M.S. Theses
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Browsing M.S. Theses by Author "Aksen, Deniz."
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Item A bilevel P-median Problem for location and protection planning of critical facilities(Thesis (M.S.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2009., 2009.) Piyade, Nuray.; Aras, Necati.; Aksen, Deniz.In this thesis, we focus on the problem of location and protection planning of critical facilities. This problem involves a Stackelberg game between a system planner (defender) and a potential attacker. The system planner aims to both find the locations of p critical service facilities and determine the ones among them that should be protected. Following this twofold action, the attacker decides which facilities to interdict having the location and protection information of the opened facilities. This problem involves strategic decisions which can be taken either sequentially or simultaneously. In this study, we consider both of these cases. In the first case, the system planner first decides on the locations and then determines the protection plan of these facilities. In the second case however, the system planner gives concurrent decisions about location and protection of the facilities. Both cases are of a bilevel nature. Therefore, we formulate this problem as a bilevel mixed-integer programming problem. We propose two solution methods. The first one is a two-phase tabu search heuristic for the case which involves concurrent decision process and a sequential solution method for the second case where the system planner prefers to give sequential decisions. Both of the methods include a binary search tree embedded into it. The efficiency of the proposed algorithms is tested on an extensive amount of randomly generated test instances each with two budget levels, namely low and high. We also consider another case where the system planner does not have any financial resources to protect the facilities from an attack. This line of vision helps system planner to determine the critical facilities from the attacker’s perspective. The results show that the protection budget plays a significant role in maintaining the service accessibility after a possible attack.Item A tabu search heuristic for the vehicle routing problem with time deadlines and asymmetric distances(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2013., 2013.) Ekmen, Pelin.; Aras, Necati.; Aksen, Deniz.Reducing transportation cost and meeting customer requirements on time are the two main targets of transportation, logistics, distribution and supply chain management. In this thesis, we focus on the Vehicle Routing Problem with Time Deadlines and Asymmetric Distances which appears as an application in transporting cash to ATMs. Two mathematical models are presented to solve this problem. Since large instances cannot be solved exactly, a metaheuristic method based on Tabu Search algorithm is also proposed to determine near-optimal routes by formulating a mixed integer linear model.The algorithm is tested on randomly generated asymmetric instances derived from the Solomon benchmark problem instances. They are solved by CPLEX 12.5 solver within GAMS suite 24.0 and compared with the results obtained by the Tabu Search heuristic. Furthermore, we convert some of Christofides et al. classical VRP test problems known as CMT in the literature available, and solve them by our algorithm. We compare our solutions with the best known results in the literature.Item A trilevel defender-attacker problem with partial interdiction(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2013., 2013.) Çoğal, İlknur.; Aras, Necati.; Aksen, Deniz.Terrorism is one of the most serious problems of today's world. Thus, the reliability and robustness of network systems with critical infrastructure are crucial. In this thesis, the best location-allocation strategy is sought to serve the customers in a network system in case of intentional disruptions caused by terrorist attacks. For this purpose, a trilevel mixed integer programming model is introduced. In the upper level, the system planner (leader) determines the facility locations, the capacities of the facilities, and the assignment of customers to the facilities. The attacker (follower) makes the choice of the interdiction fractions on the opened facilities in the middle level. In the lower level, the system planner sets the reassignment of the customers considering the capacity reduction of the facilities and decides which customers are served by outsourcing. In the upper level, a tabu search heuristic is used to locate facilities and commercial solver Cplex is employed to determine the capacities of the opened facilities and pre-attack allocations. The best interdiction strategy is found by the Electromagnetism Like Algorithm (EMLA) in the middle level. Finally, the postattack allocations are found with Cplex. This solution method gives quite satisfactory results.