E-ticaret lojistiğinde son adım dağıtım hizmetlerine yönelik çok seferli ve zaman pencereli sayısal bir yaklaşım
A numerical approach to last mile delivery distribution with multitrip and time window constraints in e-logistics
Yazar: DUYGU NURCAN
Danışman: PROF. DR. FERHAN ÇEBİ
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / İşletme Mühendisliği Ana Bilim Dalı / İşletme Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2020
Konu: Bilim ve Teknoloji = Science and Technology ; İşletme = Business Administration

Özet:
Son dönemlerde giderek artan e-ticaret uygulamaları ile birlikte e-lojistik kavramı da son yıllarda önemli ölçüde gelişmiştir. E-lojistik içerisinde maliyetlerin ürün başına en yüksek olduğu bölüm son adım teslimatı (last mile delivery) olarak adlandırılan şehir içi teslimat olarak görülmektedir. Dağıtım ya da toplama lokasyonların optimize şekilde rotalanması için literatürde sıkça karşımıza çıkan Araç Rotalama Problemleri (ARP) kullanılmaktadır.

Bu tez çalışmasında e-ticaret kavramları, e-lojistik kavramları, son adım teslimatı ve araç rotalama problemleri incelenmiştir. Problem, şehir içi lojistiği yapan perakende firmalarında zaman penceresi kısıtı (time window series) ve çoklu sefer dağıtımı (multi-trip) ile birlikte maliyetleri minimize edecek rotalama oluşturma ihtiyacı ile ortaya çıkmasıdır. Benzer çalışmalar literatürden derlenerek alınmış ve incelenmiştir.

Çalışma kapsamında, perakende ürün dağıtımı yapan bir firmanın dağıtım problemi incelenmiştir. Müşteri memnuniyetini adına her siparişi ilk müsait dağıtıcı kurye ile gönderen firma, bu sayede yüksek müşteri memnuniyeti yakalamıştır, ancak artan rekabet koşulları ve düşen karlılıklar dolayısıyla maliyet avantajı sağlayacak bir çözüm aramaktadır.

Sipariş üzerine tekil dağıtım yerine kuryelerin zaman kısıtı ve çok seferli dağıtım içerisinde en uygun dağıtım rotasının belirlenmesi amaçlanmaktadır. Model çıktısında birden fazla sipariş ile firmadan ayrılmaları ve bunun maliyete ve müşteri bekleme süresine olan etkisi incelenmiştir. Bu sayede maliyete yapılan iyileştirmelerin müşteriye teslim sürelerindeki değişikliğe etkisinin de görülmesi sağlanmış ve değerlendirme yapılmıştır.

İşletmenin problemini çözmek adına Zaman Pencereli Simetrik Kapalı Uçlu Çok Amaçlı ve Çok Seferli bir Araç Rotalama Problemi (Multifunctional Multi-trip Symmetric Closed Loop Vehicle Routing Problem with Time Windows) kullanılmıştır.

Özellikle son adım teslimatlarında görülen dinamik sipariş talebi, zaman penceresi kısıtı ve çok seferli dağıtım kavramları ile ilgili çalışmaların az olduğu görülmüş ve probleme uygun çok amaçlı tam sayılı doğrusal bir matematik model geliştirilmiştir. Gerçek hayat örneği oluşturması adına örnek işletmenin verileri alınmış ve veriler GAMS CPLEX 30.2.0 versiyonunda tamsayılı programlama yöntemi ile çözülmüştür. Çıkan değerlerle dağıtım için maliyeti minimize edecek yol bulunmaya çalışılmıştır. Son kısımda bulgular incelenmiş, tartışılmış ve gelecek çalışmalarda kullanmak üzere öneriler belirtilmiştir.

Abstract:
Along with the increasing e-commerce applications in the last period, the concept of e-logistics has developed significantly in the recent years. In e-logistics, the section with the highest costs of per piece is seen as in-city delivery which is called last mile delivery. Vehicle Routing Problems (ARP), which are quite common in the literature, are used to optimize the routing of locations.

The problem examined in this thesis study is to minimize the costs of a company engaged in-city logistics with time window constraints in the retail distribution sector and multi-trip distribution. At this point, the company, which prioritizes customer satisfaction, wants the order delivery times to be minimized as a secondary goal. For this purpose, it is aimed to create a mathematical model that will deliver the orders to the customer at the best delivery time with the minimum cost.

