Optimal division of inventory between depot and bases

We consider a problem of optimal division of stock between a logistic depot and several geographically dispersed bases, in a two‐echelon supply chain. The objective is to minimize the total cost of inventory shipment, taking into account direct shipments between the depot and the bases, and lateral transshipments between bases. We prove the convexity of the objective function and suggest a procedure for identifying the optimal solution. Small‐dimensional cases, as well as a limit case in which the number of bases tends to infinity, are solved analytically for arbitrary distributions of demand. For a general case, an approximation is suggested. We show that, in many practical cases, partial pooling is the best strategy, and large proportions of the inventory should be kept at the bases rather than at the depot. The analytical and numerical examples show that complete pooling is obtained only as a limit case in which the transshipment cost tends to infinity. © 2017 Wiley Periodicals, Inc. Naval Research Logistics, 64: 3–18, 2017     

Some approximations in multi-item,multi-echelon inventory systems for recoverable items

The optimization problem as formulated in the METRIC model takes the form of minimizing the expected number of total system backorders in a two-echelon inventory system subject to a budget constraint. The system contains recoverable items – items subject to repair when they fail. To solve this problem, one needs to find the optimal Lagrangian multiplier associated with the given budget constraint. For any large-scale inventory system, this task is computationally not trivial. Fox and Landi proposed one method that was a significant improvement over the original METRIC algorithm. In this report we first develop a method for estimating the value of the optimal Lagrangian multiplier used in the Fox-Landi algorithm, present alternative ways for determining stock levels, and compare these proposed approaches with the Fox-Landi algorithm, using two hypothetical inventory systems – one having 3 bases and 75 items, the other 5 bases and 125 items. The comparison shows that the computational time can be reduced by nearly 50 percent. Another factor that contributes to the higher requirement for computational time in obtaining the solution to two-echelon inventory systems is that it has to allocate stock optimally to the depot as well as to bases for a given total-system stock level. This essentially requires the evaluation of every possible combination of depot and base stock levels – a time-consuming process for many practical inventory problems with a sizable system stock level. This report also suggests a simple approximation method for estimating the optimal depot stock level. When this method was applied to the same two hypotetical inventory systems indicated above, it was found that the estimate of optimal depot stock is quite close to the optimal value in all cases. Furthermore, the increase in expected system backorders using the estimated depot stock levels rather than the optimal levels is generally small.     

Optimal dispatching strategies for vehicles having exponentially distributed trip times

A transportation system has N vehicles with no capacity constraint which take passengers from a depot to various destinations and return to the depot. The trip times are considered to be independent and identically distributed random variables. The dispatch strategy at the depot is to dispatch immediately, or to hold any returning vehicles with the objective of minimizing the average wait per passenger at the depot, if passengers arrive at a uniform rate. Optimal control strategies and resulting waits are determined in the special case of exponentially distributed trip time for various N up to N = 15. For N ? 1, the nature of the solution is always to keep a reservoir of vehicles in the depot, and to decrease (increase) the time headway between dispatches as the size of the reservoir gets larger (smaller). For sufficiently large N, one can approximate the number of vehicles in the reservoir by a continuum and obtain analytic experession for the optimal dispatch rate as a function of the number of vehicles in the reservoir. For the optimal strategy, it is shown that the average number of vehicles in the depot is of order N^1/3. These limit properties are expected to be quite insensitive to the actual trip time distribution, but the convergence of the exact properties to the continuum approximation as N → ∞ is very slow.     

Exact analysis of a two-echelon inventory system for recoverable items under batch inspection policy

We investigate a two-echelon (base-depot) inventory system of recoverable (repairable) items. The arrivals of demand at the bases are in a Poisson manner and the order sizes are random. The failed units can be repaired either at the base or at the depot, and the units beyond economic repair are condemned. Inspection of the failed units is carried out in the batches they arrive, that is, arrival batches are not broken up. The exact expressions for stationary distribution of depot inventory position, and of the number of backorders, onhand inventory, in-repair inventory at all locations are derived under the assumptions of constant repair and lead times. Special cases of complete recoverability, nonrecoverability, and of the unit order size are also discussed.     

A three-echelon,multi-item model for recoverable items

The main objective of this paper is to develop a mathematical model for a particular type of three-echelon inventory system. The proposed model is being used by the Air Force to evaluate inventory investment requirements for alternative logistic structures. The system we will model consists of a group of locations, called bases, and a central depot. The items of concern in our analysis are called recoverable items, that is, items that can be repaired when they fail. Furthermore, each item has a modular or hierarchical design. Briefly, the model is used to determine the stock levels at each location for each item so as to achieve optimum inventory-system performance for a given level of investment. An algorithm for the computation of stock levels for each item and location is developed and illustrated. Some of the ways the model can be used are illustrated with Air Force data.     

