Tedarik zincirinde işbirlikçi bir optimal stok kontrol modeli
A model for optimal collaborative inventory control on supply chain
Yazar: DİDEM TAMDEĞER
Danışman: DOÇ. DR. HÜR BERSAM BOLAT
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / İşletme Mühendisliği Ana Bilim Dalı
Derece: Yüksek Lisans, 2013
Konu: İşletme = Business Administration
Dizin: Lojistik hizmetler = Logistic services ; Tedarik zinciri = Supply chain

Özet:
Şirketler, düşük maliyet ile müşteri memnuniyeti sağlama gibi rekabetçi avantajlar kazanmak amacıyla tedarik zincirinde stratejik işbirliği yapma yoluna gitmektedir. İşbirliğine dayalı ilişkilerde iyi bir kar marjı elde etmek ise, etkin stok ve depolama uygulamalarını sürdürme yeteneğine bağlı olarak değişmekte; bu konuda uzmanlaşan üçüncü parti lojistik firmalarının müşteri ihtiyaçlarına göre şekillenen faaliyetlerini öne çıkarmaktadır. Bu çalışmada, son yıllarda tanınırlığı artan ?Tedarikçi Yönetimli Stok uygulamalarından yola çıkılarak üçüncü parti lojistik firmasının da bulunduğutedarik zincirindeki maliyetleri hem üretici firma hem de üçüncü parti lojistik firma açısından optimal yapan ilişki tipleri incelenmiştir. Üçüncü parti lojistik firması ile yapılan işbirliği sonucu meydana gelen değişiklikleri incelemek için üç aşamalı tek üretici- üçüncü parti lojistik firması-tek alıcı firmadan oluşan bir tedarik zinciri yapısı göz önüne alınmıştır. Zincir boyunca oluşan stokla ilgili taşıma, depolama maliyetlerini en küçükleme amacı altında üçüncü parti lojistik firmasının üreticiden aldığı, depoladığı ve alıcılara ikmal yaptığı optimal miktar ile taraflar arasında anlaşılan kontrat fiyatını belirlemek için matematiksel bir model geliştirilmiştir. Modelde ele alınan karar değişkenleri kurgulanan senaryolar üzerinden oluşturularak, her bir senaryonun uygulanabilirliğini test etmek amacıyla tekstil sektöründe faaliyet gösteren ABC firmasından alınan sayısal değerlerkullanılmıştır. Senaryolardan birinde sadece üretici firmanın zincir boyunca verilecek kararlarda söz sahibi olduğu varsayılırken, bir diğer senaryoda sadece üçüncü parti lojistik firmasının karar verici konumda olduğu varsayılmıştır. Bu iki senaryoya karşı, farklı sektörlerde faaliyet gösteren bu şirketlerin işbirliği yaparak zincir maliyetini minimum yapacak şekilde birlikte karar vermesi varsayımı altında üçüncü bir senaryo kurgulanmıştır. Her bir senaryo çalışmasında, değişen parametrelerin ikmal sıklığı, kontrat fiyatı ve toplam maliyet yapısına etkisi incelenerek duyarlılık analiz çalışması yapılmıştır. Üretici firma ve üçüncü parti lojistik firması oyun teorisi yaklaşımında bahsi geçen oyuncular olarak kabul edilip kurgulanan her bir senaryo çalışmada ele alınan tarafların birbirlerine karşı uygulayabileceği birer strateji olarak düşünülmüştür. Elde edilen sonuçlar oyun teorisi yaklaşımında kullanılan ödemeler matrisi ile oluşturulmuş ve bu yaklaşımda kullanılan çözüm yöntemleri ile sonuçlar değerlendirilmiştir.

Abstract:
Under increased competition conditions, on the one hand firms should transport right product on time to its potential customers on the other hand they should work with suppliers efficiently for supplying required raw material or products with minimum cost. In accordance with this necessity, firms target to create a advantage against their competitors for providing customer satisfaction with minimum cost by restructuring their supply chain management methods. In this competitive enviroment new management trends are developed, cost reduction and service level improvement become significative goals. To reach these goals, efficient inventory management is the most effective tool for supply chain activities. Despite efficient inventory management support goals of supply chain partners, uneffient management may destroy firms and its other partners on the same chain. Firms serch a solution for when and how many stock they should hold based on estimated demand informations with the equilibration goal between service level and inventory costs. With this serach, they choose to restructure their logistic infrastructures and inventory management activities by outsourcing for reducing operational cost. At this point, firms should make a cooperative attempt for both strengthening their relationships with other firms and coordinate its internal activities. Therefore they are willing to form strategic cooperation with the aim of gaining competitive advantages as lower logistics costs and higher customers? loyalty. In the collaborative systems, obtaining a good profit margin depends on the ability of maintaining efficient inventory and warehousing practices which increases the importance of third party logistics firms' activities which are determined by the specific requirements of the clients. In this study, we investigate relationship types between third party logistic and manufacturer firm which makes supply chain costs optimal in terms of both manufacturer and logistic firm based on Vendor Managed Inventory (VMI) practices, a widely discussed supply chain collaboration concept in recent years.Vendor managed inventory practice reorganizes the traditional system of ordering and replenishment characterized by transfer of purchase orders from the distributor to the supplier. The supplier himself indeed decides the quantity to be delivered on the basis of information about sales and the stock level in the distribution center, taking into account the orders already acquired by outlets and following a pre-established programme of replenishment. The distributor, on the other hand, has to guarantee a continuous flow of information to enable the manufacturer to formulate realistic order proposals and make reliable provisions. The key characteristics of VMI are thus short replenishment lead times, and frequent and punctual deliveries that optimize production and transport planning. As a result, bullwhip effect go away with the information sharing between partners.xxVendor managed inventory aims to reduce inventory levels along the supply chain while meeting customer satisfaction from manufaturer to the area where buyer presents product to his customer. It is assumed that this goal can be achieved by making cooperation between supplier and buyer. Many benefit like cost reduction on warehousing and ordering, increase on turnover rate and customer satisfaction, reduction on stock