Mathematical Problems in Engineering

Volume 2017 (2017), Article ID 6710929, 15 pages

https://doi.org/10.1155/2017/6710929

## Solving a Two-Stage Stochastic Capacitated Location-Allocation Problem with an Improved PSO in Emergency Logistics

College of Field Engineering, The PLA University of Science and Technology, Nanjing 210000, China

Correspondence should be addressed to Wanhong Zhu

Received 12 November 2016; Revised 12 March 2017; Accepted 23 March 2017; Published 31 May 2017

Academic Editor: Jorge Magalhaes-Mendes

Copyright © 2017 Ye Deng et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

A stochastic expected value model and its deterministic conversion are developed to formulate a two-stage stochastic capacitated location-allocation (LA) problem in emergency logistics; that is, the number and capacities of supply centers are both decision variables. To solve these models, an improved particle swarm optimization algorithm with the Gaussian cloud operator, the Restart strategy, and the adaptive parameter strategy is developed. The algorithm is integrated with the interior point method to solve the second-stage model. The numerical example proves the effectiveness and efficiency of the conversion method for the stochastic model and the proposed strategies that improve the algorithm.

#### 1. Introduction

A location-allocation (LA) problem, also known as facility location problem (FLP), involves locating a number of facilities to which customers are allocated to minimize the cost of satisfying customer demands. It is an important problem in supply chain or logistics management and greatly affects long-term transportation and storage decisions. Many enterprises and government departments focus on this problem to reduce cost and to improve efficiency, especially in emergency logistics given the frequent occurrence of disasters, epidemic, security incidents, and other emergencies nowadays. This problem has complex and uncertain features, such as the changing demands, allocations, and locations of customers or facilities.

Many studies have been conducted on the basic LA problem since the proposal of Cooper [1] in 1963. However, most of the studies were for deterministic cases and not for uncertain occurrences. In the recent two decades, several models for uncertain occurrences have been proposed and solved by different algorithms. Logendran and Terrell [2] first considered a stochastic uncapacitated LA problem and proposed an expected value model (EVM) to maximize the net profits. Carrizosa et al. [3, 4] proposed a LA problem that considers the locations of both customers and facilities, which may be regions that have several probability distributions. Liu [5, 6] contributed to the uncertainty theory by proposing three stochastic models [7] and three fuzzy programing models [8] for the capacitated LA problem. A hybrid intelligent algorithm that consists of a network simplex algorithm, a simulation, and a genetic algorithm was developed to solve the stochastic and fuzzy models above. Silva and De La Figuera [9] studied the capacitated facility location problem with constrained backlogging probabilities and solved it using a heuristic method based on a reactive greedy adaptive search procedure. Wang and Shi-Wei [10] proposed a robust optimization model for a logistics center LA problem and compared it with stochastic and deterministic optimization models. Two algorithms, namely, an enumeration method and a genetic algorithm, were adopted to solve the problem. Yao et al. [11] considered a joint facility LA and inventory problem with stochastic demands. The problem involves identifying the best locations of warehouses and the inventory levels and allocating customers. A heuristic integrated approximation and transformation technique was developed to solve the problem. Wen and Kang [12] considered a facility LA problem with random fuzzy demands. They proposed a hybrid intelligent algorithm, similar to the method of Zhou and Liu [8], which consists of the simplex algorithm, a random fuzzy simulation, and a genetic algorithm. A similar method was also adopted by Mousavi and Niaki [13] in solving a LA problem with a fuzzy variable and customer location demands, which were normal random variables. Vidyarthi and Jayaswal [14] proposed a nonlinear integer programing model to solve a LA problem with immobile servers, stochastic demands, and congestions. Pereira et al. [15] presented a probabilistic maximal covering LA problem and proposed a hybrid algorithm to solve it. They formulated a linear programing model to efficiently solve small and medium problems and a flexible adaptive large neighborhood search heuristic to solve large problems. Alizadeh et al. [16] considered a capacitated multifacility LA problem with stochastic demands. The capacitated subsources of each facility could be utilized when the number of demand points, that is, the planned total requirements, are exceeded. Alizadeh et al. [17] transformed the mixed-integer nonlinear programing model to a simple formulation model and proposed a genetic algorithm (GA) and a colonial competitive algorithm (CCA) to solve medium and large problems.

In this study, we consider a two-stage stochastic capacitated LA problem (SCLAP) in the context of emergency logistics. In the management of emergency logistics, the core problem is utilizing the limited relief supplies rapidly and efficiently. Hence, predesigned supply centers, that is, emergency logistics distribution centers, are important. The “appropriate” number, size, and location of supply centers have become a comprehensive decision problem that should not be addressed separately. The demands of different customers are mostly uncertain and depend largely on different scenarios. Hence, we focus on the uncertainty in demand quantity, which is assumed as the only independent stochastic variable in this paper that follows a given regular stochastic distribution. To solve SCLAP two conditions must be satisfied: the constraint of stochastic quantity demands and the minimization of the generalized cost. The generalized cost consists of two parts, namely, the sum of the costs of building and maintaining supply centers and the stochastic costs of transportation to each customer from each supply center. Therefore, we determine the appropriate number, capacities, and locations of supply centers in this paper.