A stock redistribution model

Multi-depot supply systems are subject to stock distribution imbalances; i. e., the fraction of total system stock located at a depot may be too small to support the fraction of system demand expected to be placed on it. In the supply system of concern, a cutomer is always satisfied if there is stock anywhere in the system. Stock redistributions to correct imbalances may reduce both transportation costs and customer waiting times. A model for determining optimum redistribution quantities is formulated, and a practical method of solution for the two depot case is described. Selected numerical illustrations are given.     

中小型石油库(站)CAD系统中快速设计的实现

如何提高中小型石油库(站)设计的效率是工程设计人员非常关注的一个问题,也是中小型石油库(站)工程设计CAD软件开发的基本目标之一,但目前在国内研究较少。分析归纳了中小型石油库(站)实际设计中的典型设计方法,提出了以外部数据库和图形数据库为基础,采用基于工程参数驱动图块的设计技术、基于单元图的设计技术、基于工程定型图的设计技术,并结合根据需要开发的修改编辑工具,为设计人员提供了一整套设计工具,帮助其实现快速设计。试用效果证明,该思路切实可行,并已在中小型石油库(站)CAD系统的开发中得以实现。     

Multiechelon inventory systems with lateral supply

We develop approximations to estimate the expected backorders in a multiechelon system in which lateral supply actions between bases are allowed when a backorder occurs. These approximations are easy to compute, and the average absolute error over a wide range of parameter values is less than 4% when items are depot repairable, even when bases are dissimilar. With lateral supply, backorder reductions of 30-50% are not uncommon, and a 72% reduction was observed in two cases. Lateral supply becomes more important with low demand rates. A similar approach was unsuccessful for base-repairable items. However, lateral supply has a beneficial effect only when the lateral supply time is very short, 1/4 or less of the average base repair time. Even in such cases lateral supply is unlikely to be important in an actual application, because base management can expedite repair of critical items.     

New purification algorithms for linear programming

Two new algorithms are presented for solving linear programs which employ the opposite-sign property defined for a set of vectors in m space. The first algorithm begins with a strictly positive feasible solution and purifies it to a basic feasible solution having objective function value no less under maximization. If this solution is not optimal, then it is drawn back into the interior with the same objective function value, and a restart begins. The second algorithm can begin with any arbitrary feasible point. If necessary this point is purified to a basic feasible solution by dual-feasibility–seeking directions. Should dual feasibility be attained, then a duality value interval is available for estimating the unknown objective function value. If at this juncture the working basis is not primal feasible, then further purification steps are taken tending to increase the current objective function value, while simultaneously seeking another dual feasible solution. Both algorithms terminate with an optimal basic solution in a finite number of steps.     

War reserve spares kits supplemented by normal operating assets

In peacetime, base stock levels of spares are determined on the assumption of normal resupply from the depot. In the event of war, however, a unit must be prepared to operate from stock on hand for a period of time without being resupplied from the depot. This paper describes a mathematical model for determining such war reserve spares (WRS) requirements. Specifically, the model solves the following kind of optimization problem: find the least-cost WRS kits that will keep the probability of a stockout after K cannibalizations less than or equal to some target objective α. The user of the model specifies the number of allowable cannibalizations, and the level of protection that the kit is supposed to provide. One interesting feature of this model is that in the probability computation it takes into account the possiblility of utilizing normal base operating assets. Results of a sensitivity analysis indicate that if peacetime levels were explicitly taken into account when designing a WRS kit, a cost saving of nearly 40 percent could be effected without degrading base supply performance in wartime.     

基于二元语义群决策的油料供应站选址方法

针对油料供应站的选址决策问题,提出了一种基于二元语义群决策的选址方法。构建了油料供应站选址指标体系,在简要介绍二元语义概念的基础上,给出了基于二元语义群决策的油料供应站选址步骤,通过该方法可以得到以二元语义形式给出的油料供应站评价值。并通过实例说明了该选址方法的有效性。     

Risk pooling in a two-period,two-echelon inventory stocking and allocation problem

The object of this article is to investigate the risk-pooling effect of depot stock in two-echelon distribution system in which the depot serves n retailers in parallel, and to develop computationally tractable optimization procedures for such systems. The depot manager has complete information about stock levels and there are two opportunities to allocate stock to the retailers within each order cycle. We identify first- and second-order aspects to the risk-pooling effect. In particular, the second-order effect is the property that the minimum stock available to any retailer after the second allocation converges in probability to a constant as the number of retailers in the system increases, assuming independence of the demands. This property is exploited in the development of efficient procedures to determine near-optimal values of the policy parameters.     

