A model for designing a procurement-inventory system as a defence against a recurring epidemic

The COVID-19 pandemic has caused a general shortage of personal protection products and therapeutic devices, which has highlighted the need for each country to have its own production resources and not depend solely on imports. Given the time that elapses between the onset of an epidemic and its detection, as well as the time required to activate production and the lead time of purchasing operations, it is necessary to have a permanent reserve, which we call shield stock, in order to immediately meet the demand for equipment at the beginning and throughout the course of the epidemic. This situation is analysed in order to identify the most relevant decisions in the scenario described, formulate a cost optimisation model and develop procedures to find the most economical combination of shield stock, domestic production capacity and imports to guarantee the immediate satisfaction of demand and the restoration of the shield stock after the epidemic, as a preventative measure. The procedure is illustrated with a specific pattern of the spread of the epidemic and some numerical examples.Postprint (published version

Using discrete-time mathematical programming to optimise the extraction rate of a durable non-renewabre resource with a single primary supplier

A non-linear discrete-time mathematical program model is proposed to determining the optimal extrac- tion policy for a single primary supplier of a durable non-renewable resource, such as gemstones or some metals. Karush, Kuhn and Tucker conditions allow obtaining analytic solutions and general properties of them in some specific settings. Moreover, provided that the objective function (i.e., the discounted value of the incomes throughout the planning horizon) is concave, the model can be easily solved, even using standard commercial solver. However, the analysis of the solutions obtained for different assumptions of the values of the parameters show that the optimal extraction policies and the corresponding prices do not exhibit a general shape.Peer ReviewedPostprint (author's final draft

Assigning Orders to Suppliers with Linear Piecewise Concave Costs

Purpose: Once a set of suppliers has been determined, according to criteria of quality, price and reliability, among others, there remains the problem of assigning orders to the selected suppliers, in order to cover the needs at the lowest cost. We consider the case in which the needs of a component for a set of plants should be covered by suppliers with linear piecewise concave cost functions, a lower bound on the order size for the non-zero deliveries and a capacity constraint. The purpose is to design procedures for solving this problem. Design/methodology/approach: With the aim of providing practical tools to solve the problem of assigning orders to suppliers with linear piecewise concave costs, two mixed integer linear programs are proposed. Findings: The two MILP models are compared through an extensive computational experiment. This shows that both models, with a slight advantage for one of them, can be solved within a very short time, even when the dimensions of the instance largely exceed those that can occur in real cases. Originality/value: The paper proposes novel models that can be used to solve the problem to optimality in reasonable times and with standard optimization software.Peer Reviewe

Optimal manufacturing-remanufacturing policies in a lean production environment

This study analyses a production-management model that considers the possibility of implementing a reverse-logistics system for remanufacturing end-of-life products in a lean production environment (as opposed to models than use EOQ approaches). Decisions variables are identified (including manufacturing and remanufacturing capacities and return rates and use rates for end-of-life products) and optimal policies are determined. Moreover, the structure of these optimal policies is analysed. The conclusion draw is that, in may realistic scenarios, mixed policies (that is, with return rates and use rates strictly between 0 and 1) can be optimal. This conclusion is contrary to results published in earlier studies, which are based on more restrictive assumption

Research into the area of supply chain

Prospective doctoral students or anybody who wishes to undertake research activities find often difficult to know what research is about, how the settings are where research takes place, and how results are presented and communicated. In addition, for people who want to make innovative contributions within an organization is important to tell apart research from what is not, and where to find the results. In the supply chain field, there are two additional difficulties. On the one hand, the lack of a generally accepted definition of supply chain. On the other, the fact that, alongside research publications, others proliferate that, although being rather informative, speculative or even falling in self-help literature field, are sometimes presented as if they were search results. The text is a tool to answer the questions implicit in the above considerations, in order to reduce the time elapsed between the moment the student decides to do research and when he or she starts to really do it, and thus contribute to improve productivity and quality of research.Postprint (published version

Resolución del Product Rate Variation Problem (PRVP) de grandes dimensiones como un problema de afectación.

El PRVP es un problema que se presenta en las líneas de producción mixtas. Una de las formas de resolverlo consiste en reducirlo al problema de afectación y aplicar entonces algoritmos específicos para este último. Este trabajo presenta un estudio de lo

Embodying decisions on work shifts into strategic manufacturing capacity planning

© 2018 Informa UK Limited, trading as Taylor & Francis Group In spite of the obvious impact that decisions on workforce planning have on the capacity of a production system, they are rarely mentioned in strategic capacity planning literature. This paper analyses the implications of embodying decisions on work shifts in strategic capacity planning and proposes a mathematical programming model that allows treating any type of relationship between the intensity in the use of the production equipment and maintenance and variable production costs. A computational experiment shows that the model can be solved in acceptable computing times for moderate values of the number of types of equipment and of work shifts. Using the model, the optimal solutions for diverse scenarios are presented and compared with those obtained under the assumption that the work shifts must be kept invariable over the entire horizon.Peer ReviewedPostprint (author's final draft

LETRIS: Staffing service systems by means of simulation

Purpose: This paper introduces a procedure for solving the staffing problem in a service system (i.e., determining the number of servers for each staffing period). Design/methodology: The proposed algorithm combines the use of queueing theory to find an initial solution with the use of simulation to adjust the number of servers to meet previously specified target non-delay probabilities. The basic idea of the simulation phase of the procedure is to successively fix the number of servers from the first staffing period to the last, without backtracking. Findings: Under the assumptions that the number of servers is not upper-bounded and there are no abandonments and, therefore, no retrials, the procedure converges in a finite number of iterations, regardless of the distributions of arrivals and services, and requires a reasonable amount of computing time. Originality / value: The new procedure proposed in this paper is a systematic, robust way to find a good solution to a relevant problem in the field of service management and it is very easy to implement using no more than commonly accessible tools.Peer Reviewe

Using optimal control to optimize the extraction rate of a durable non-renewable resource with a monopolistic primary supplier

The problem dealt with in this paper is that of optimizing the path of the extraction rate (and, consequently, the price) for the monopolistic owner of the primary sources of a totally or partially durable non-renewable resource (such as precious metals or gemstones) in a continuous-time frame, assuming that there is an upper bound on the extraction rate and with an interest rate equal to zero. The durability of the resource implies that, unlike the case of non-durable resources, at any time there is a stock of already-used amounts of the resource that are still potentially reusable, in addition to the resource available in the ground for extraction. The problem is addressed using the Maximum Principle of Pontryagin in the framework of optimal control theory, which allows identifying the patterns that the optimal policies can adopt. In this framework, the Hamiltonian is linear in the control input, which implies a bang-bang control policy governed by a switching surface. There is an underlying geometry to the problem that determines the solutions. It is characterized by the switching surface, its time derivative, the intersection point (if any) and the bang-bang trajectories through this point.Peer ReviewedPostprint (published version

MILP model for the Visibility Windows Assembly Line Balancing Problem (VWALBP): the case of the Müller- Hannemann & Weihe problem

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