A queueing model for the simulation of a port grain elevator

Abstract

Transferring grain from land-based modes of transport to ocean carriers, port elevators represent a substantial portion of the total transportation charges for moving grain from the surplus producing regions of the United States to deficit consuming regions abroad. Port elevators consist of rail car unloading and ship loading facilities, which are linked by grain storage capacity. The optimal combination of these 3 components would minimize the total cost of moving grain through the port, considering both the cost of carriers' time in port and of investment in grain handling and storage facilities for a given volume of exports. A computer simulation model of the port grain elevator system was programmed in GPSS/360. The discrete-event computer simulation program generates ship arrivals and assigns each ship a cargo size and vessel type in a Monte-Carlo fashion from observed statistical distributions of ship arrivals, cargo sizes, and ship types. Depending on the respective size and type of ship, the simulation model loads ships at one of 6 rates. Rail cars arrive in advance of the ship for which the grain is designated; the grain is placed in storage and then loaded on ship. The model's performance index is the average total cost of moving grain through the port, including both the cost of carriers' time and the fixed and variable cost of elevator operation. Through simulation of the Farmers Export Company elevator at the Port of Galveston, Texas, each of the 3 major factors affecting cost (i.e., ship loading rate, elevator storage, and rail car 3 unloading) was studied at two levels and arranged as a 2 factorial experiment. Three-way analysis of variance indicated that a 12 percent increase in the rate of ship loading significantly reduced the cost of moving grain through the port while a 2 ,000,000 bushel increase in grain storage capacity significantly increased intermodal transfer cost. Two additional rail dumps did not affect cost of grain transfer at the port. Increasing the ship loading rate was the most attractive investment alternative. The model further indicated that the expense of carriers' time in port comprised nearly 40 percent of the total intermodal transfer cost of bulk grain at the Port of Galveston.