Performance evaluation of a multi-product system under CONWIP control

Analytical models of multi-product manufacturing systems operating under CONWIP control are composed of closed queuing networks with synchronization stations. Under general assumptions, these queuing networks are hard to analyze exactly and therefore approximation methods must be used for performance evaluation. This research proposes a new approach based on parametric decomposition. Two-moment approximations are used to estimate the performance measures at individual stations. Subsequently, the traffic process parameters at the different stations are linked using stochastic transformation equations. The resulting set of non-linear equations is solved using an iterative algorithm to obtain estimates of key performance measures such as throughput, and mean queue lengths. Numerical studies indicate that the proposed method is computationally efficient and yields fairly accurate results when compared to simulation.     

The trans-shipment problem in a two-echelon,multi-location inventory system with lost sales

In this paper, we address a two-echelon, multi-location pooling inventory system that consists of an outside supplier, a warehouse and two retailers. To control their inventories, both warehouse and retailers use (R,?s,?S) policy. The retailers face stochastic customer demands for a single product and the warehouse receives only the replenishment orders of retailers. In case of stock-out at retailers, emergency trans-shipments are used to satisfy the unmet demand at one retailer with the surplus from the other retailer. When the stock is insufficient at the warehouse, we propose two rationing policies to allocate on-hand stock between retailers. The demand that cannot be satisfied neither by stock on-hand nor by trans-shipment from retailers is considered lost. Our work has two objectives. First, we propose an inventory model based on three components: the optimisation inventory model, the trans-shipment policy and the rationing policies for determining the best values of (s,?S) at each location that minimise total system cost. Second, we validate this model via an empirical simulation study that allows us to identify the influential parameters on trans-shipment benefits.     

A geometric process repair model for a repairable cold standby system with priority in use and repair

In this paper, a deteriorating cold standby repairable system consisting of two dissimilar components and one repairman is studied. For each component, assume that the successive working times form a decreasing geometric process while the consecutive repair times constitute an increasing geometric process, and component 1 has priority in use and repair. Under these assumptions, we consider a replacement policy N based on the number of repairs of component 1 under which the system is replaced when the number of repairs of component 1 reaches N. Our problem is to determine an optimal policy N^* such that the average cost rate (i.e. the long-run average cost per unit time) of the system is minimized. The explicit equation of the average cost rate of the system is derived and the corresponding optimal replacement policy N^* can be determined analytically or numerically. Finally, a numerical example with Weibull distribution is given to illustrate some theoretical results in this paper.     

Safety Management Assessment System (SMAS): a process for identifying and evaluating human and organization factors in marine system operations with field test results

Eighty percent or more of major marine accidents are caused or influenced by humans and organizations. Research has identified characteristics of high reliability organizations (HRO), organizations which have had few accidents while involved in operations where failure would result in severe consequences. These characteristics have important effects on the safety of marine systems. Assessing systems for these HRO characteristics is the first step in reducing accidents caused by human and organization factors (HOF). The Safety Management Assessment System (SMAS) was developed specifically to assess marine systems (offshore platforms, marine terminals, ships) for HOF. SMAS is a screening method that selects and trains operators of the system to conduct a self-assessment. The assessment process takes five days and has the assessors making comparisons and evaluating HOF by selecting ranges and providing comments to capture the uncertainty. Included in the process is a visit to the system. A computer program was developed to assist in the assessment process. A field test of SMAS was conducted at a marine terminal in California. Two teams, with members from the terminal and the regulatory agency, along with a facilitator, were selected and trained. These two independent teams conducted separate evaluations on the same marine terminal. A comparison of their assessments showed that the use of ranges and comments were very helpful. An analysis of field test data shows that SMAS can produce results more consistent than randomness. SMAS shows promise as an efficient and practical method to assess humans and organizations.