outcan be provided with Vendor Managed Inventory program. Beside these benefits, if this program implements uncorrectly, it has been seen that there are negative effects and unsuccesful ends of initiatives. For this reason, partners on the supply chain who will make cooperation under Vendor Managed Program should trust each other and share information completely. To adopt this cooperation under VMI practice, there are some significative environmental effects. Cooperation between partners becomes stronger when competition increases on supplier or manufacturer market. Besides that, cost of supplier or manufacturer inceases when demand structure of buyer have changable and uncertain pattern leading to destroy cooperations.In literature review, it is seen that numerious models conducted on VMI over various assumptions through cooperative practices on inventory management models. Studies which are conducted by different authors are examined what assumptions they have built on, how they are formed and resulted. In literature it is mentioned that with the increasement of supplier firms and extension of supply chain, coordination problems of supplier occur between its buyers. This reveals that an intermediary firm may be need between manufacturer and supplier firms. Based on this necessity, third party logistic firms are thought as intermediary firm under contract based cooperative approachs. In this study, three echelon supply chain is modelled by adding third party logistic firm which performs as a connecter between two partners to two echelon supply chain that implements vendor managed inventory strategy. In this study, we purpose to examine the changes under collaboration between third party logistic firm and manufacturer firm. With the aim of minimizing transportation, warehousing cost and inventory related costs in the supply chain, a mathematical model is proposed to determine optimal replenishment quantities (that third party logistic firm take from manufaturer area, hold and replenish to buyer area) and contract prices (that is agreed between partners). Firstly decision variables, parameters and assumptions are built and cost functions of manufacturer third party logistic firm are formed. To find the optimal relationship between manufacturer and logistic firm we fictionalised three scenarios that differs from each other on inventory management responsibility and policies. Common assumptions that are valid for all scenarios are that logistic firm replenish according to daily demands of buyer and hold inventory for manufacturer firm; in return manufacturer pay a service price per product to logistic firm. In one of the scenario, it is assumed that only manufacturer firm has the responsibility to make decision on stock management, by contrast, it is assumed that responsibility on decisions belongs to only third party logistic firm in the other scenario. Over against these assumptions, third scenario is fictionalised under cooperation between manufacturer and logistic firm that makes supply chain costs minimum. To test the applicability of each scenario in the model, numerical datas taken from ABC firm which operates in textile sector, has production centers and buyer stores both in domestic and foreign. We assumed that this firm manufactures product foreign and present them in its own buyer areas. Impact of varying xxiparameters on replenishment frequencies, pricing and total cost structures are investigated and sensitivity analysis is made in each scenario study. Manufacturer and third party logistic firm are accepted as players that is mentioned on game theory approach and each scenario fictionalised in the model are utilized as a strategy that can be implemented by partners to each other. With this perspective, we decide to evaluate the results by game theory approach.As a result of model; it is found if manufacturer firm manage supply chain inventory without considering third party logistic firm?s decision or costs, contract price that logistic firm offer in return for the warehousing and transportation services and total cost of supply chain increases. On the other hand, when logistic firm has the right to comment on warehousing and replenishment decisions it is seen that supply chain costs decreases. It is seen that costs like carrying, transportation and opportunity have different effects on the supply chain system. With frequent replenishments, logistic firm drops stock level in its warehouse as well as lowering operational difficulty in buyer?s area. However transportation and ordering costs are the costs that should not be forgotten. Let?s assume that manufacturer firm decide to take advantage of scale economy with increased demand trend of its firm so he can reduce production cost per unit. Under this condition, our model is evaluated again. It is observed that total supply chain cost and seervice price which is offered by logistic firm decreases until a certain point and then increase again when only manufacturer firm has the responsibility on inventory management. At this point, significance of cooperation between two different firm on the same supply chain and conjugate movement on the inventory control along supply chain is emphasised again. When we evaluate the results with game theory approach and solution methods considering opportunity cost to each other and only one strategy pair is found. Expected value of the game is get under collaborative condition and same comments are seen that if manufacturer firm act independently and has responsibility on inventory control himself, it would be disadvantageous and unsuccesful decision for him. This model assists to manufacturer and third party logistic firms for making decision on different cost structures and conditions. Our model can be developed with different perspectives on future studies. Model can be built with more than one buyer firm and stockout condition. Deciding optimum transportation quantities can be evaluated again with limited capacity of third party logistic firm. Third party logistic firm would provide service with increased demand. According to this, logistic firm can make different discount rates on contract price on different demand levels. Under this assumption, quantitiy discount stock control model can be inserted to this model and fictionalised scenarios can be evaluated again.