Design of an automated warehouse for air cargo: The case of

Journal of Business Logistics, 1994 by van Oudheusden, Dirk L, Boey, Peter

In the last few years, warehouse automation has progressed at a rapid pace, and in a number of papers the design of such systems has been analyzed. However, few texts can be found on automated warehousing for air cargo terminals. An air cargo terminal warehouse has unique characteristics and needs; its design must reflect these individual requirements. In this paper, a real-world case study is made on the design of an automated warehouse for the Thai Airways Cargo Terminal at Don Muang International Airport, Bangkok, Thailand. First, air cargo operation is introduced and the problem of the case under study is described. Second, since the existing literature does not deal with air cargo, the adaptation and modifications needed in the formulation and solution methodology for optimal design are explained. The design obtained is further tested by simulation. Finally, the results are evaluated to give suggestions for the implementation of such a system.

AIR CARGO TERMINAL OPERATIONS

Air cargo terminal operation is a complex combination of information and material flows, having many processing stages along the flow lines. These processing stages are customs inspection, documentation, build up and break down of air cargo containers (known as unit load devices, or ULD), transportation, and other material handling operations. The arrival and service rates vary throughout the day and the storage systems should be capable of dealing with the uneven elements in the flow process. Because of the different sizes of cargo handled, there exist three main storage systems in a cargo terminal: unit load storage, regular-sized cargo storage, and mini-load storage. The unit load storage system is designed to store ULDs. The system comprises elevated transfer vehicles (ETV), specialized material handling equipment that facilitates rapid storage and retrieval of ULDs from the racks. The regular-sized cargo storage system deals with bin storage: individual consignments before the build up and breakdown operation of the ULDs are stored in bins on the warehouse racks. The mini-load storage system serves small consignments and mail cargo. Many modern terminals have automated sorting systems and carousel storage systems. However, manual systems are still widely used for mini-loads in many large terminals.

Among these three storage systems, the regular-sized cargo occupies more than 90% of the total cargo handled by the air cargo industry, and this storage system in known as the cargo warehouse. The purpose of this warehouse is to serve as a temporary storage before the next operation can be carried out. The storage and retrieval rates are high. An efficient storage retrieval system is vital or else an excessive queue line will form. The queue line can hamper terminal flow operation. Thus, automated storage retrieval systems (AS/RS), also known as automated warehouses, are now introduced at air cargo terminal warehouses to meet the high throughput rate.

THAI AIRWAYS CARGO TERMINAL WAREHOUSE

Over the centuries, Siam was a crossroad where Eastern Hemisphere traders and voyagers met on their east-and-west bound journeys. Today, its descendant, Thailand, is the gateway to Asia and the Pacific Rim from the other sides of the globe. Not only its geographical advantage makes its airport cargo terminal the aviation hub of the Far East Region, but also the tremendous growth the Kingdom encountered in recent years, resulting in an excessive increase in cargo handling. The tonnage handled by Thai Cargo increased by about 300% in the late 1980s. By the end of 1991, tonnage had reached 0.37 million tons per year, as shown in Figure 2. (Figure 2 omitted)

Over the last years, the facilities of Thai Cargo have been extended due to the rate of increment in tonnage handled. In 1986, a new terminal was opened that covered 25,920 m sup 2 . Such modern air cargo container handling facilities as elevated transfer vehicles (ETV) were installed. ETVs are specialized material handling equipment that store and retrieve the ULDs. Although the ULD handling facilities were upgraded in the new terminal, the main warehouse, which handles almost 90% of total cargo, remains non-automated. Therefore, during the peak hours, the cargo arrival rate to the warehouse exceeds by far the service rate of the forklifts that handle the storage retrieval operations in the classical warehouse. Excessive queuing causes terminal congestion and results in poor handling and delays.

As the demand grew, additional storage space was required. An extension of 22,000 m sup 2 of the terminal was proposed, but was not approved by the Airport Authority of Thailand. Space utilization is a crucial issue for the airport planners because of the location of the airport in a suburban area of congested Bangkok. These circumstances leave the management with few options except to introduce an AS/R system so that not only efficient space utilization can be achieved, but also that prompt storage retrievals can be realized. However, AS/R systems arc very expensive and once installed, the physical configuration can hardly be changed To find an optimal design that minimizes the total cost while ensuring smooth operation of the terminal is of primal managerial interest in the early stage of planning.