In a traditional capacitated LA problem (CLAP), each customer can be supplied by existing supply centers and can be supplied by more than one center at the same time. Hence, the problem becomes NP-hard and difficult to solve [7]. Moreover, in the two-stage SCLAP, the number, the capacities, and the locations of supply centers are considered decision variables. The research on this model is extremely weak, and few relevant papers have been found [18–23]. Furthermore, most of the research considered the number and the capacities of supply centers separately, and no research that considered both variables together has been found.

To efficiently solve this model, an improved particle swarm optimization (PSO) algorithm is proposed. The algorithm consists of three improvement strategies: the Gaussian cloud operator, the Restart strategy, and the adaptive parameter strategy. The second stage of the problem is modeled as a linear program. Hence, we adopt the interior point method of the time-consuming simplex method. We convert the initial stochastic programing model to a crisp model, thus reducing the computing time dramatically based on the assumption of the demands with independent regular distributions and the uncertainty theory proposed by Liu [6].

The remainder of this paper is organized as follows. Section 2 describes the random EVM and the crisp model for the two-stage SCLAP. Section 3 presents the details of the hybrid algorithm solution to the model. Section 4 introduces a case study of the new model and verifies the algorithm efficiency with the improvement strategies. Section 5 concludes with the contributions and innovations of this paper and presents the future research directions.

#### 2. Model Formulation

##### 2.1. Problem Description and Theoretical Foundation

To model the two-stage SCLAP, the following assumptions should be considered: the graph of all the nodes is a complete graph, each customer node can be connected with all supply nodes but cannot be connected with another customer node, the weight of each edge between two nodes is measured by the Euclidian distance plus the transportation volume, the locations of customer nodes are fixed and the demand quantities are stochastic, and the capacity constraint is only imposed on supply nodes. The notation and variables used in the following formulations are defined in Descriptions of Notations and Variables.

To model the two-stage SCLAP, we first apply the EVM introduced by Zhou and Liu [7] to the SCLAP. Then, we extend the classic one-stage EVM to a two-stage model and provide a deterministic equivalent form. We introduce several basic definitions, theorems of probability, and uncertainty theories.

*Definition 1 (see [5]). *Let be a nonempty set and the -algebra of the subsets (called events) of . The set function Pr is called a probability measure if it satisfies the following conditions.

*Axiom 1 (normality). *

*Axiom 2 (nonnegativity). *

*Axiom 3 (countable additivity). *For every countable sequence of mutually disjoint events , we obtain

*Definition 2 (see [5]). *Let be a nonempty set, the -algebra of the subsets of , and Pr the probability measure. Then, the triplet is called a probability space.

*Definition 3 (see [5]). *A random variable is a measurable function from the probability space to the set of real numbers; that is, for any Borel set of real numbers, the set is

Liu [6] also proposed the definitions for* measure inversion theorem*,* regular uncertainty distributions,* and* inverse uncertainty distribution* as supplements to the uncertainty theory. The uncertainty distributions are specifically presented as stochastic distributions, and the uncertainty measure can also be replaced by the probability measure Pr. He proposed several theorems that can help in the conversion of the stochastic model into a crisp model based on these definitions. The related definitions and theorems are listed as follows.

*Definition 4 (see [6] measure inversion theorem). *Let be an uncertain variable with an uncertainty distribution . Then, for any real number , we have

*Definition 5 (see [6] regular uncertainty distribution). *An uncertainty distribution is regular if it is a continuous and strictly increasing function with respect to at which and

*Definition 6 (see [6] inverse uncertainty distribution). *Let be an uncertain variable with a regular uncertainty distribution . Then, the inverse function is the inverse uncertainty distribution of .

Theorem 7 (see [6]). *Let be independent uncertain variables with regular uncertainty distributions , respectively. If is a strictly increasing function, then has an inverse uncertainty distribution*

Theorem 8 (see [6]). *Let and be independent normal uncertain variables and , respectively. Then, the sum of is also a normal uncertain variable ; that is,*

##### 2.2. Initial Expected Value Model

The mathematical formulation, which is an expected value model (EVM), proposed by Zhou and Liu for the initial SCLAP can be defined as follows [7]:which is subject towhere denotes the expected value of uncertain variable . If (9) will be reformulated as follows:

In this mathematical model, the decision variable is the location of the supply centers , and the objective function is the minimum value of the sum of the demand and distance from each supply center to the customer in (9) or (16). The first constraint (see (10)) states that a customer’s stochastic demand must be satisfied. The second constraint (see (11)) states that the capacity of each supply center must be sufficient to supply all customers with demands. Equations (12), (13), and (14) are used to keep the variables within a certain range.