Within the scope of the study, the distribution problem of a company that distributes retail products in Şişli district of Istanbul has been examined. The company takes many orders due to its location. The company, which prioritizes customer satisfaction, has adopted the principle that the incoming order will be delivered immediately with the first available courier and that each courier will reach the customer in the fastest way with a single order. Thanks to this, the company, which has achieved high customer satisfaction, does not want to compromise on its values as it associates order continuity to this customer satisfaction. However, this company is looking for a solution that will provide cost advantage due to increasing competition conditions and falling profitability.

In-city logistics solutions have become a significant subject especially with the development of e-commerce in the recent period. Especially time-window and multitrip distribution routing requires service in a short time and under difficult constraints. There are many different optimization models for in-city logistics. Our aim is to create a suitable distribution model for a routing problem with a multitrip and time-windowed. This model should be able to work in case of different time window and high repeat orders. In particular, a data set has been designed that can respond to different data sets. With the distribution model created for real life, the evaluations of the findings will be made with more clear way.

It is aimed to determine the most appropriate distribution route within the time constraints and repetitive trips of the couriers instead of the private distribution made for the purpose of customer satisfaction. In the model output, leaving the company of the couriers with more than one order and its effect on cost and customer waiting time are examined. In this way, it is ensured that the effects of cost improvements on the change in delivery times to the customer are seen and evaluated.

Due to the general nature of the problem, customers order products online and the company tries to deliver these orders at certain time intervals. The main purpose of the problem solution is to create the lowest possible logistic cost in the process of delivering the orders to the customers by the motorcycled couriers depending on the time constraint. As a secondary goal, our solution aims to minimize the delivery time to customers. In this respect, our model is a multi-functional mathematical model.

Our problem has been addressed as a vehicle routing problem due to the goal of distributing customer demands on the most suitable route with time and capacity constraints by leaving a warehouse with one or more distributor vehicles depending on a specific route. Vehicle routing problems are NP-difficult class problems. Especially as in real life examples, the solution of more than one variable becomes more difficult.

In the study, e-commerce concepts, e-logistics concepts, last mile delivery and vehicle routing problems are examined. Similar studies have been compiled from the literature and examined.

In order to solve the problem, it has been decided to continue with Multifunctional Multi-trip Symmetric Closed Loop Vehicle Routing Problem with Time Windows.

Unlike similar studies, the model was created based on the departure and arrival times of the courier instead of the order arrival and delivery times. In this way, conditions are defined for multiple orders delivery at once.

The details of the model are shown mathematically and written in accordance with the GAMS model. All details are shown in detail in the study.

The studies on the concepts of dynamic order demand, time window constraint and multi-trip distribution, especially seen in the last step deliveries, have been found to be limited and a new integer linear mathematical model has been developed. In order to create a real life example, the data of a sample retail company has been taken and solved in GAMS CPLEX 30.2.0 version with integer programming method. With the output values, it has been tried to find a way to minimize the cost for distribution. 

The current situation of the company was examined with the prepared model. According to the solutions offered for the problem, the model was run and approximately 32% cost advantage was provided. This advantage is important for the competitiveness that the company wants to achieve.

In the study, the cost advantage of combining orders has been seen, and an integer linear mathematical model has been developed that can be used for repeat and different evaluations. With the use of the model, it has been an idea on many issues such as the number of courier, delivery time and order distance limits.

The findings were shared with the company at the end of the study, and it was found that the study and the model developed were suitable for the solution of the company's problem. It is provided to see the opportunity cost lost by the business. The model will give an idea to the company for the future as well as the current situation.


It is thought that the study will also contribute to reducing the city traffic population and reducing carbon emission values thanks to less distance.

In addition, instead of taking the order delivery time 45 minutes as in the study, the effect of shorter delivery time selection on customer perception, or the effect of longer delivery time selection on reducing the operating costs can be examined. Similarly, the effect of keeping distribution distances narrower or wider than determined in the study on customer loss and cost advantage can be evaluated.

On the critization of the study, it is a constraint that the capacity limit is determined and modeled as two. In order for this study to be analyzed in different capacity constraints, improvements must be made in the model. This is actually one of the forward-looking suggestions.

Another criticism is that although there is a free choice between tours in routing, the selection in the tours is not released. The restriction of the first in first out in the model has prevented freedom of choice. However, it can be evolved into a model in which the model and the constraint evaluate multiple options with cycles and iterations, thus allowing freedom of choice. This criticism correction is an important suggestion for future studies, especially in order to prevent losses due to this limitation in studies on big data.