移动式综合避雷装置防雷机理与应用

针对石油库重点设施设备的防雷保护特点,提出运用移动式综合避雷装置对油库不同区域进行雷电机动防护。分析了移动式综合避雷装置系统的组成和特点,并应用于实践。移动式综合避雷装置应用于某油库一些重点区域的综合防雷实践中,具有安全、可靠、实用、方便等特点,值得推广应用。     

Steady-state approximation of a multiechelon multi-indentured repairable-item inventory system with a single repair facility

We model a two-echelon multi-indentured repairable-item inventory system where each “base” has a maximum number of identical online machines, and each machine consists of several module types. Machine failures are due to module failures and occur according to an exponential distribution. When a machine fails, the failed module is replaced by an identical spare module if one is available. Otherwise, the module is backordered. All failed modules go to a single “depot” repair facility which consists of a finite number of identical repairmen who are able to repair any module type in an exponentially distributed time, although the repair rates for different module types may differ. The principal contribution of this article is an approximation algorithm for calculating the steady-state characteristics of the system. In comparison with simulation results, the algorithm is quite accurate and computationally efficient. © 1993 John Wiley & Sons, Inc.     

油料储存罐火灾初期模式实验研究

油库是油料的储存场所,绝大部分油料以油罐作为储存容器,而油料洞库的安全一直以来是各级主管部门非常关心又急待解决的难题。因此,研究油罐的储存安全问题是解决这一难题的关键。针对油库常用的储油装置——油罐,在其模拟实验装置上,以汽油作为燃烧工质,进行了系统实验研究与分析。并在此基础上,得出了以下结论:油气体积分数是油罐火灾模式的决定因素,若没有氧气补充,爆炸后的复燃或二次爆炸的可能较小,油罐开口条件下的燃烧危险性大,必须采取有效的主动消防措施。这些结论对研究油料储存罐的安全、消防提供了一定价值的依据。     

基于ISM的油库安全事故成因分析

分析油库安全事故成因,划分事故成因类型,对于有效控制事故发生、保障油库运行安全是极其重要的。运用系统工程思想,对油库安全事故成因要素的相互关系进行分析,建立了油库安全事故成因多层递阶解释结构模型（ISM）。分析和计算结果表明：油库安全事故成因大致可分为根本原因、直接原因和间接原因三个类型（层次）。针对不同层次结构的事故成因,可以制定相应的预防和控制措施,为军用油库抓住安全管理重点、有效防止事故发生提供理论依据。     

基于ASP．NET技术的动态网页实现

ASP .NET为Web应用系统开发提供了一种全新的灵活的基础结构,是迄今为止最先进的Web应用服务系统开发平台.动态网页是Web应用服务系统的核心.提出了采用ASP .NET技术实现动态网页的方法,在数字化油库与油库信息可视化项目研究中得到很好的应用.     

油料洞库通风系统探讨

油料洞库通风系统是油料洞库的有机组成部分,其主要作用在于降低洞库油气浓度、减小洞库湿度以及进行油罐清洗通风,从而提高油料洞库的安全性,减少金属设备的腐蚀.分析了油料洞库的基本结构型式,探讨了油料洞库通风系统工艺布置,分析了通风系统工艺设备组成、通风模式及设备选型,给出了油料洞库通风流量、通风管道规格及通风机全风压的计算...     

直升机野战油料保障模拟优化研究

野战条件下,场站通常在前沿阵地开设野战直升机场实施伴随保障,其中油料保障主要采取飞机加油车先对直升机实施加油,后到野战机场油库领取油料这一方式进行。若将野战直升机场油料保障作为一个系统来研究,要实现适时、适量的油料保障,确定合适的飞机加油车及野战机场油库发油鹤位数量是一个关键问题。利用GPSSW语言对这一问题进行模拟,得出模拟结果并进行统计分析,经反复模拟和参数比较得到适合的参数,为部队保障提供依据。     

A two-echelon multi-station inventory model for navy applications

The two inventory echelons under consideration are the depot, D, and k tender ships E₁, …, E_k. The tender ships supply the demand for certain parts of operational boats (the customers). The statistical model assumes that the total monthly demands at the k tenders are stationary independent Poisson random variables, with unknown means λ₁, …, λ_k. The stock levels on the tenders, at the heginning of each month, can be adjusted either by ordering more units from the depot, or by shipping bach to the depot an excess stock. There is no traffic of stock between tenders which is not via the depot. The lead time from the depot to the tenders is at most 1 month. The lead time for orders of the depot from the manufacturer is L months. The loss function due to erroneous decision js comprised of linear functions of the extra monthly stocks, and linear functions of shortages at the tenders and at the depot over the N months. A Bayes sequential decision process is set up for the optimal adjustment levels and orders of the two echelons. The Dynamic Programming recursive functions are given for a planning horizon of N months.