کنترل موجودی قطعات خدماتی با انتقال جانبی و خط لوله انعطاف پذیر

In equipment-intensive industries such as truck, electronics, aircraft and dredging vessel manufacturing, service parts are often slow moving items for which the transshipment time is not negligible. However, this aspect is hardly considered in the existing service logistics literature. In this paper, we consider this aspect and propose a customer-oriented service measure which takes into account pipeline stock and lateral transshipment flexibility. We provide an approximation method for optimizing the stock allocation subject to this service measure. Via extensive numerical experiments, we show that our approximation performs very well with respect to both system performance and costs. Moreover, our numerical experiments indicate that including lateral transshipments and pipeline stock flexibility in inventory decisions is more beneficial than lateral transshipments alone. This effect is larger for high demand rates and high lateral transshipment costs. Results from a case study in the dredging industry confirm our findings. We therefore recommend introduction of pipeline stock information such as the track and trace information from freight carriers in existing ERP systems.

Research on service parts inventory control with lateral transshipments has been motivated by needs from various industries, including equipment-intensive industries such as truck, electronics, aircraft and dredging vessel manufacturing. Facing stochastic demand of critical service parts, a multi-location inventory control system often allows movement of stock between locations at the same echelon level or even across different levels in order to fulfil customers' demand in time. Many of these critical service parts are slow moving and heavy items for which, in some cases, air transport is impossible or prohibitively expensive. For example, in dredging industry, critical service parts usually weigh more than 1700 kg and therefore are way too costly to transport by air. The lateral transshipment time for these items can be longer than 3weeks, and is not negligible compared to lead times (around 7weeks). Moreover, if transportation is slow, there may be considerable amounts of pipeline stock being transported between a production plant and local bases. In some cases, the average pipeline stock can be up to half of the average stock on hand. In these cases, it may be more profitable to wait till the pipeline stock arrives than to order via lateral transshipments. As a result, the timing of transshipments and normal replenishments becomes an important factor in decision making. To the best of our knowledge, this aspect is not much considered in the existing service parts literature. Good customer-oriented performance measures are also lacking in the literature. The standard service levels, such as fill rates, are supplier-oriented; whereas customers only observe deliveries with no delays and deliveries within a certain response time in case of delays. Some studies (Kutanoglu and Mahajan, 2009) introduce more customer-oriented service levels by distinguishing the availability of items from different sources with different response times. However, since these studies ignore the fact that the pipeline stocks may arrive and be delivered to customers sooner than other emergency shipments, they still emphasize the operational processes of service suppliers. Inventory sharing by lateral transshipments between local bases makes stock more valuable as the stock may be used by different bases. The benefits are clear when there is an agreement with a customer on the response time and lateral transshipment times are negligible. On the other hand, if transshipment times are not negligible, lateral transshipments could be detrimental for service levels, as products spend more time in transportation before reaching customers. Hence, the total system cost may be higher in this case due to higher requirements of base stocks. 1.1. Statement of contribution The contribution of this paper is as follows: First, we propose a customer-oriented performance measure where both pipeline stock and lateral transshipment flexibility contribute to service performance. Second, we provide an approximation method for optimizing stock levels subject to this measure. The quality of this approximation and the benefits of lateral transshipment and pipeline stock flexibility are assessed via extensive numerical experiments. Based on these experiments, we find that the use of pipeline stock information improves the performance and costs of systems with lateral transshipments. We conclude that including lateral transshipment and pipeline stock flexibility in inventory decisions is more beneficial than lateral transshipments alone. Subsequently, we apply our method in a case study from dredging industry and confirm our findings in practice. Finally, we offer managerial insights on the lateral transshipment decisions when the transshipment time is non-negligible. The paper is organized as the following. In Section 2, we review the inventory control literature on lateral transshipments. In Section 3 we present the inventory model. In 4 and 5, we give an approximation for the customer-oriented service measure in the context of a single-echelon inventory system with and without lateral transshipments between the local bases. In Section 6 we minimize the average inventory cost subject to service level constraints. We validate our methods and we assess the benefits of lateral transshipment and pipeline stock flexibility via extensive numerical experiments in Section 7. In Section 8, we apply our method in a case study for a global market leader in the dredging industry. In the last part, we draw our conclusion and offer managerial insights.

This paper assesses the effect of pipeline stock flexibility in one or two-echelon inventory models where the lateral transshipment time is not negligible. We introduce customer-oriented service levels, expressed by the probability of instantaneous service and the probability of service within a certain response time. We propose approximations based on queuing models with state dependent arrival rates. Via extensive numerical experiments, we show that our approximations perform well in terms of both system performance and costs. Our numerical experiments indicate that including both lateral transshipment and pipeline stock flexibility in inventory decisions is more beneficial than including lateral transshipments alone. The magnitude of this effect is higher for high demand rates and high lateral transshipment costs. This conclusion is also supported by our findings in a case study for a market leader in dredging industry. As a result of our research, we recommend the introduction of pipeline stock information such as the track and trace information from freight carriers in existing ERP systems.