A novel cross-docking EOQ-based model to optimize a multi-item multi-supplier multi-retailer inventory management system

Nowadays, the retail industry accounts for a large share of the world’s economy. Cross-docking is one of the most effective and smart inventory management systems used by retail companies to respond to demands efficiently. In this study, the aim is to develop a novel cross-docking EOQ-based model for a retail company. By considering a two-stage inventory procurement process, a new multi-item, multi-supplier, multi-retailer EOQ model is developed to minimize the total inventory costs. In the first stage, the required items are received from suppliers and are held in a central warehouse. In the second stage, these items are delivered to several retail stores. The total inventory costs include four main parts, i.e., holding costs at the central warehouse, holding costs at the retail stores, fixed ordering costs from the suppliers, and fixed ordering costs from the central warehouse. The optimal inventory policy is obtained by analyzing extrema, and a numerical example is used to confirm the efficiency of the proposed model. Based on the obtained results, it is evident that the proposed model produces the optimal policy for the cross-docking system. Furthermore, the model enables managers to analyze the effects of key factors on the costs of the system. Based on the obtained results, the annual demand of each retailer, the ordering cost by the central warehouse, the ordering cost at each retail store, and the holding cost at each retail store have a direct impact on the optimal cost. Furthermore, it is not possible to describe the effects of the holding cost at the central warehouse on the optimal cost of the system generally

Production-inventory system with finite production rate, stock-dependent demand, and variable holding cost

In general, traditional production-inventory systems are based on a number of simplifying – but somewhat unrealistic – assumptions, including constant demand rate, constant holding cost, and instantaneous order replenishment. These assumptions have been individually challenged in numerous variations of production-inventory models. Finite production rate models, such as economic production quantity (EPQ) systems consider gradual order replenishment. Stock-dependent demand models assume the demand rate to be an elastic function of the inventory level. Variable holding cost models assume the holding cost per unit per time period to be a function of the time spent in storage. In this paper, the three simplifying assumptions are simultaneously relaxed in a new production-inventory system with a finite production rate, stock-level dependent demand rate, and variable holding cost. Mathematical models and optimum solution procedures, including nonlinear programming, are presented for two functional forms of holding cost variability. The main contribution of this paper is the formulation and solution of a new production-inventory model that more closely represents real-world situations. The realistic assumptions and efficient solution algorithms should make the model practical and useful for industrial applications

COMPRESSED WORKWEEK SCHEDULING WITH CONSECUTIVITY, FREQUENCY AND STRETCH CONSTRAINTS

ABSTRACT In this paper we consider a three-day workweek scheduling problem with three realistic daysoff scheduling constraints: (1) at least two off days per week must be consecutive, (2), employees must get a given proportion of weekends off, and (3) the number of consecutive workdays in any work stretch cannot exceed four. An integer programming model is formulated and efficiently solved by an algorithm that involves three stages: (1) determining the minimum workforce size by primal-dual relations, (2) adding a workforce-size constraint to the integer programming model to expedite its solution, and (3) constructing fair and feasible multiple-week rotation schedules

Operator staffing and scheduling for an IT-help call centre

This paper describes the staffing and scheduling of IT help desk operators for a large petrochemical company. The objective is to reduce the labour cost by determining the best staffing level and employee weekly tour schedules required to meet the workload that varies over a 24-hr operating period. Several steps are taken for the staffing and tour scheduling of an IT help desk agents. First, data on the number of calls is used to estimate hourly labour requirements. Next, new scheduling options are proposed to better match these requirements. An Integer Programming (IP) model is then formulated and solved to determine tour scheduling assignments. Finally, alternative schedules are evaluated in terms of tradeoffs between workforce size and cost, service level and employee utilisation. The chosen tour schedules provide better service with a lower cost and a smaller number of employees.[Received 10 November 2006; Revised 23 April 2007; Accepted 18 June 2007]employee scheduling; staffing; queueing models; stochastic demand; call centre scheduling; integer programming; information technology; IT help desk; help desk operators; petrochemical industry; labour costs; cost reduction.

Optimum multi-period, multi-plant, and multi-supplier production planning for multi-grade petrochemicals

A multi-period production and inventory control problem for a multi-grade, multi-supplier petrochemical product is formulated and optimally solved. Raw materials are available from several suppliers, and several plants (chemical reactors) are used for making the petrochemical product. Several grades of the petrochemical product can be produced by changing the conditions inside each reactor. During transition from one grade to another, a certain amount is produced of off-spec material. The quantity of off-spec production is sequence dependent, i.e. it depends on the two grades that the transition takes place between. The problem is formulated and solved by means of a graphical network model as well as a mixed-integer programming (MIP) model. The MIP model determines, for each time period, the sequence and quantities of different grades produced in each plant, the amounts of raw materials purchased from each supplier, and the inventory levels of each grade. The objective is to maximize the total profit, which is equal to the sale revenue of all regular grades and off-spec materials, minus the raw material costs and inventory holding costs