developed multi-period, multi-product model with the objective of profit maximization reflects the characteristics of both departments. Demand for each product in each period is generated through the use of demand functions. The advertising efficiency and price of the products are determined within the model. The cash outflow for advertising expenditures is approximated by a quadratic cost function, and the production cost exhibits economies of scale. Also, stockouts are taken into consideration in the form of backlogging. A solution methodology consisting of two phases is developed, and cases displaying some important characteristics are generated and solved in order to test the performance of the proposed model. This methodology incorporates linear mixed integer programming which is embedded in two different search techniques, along with a dynamic programming-based heuristic procedure.
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    A mathematical model for the delivery routing problem via drones
    (Pamukkale Üniversitesi, 2019) Han, Sinem; Özer, Begüm; Alioğlu, Barış; POLAT, ÖZGÜR; AKTİN, AYŞE TÜLİN
    Logistics sector aims to distribute merchandise to designated places on time, while focusing on the entire process that starts from storing the goods at warehouses. Transportation by land is the most common and preferred way in logistics in terms of cost and service. Studies have been going on for drone transportation with the purpose of reducing the service time and making the process automated. Advancement of this technology has led to an increase in transportation with drones. Traffic is a huge problem in land transportation causing delays in delivery time. Furthermore, nature and economy are affected because of the CO2 emission, which is a consequence of intense traffic. In this study, utilizing drones for delivery is proposed with the aim of increasing customer satisfaction and minimizing transportation cost per delivery in a green way. Istanbul's Bakirkoy district is selected as the implementation region, and daily drug delivery to pharmacies with drones is examined. In the first stage, a mathematical model that uses pharmacy coordinates as an input is developed for clustering analysis. In the second stage, one of these clusters is selected, and the location of the drone center that will serve pharmacies in the cluster is obtained by center of gravity method. Then, a vehicle routing model is proposed for finding the drone routes and calculating the total distance travelled within this cluster. Mathematical models are solved with GAMS/CPLEX solver. It is believed that this study will trigger new studies in "green delivery" literature in near future.
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    A hierarchical planning approach for a production-distribution system
    (TAYLOR & FRANCIS LTD, ONE GUNPOWDER SQUARE, LONDON EC4A 3DE, ENGLAND, 1999-11-10) Özdamar, Linet; AKTİN, AYŞE TÜLİN; 57877; 109203
    A production-distribution model involving production and transportation decisions in a central factory and its warehouses is developed. The model is based on the operating system of a multi-national company producing detergents in a central factory from which products are distributed to geographically distant warehouses. The overall system costs are optimized considering factory and warehouse inventory costs and transportation costs. Constraints include production capacity, inventory balance and fleet size integrity. Here, a hierarchical approach is adopted in order to make use of medium range aggregate information, as well as to satisfy weekly fluctuating demand with an optimal fleet size. Thus, a model which involves an aggregation of products, demand, capacity, and time periods is solved. In the next planning phase, the aggregate decisions are disaggregated into refined decisions in terms of time periods, product families, inventory and distribution quantities related to warehouses. Consistency between the aggregate and disaggregation models is obtained by imposing additional constraints on the disaggregation model. Infeasibilities in the disaggregated solution are resolved through an iterative constraint relaxation scheme which is activated in response to infeasible solutions pertaining to different causes. Here, we investigate the robustness of the hierarchical model in terms of infeasibilities occurring due to the highly fluctuating nature of demand in the refined time periods and also due to the aggregation process itself.
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    An Analytical Approach to Machine Layout Design at a High-Pressure Die Casting Manufacturer
    (Springer-Verlag Singapore Pte Ltd., 2022) EMİR, OĞUZ; AKTİN, AYŞE TÜLİN
    Sahin Metal was founded in 1975 on a 7500 m(2) area in.Istanbul to supply high-pressure aluminum casting parts to a variety of industries, especially automotive manufacturers. 64 different products are produced with various routings through 14 workstations in the plant. Being a Tier 2 company, these products are then sent to Tier 1 firms and finally reach the leading vehicle manufacturers. Nowadays, increasing competition, changing customer demands, and quality targets bring the necessity of restructuring internal processes. In this regard, Sahin Metal plans to rearrange the existing machine layout to minimize the distance traveled between departments by taking into account the material flow. This study aims to determine an efficient machine layout design by implementing analytical approaches. The study is started by visiting the production facility and meeting with the company's engineers to determine the project roadmap. Following that, the data collection process is initiated, and ABC analysis is performed to define product classes. After this identification, two approaches are utilized simultaneously. The Hollier method is employed to find a logical machine arrangement. In addition, a mathematical model based on the Quadratic Assignment Problem (QAP) is developed to obtain the optimum machine layout. The developed integer nonlinear model is solved by CONOPT using GAMS software under various scenarios. Finally, these results are compared with the existing system, and a convenient layout design is proposed to the company.
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    An Analytical Approach to the Inventory Management System of Turkish Technic
    (2018) Karagöl, Belma Nüvit; Çöpürkaya, Derya; Kodaz, Kubilay; Ünek, Simge; AKTİN, AYŞE TÜLİN; 109203
    Time, which constitutes an important limitation in a company’s operations, is a factor that affects the supply chain processes and cash flow. Another element that has an influence on cash flow is customer satisfaction. Sector leaders are mostly companies having high customer service levels. Moreover, there exists a strong bond between customer satisfaction and inventory. This study aims to propose an analytical approach to the inventory management policy of a company operating in the aviation industry. Founded in 2006, Turkish Technic has been established to support Turkish Airlines and other aviation companies in their maintenance, repair and overhaul (MRO) services. Aircraft are served in the hangars located at Istanbul Atatürk and Sabiha Gökçen Airports. As a result of their high safety and quality standards, Turkish Technic has become the leader MRO company in the region. Being an exemplary model in its sector, the company relies on the effective management of the diverse parts held in stock both as an indicator of the existence of a robust inventory system, and for a sustainable reputation. Since Turkish Technic holds a variety of parts, initially an ABC analysis will be performed in selecting the most critical ones. Using the past data acquired from the company and Arena Input Analyzer, demand distributions will be obtained and demand quantities for 2018 will be forecasted for each selected part. In the next step, optimum order sizes, safety stock levels, and reorder points of these parts will be calculated. Mathematical modelling and Excel will be used to determine these quantities. The aim is to minimize inventory holding costs, while meeting a preset cycle service level (CSL). The proposed model will be solved using GAMS/CPLEX software. Scenario analysis will be performed to observe and compare the effects of different CSLs on the behaviour of the company’s inventory system.
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    THY Teknik’te Motor Kiralama Sürecinin Değer Akış Haritalama Metoduyla Analizi
    (2016) Çağlayangil, Özlem; Çoban, Doğan Can; Taşkın, Ayşe Nur; AKTİN, AYŞE TÜLİN; 263817