According to Zhou and Liu [7], the EVM can be easily solved by a hybrid intelligent algorithm that consists of the network simplex algorithm, stochastic simulations, and the GA. However, in emergency logistics management, in addition to making a decision on supply center locations, the number and capacities of supply centers should also be considered. Therefore, the SCLAP should be extended into a two-stage model. Variables and are both fixed in the initial EVM, whereas, in the two-stage EVM, these variables are decision variables.

##### 2.3. Two-Stage Expected Value Model

In the two-stage EVM for SCLAP, the decision variables extend to the number , the capacities , and the locations of supply centers . The first stage aims to determine the minimum value of the generalized cost and generates the number and capacity values for the second stage. The second stage utilizes the number and capacity values from the first stage and determines the minimum value of the stochastic demand cost of transportation. In addition, to provide a comprehensive description of the optimization objective in emergency logistics, we introduce a nonlinear function based on the objective function in the initial model to calculate the generalized cost. The two-stage EVM for SCLAP can be formulated as follows.

First stageis subject toSecond stageis subject towhere () denotes the fixed cost of constructing a supply center and denotes the variable cost coefficient to maintaining an operating supply center. Equation (17) represents the expected generalized cost. The first portion, , is the sum of the cost of construction and the cost of maintaining the supply centers, and the latter portion, , is the sum of the costs of the demands for transportation from each supply center to the customer, similar to the initial EVM. The first constraint in (18) ensures that the capacity of each supply center is limited within a reasonable range from to . Equation (19) stipulates that the total capacity of supply centers must exceed the total expected demands of customers regardless of the value of or the number of supply centers . The formulations in the second stage denote the same items as those in the initial model, except that fixed constants and become variables.

##### 2.4. Deterministic Equivalent Models

Evidently, the two-stage EVM model is much more complicated than the initial model, and the stochastic simulation method is extraordinarily time-consuming because the number and the capacities of supply centers are both unknown. However, on the basis of the definitions and theorems in Section 2.1, if we assume that the demand variables are nonnegative normal distributions, that is, , we can convert the two-stage EVM of the SCLAP into a deterministic two-stage model. The following equivalent formula of the first stage model can be expressed as and subject to

*Proof. *Customer demand follows the normalized distribution (Theorem 8), which belongs to the* regular uncertainty distribution*, and is a strictly increasing function. Hence, one has the following.

(1) Objective conversion is as follows: (2) Constraint conversion is as follows:*∵* and according to Theorem 8*∴**∴*The following equivalent formula of the second-stage model can be established as and subject to

*Proof. *Customer demand follows the normalized distribution (Theorem 8), which belongs to the* regular uncertainty distribution*. Hence, one has the following.

(1) Objective conversion is as follows: (2) Constraint conversion is as follows:After the conversion of the two-stage EVM, we can obtain a two-stage deterministic model, which can be solved much easily. Similarly, we can also convert the chance-constrained programing (CCP) model into a deterministic model. Assuming that the supply centers satisfy the demand of customer with a probability , the total supply capacity satisfies the total demand of customers with probability , and the demand variable complies with the same kind of regular stochastic distribution , the CCP of the two-stage SCLAP can be formulated as follows (only the values different from the EVM are listed for simplicity).

Equation (19) is reformulated asand (21) is reformulated as According to Theorem 7, the stochastic function is strictly increasing. Therefore, one has the following.

Equation (33) can be converted intoand (34) can be converted intoHowever, if the SCLAP is a dependent-chance programing (DCP) model (only the values different from the EVM are listed for simplicity), (17) is reformulated asand (19) is reformulated asThe objective is to maximize the probability, which cannot be converted into a deterministic model. Hence, the stochastic simulation process must be activated to calculate the probability. In this paper, we only focus on the EVM and develop an improved PSO (IPSO) algorithm to solve this problem given that the two-stage SCLAP is newly proposed.

#### 3. Improved IPSO

The LAP has been proven to be NP-hard [24]. Hence, it is a two-stage SCLAP. Heuristic methods are the best at handling large NP-hard problems, especially under uncertain environments. We develop an improved IPSO combined with the interior point method [25] to solve the deterministic equivalent conversion of the two-stage EVM.

##### 3.1. Design of the IPSO

In 1995, Kennedy and Eberhart [26] first proposed a swarm intelligence optimization algorithm, which was inspired by the flocking of birds and schooling of fish against predation, called the particle swarm optimization (PSO). PSO has been successfully applied to parameter optimization [27], combinatorial optimization [28], pattern recognition [29, 30], data mining [31], and other fields [32, 33]. However, local optimal solution is often obtained when general PSO algorithm is applied to solve the continuous location of distribution centers. Therefore, scholars have proposed many improvement strategies for a much efficient and effective PSO [34–37]. Recently, Zhan et al. [38] proposed an improved PSO based on neighbor heuristic and Gaussian cloud learning, and the results proved its superiority over many PSO variants. We also adopt a Gaussian cloud operator combined with an adaptive parameter strategy and a Restart strategy to improve the general PSO algorithm of the two-stage SCLAP. For a detailed description of the IPSO algorithm, an algorithm flow chart is provided as a supplement in Figure 1.