تقاضا برای انرژی در طول چند دهه گذشته افزایش یافته است. صنعت گاز طبیعی مایع[1] به عنوان یکی از راه‌های صدور گاز به مناطق دور دست شناخته شده که با توجه به فاصله مناطق تولید و مصرف گاز طبیعی، مزایای اقتصادی و زیست محیطی استفاده از گاز نسبت به سایر سوخت‌های فسیلی و توسعه تکنولوژی مایع‌سازی گاز مورد توجه واقع شده‌است. از آنجا که مقدار زیادی از پول سرمایه‌گذاری‌شده در صنعت گاز طبیعی مایع صرف حمل‌و‌نقل می‌شود، تصمیم برای مسیریابی و زمان‌بندی کشتی‌ها اهمیت بسیاری دارد. در این مطالعه بر روی بنادر مبدا و مقصد و حمل‌و‌نقل، از زنجیره تأمین گاز طبیعی مایع تمرکز شده‌ و هدف از این تحقیق ارائه یک مدل کارآمدتر با هدف به حداقل‌رساندن هزینه‌ی ارسال گاز طبیعی از بنادر اولیه به بنادر مقصد، با در نظر گرفتن تنوع محصول بوده است. برای حل مدل برنامه‌ریزی عدد‌صحیح مختلط پیشنهادی از روش شاخه و کران (نرم‌افزار [2]GAMS) استفاده شده‌است. با استفاده از نتایج حاصله می‌توان مسیریابی دقیق هر یک از کشتی‌ها، تعداد هر نوع محصول که توسط هر کشتی بارگیری شده و زمان‌بندی حرکت کشتی‌ها از بندر اولیه به بنادر مقصد و از مقصد به یکی از بنادر مبدا را مشخص نمود پرونده مقاله

این مقاله مدلی ریاضی برای مکان یابی بهینه شعب تامین اجتماعی در شرق تهران با استفاده از داده های 1390 می‌پردازد. درابتدا به کمک مدل ریاضی صفر و یک مشخص می‌شود که کدام مناطق نیاز به تاسیس شعبه جدید دارند. سپس به تعیین مکان دقیق شعبه در منطقه مورد نظر پرداخته می‌شود. برای این منظور، عوامل موثر در مکان یابی تعریف می‌گردند. این عوامل عبارتند از: قیمت زمین، جمعیت بیمه شده، نزدیکی به پارکینگ ها، نزدیکی به بانکهای عامل، نزدیکی به کارگزاری‌ها ، نزدیکی به درمانگاه‌ها، نزدیکی به مترو ، نزدیکی به ایستگاه های اتوبوس و نزدیکی به بزرگراه. با توزیع پرسشنامه و انجام مقایسات زوجی و روش بردار ویژه، ضریب اهمیت هر عامل تعیین گردیده و اوزان حاصل به عنوان ضرایب تابع هدف یک مدل دیگر مورد استفاده قرار می‌گیرند. این مدل بهترین مکان برای تاسیس شعب جدید را مشخص می‌کند. نتایج مدل اول نشان می‌دهد که از بین 18 منطقه حال حاضر در شرق تهران ، 14 منطقه نیاز به احداث شعبه جدید دارند. در نهایت، مکان بهینه شعب جدید با حل مدل دوم مشخص گردید. پرونده مقاله

Inappropriate gate assignment has some consequences such as flight delays, inefficient usage of resources and customer’s dissatisfaction. Airports are service sectors and provide services to their customers. Passengers and airlines are two main customers of an airport. Current research presents a novel mathematical model for gate assignment problem to minimize customers’ dissatisfaction. In other words, passengers walking time as well as flight delays are minimized in as objective function. In this model, transit and non-transit passengers and also arrival and departure flights are considered. Moreover, operational safety constraints are inserted to avoid collision of large aircraft. The proposed model is solved by Benders decomposition approach. The model is applied in San Francisco International Airport for a 24-hour horizon. The result shows that walking time of non-transit passengers, walking time of transit passengers, delay in departure flights and delay in arrivals include 74.55, 23.32, 0.96 and 1.17 percent of the objective function, respectively. پرونده مقاله

The present study developed a bi-level mathematical model to determine optimal routes for repair teams in charge of inspecting urban traffic lights. In this model, the municipality as the leader locates the construction sites of urban spare parts warehouses and repair centers to minimize the costs of constructing the facilities. At the follower level, the contractor determines the optimal routes for the repair teams. Bi-level models are strongly NP-hard in type. A heuristic algorithm is therefore developed to solve numerical examples, in which the leader first determines different decision-making strategies for the follower through generating a set of justified solutions. In response to the leader’s set of strategies, the follower presents a set of corresponding solutions. The individual solutions of the follower are then entered into the leader model and the corresponding values of the objective function are calculated. A solution with the optimal numerical value for the leader is ultimately selected as the Stackelberg equilibrium. The efficiency of the proposed model and algorithm was evaluated by presenting the computational results obtained from solving several random numerical examples of small, medium and large dimensions through generating the Stackelberg equilibrium and establishing a relationship between the leader and follower levels. The present findings are recommended to be used as a management tool by policymakers in the urban management sector. پرونده مقاله

The companies in the aviation industry require exact scheduling and operation due to their complex and costly activities. The aircraft routing problem (ARP) which meets all of the requirements related to maintenance operations and achieves the minimum costs is among the significant issues for an airline. Solving the ARP includes creating all of the routes and defining aircraft maintenance inspections. The present study aims to review and categorize the recent research on ARP and maintenance operation. To this aim, four significant categories including type of model, maintenance and repair factors, disruption and robustness, as well as objective function and solution approach were defined. Based on the literature review, the integrated study of the airline schedule steps provides better results than the multi-stage review. In addition, defining the combined framework of different maintenance factors generates a more accurate schedule to control the maintenance requirements. Further, applying multiple hybrids meta-heuristic approach leads to significant results. پرونده مقاله

Logistics in upstream oil industry is a critical task as rigs need consistent support for ongoing production. In this paper, a multi-period, multi-product and multi-hub routing and scheduling model is presented for offshore logistics problem. As rigs can be served in specific time intervals, time windows constraints are considered in the proposed model. Despite classic VRP models, vessels are not forced to return hubs at the end of duty days. Also, a vessel may leave and return back to hubs several times during the planning horizon. Moreover, the model determines which vessels are applied in each day. In other words, a vessel may be applied in some days and be inactive in other days of planning horizon. To develop a compromise model, fueling issue is considered in the model. As a rig can be supplied by different vessels in real world cases, the proposed model is split delivery. Based on these challenges and contributions, this research deploys an integrated optimization of routing and scheduling of vessels for offshore logistics. This paper deals with a combinatorial optimization model which is NP-hard. Hence, Genetic Algorithm is applied as the solution approach. The average gap between objective functions of GAMS and GA is only 1.18 percent while saving CPU time in GA is much more than GAMS (about 78.16 percent on average). The results confirm the applicability and efficiency of the GA. پرونده مقاله