• A hybrid condition-based maintenance policy for continuously monitored components with two degradation thresholds

      Poppe, Joeri; Boute, Robert; Lambrecht, Marc (European Journal of Operational Research, 2018)
      Condition-based maintenance (CBM) makes use of the actual condition of the component to decide when to maintain and/or replace the component, thereby maximising the lifetime of the machine, while minimising the number of service interventions. In this paper we combine CBM on one (monitored) component, with periodic preventive maintenance (PM) and corrective maintenance (CM) on the other components of the same machine/system. We implement two thresholds on the degradation level to decide when to service the monitored component: when the degradation level of the monitored component surpasses a first ‘opportunistic’ threshold, the monitored component will be serviced together with other components, for instance with a (planned) PM intervention, or upon breakdown of another component, requiring CM. In case none of these opportunities have taken place, and the degradation level surpasses a second ‘intervention’ threshold, an additional maintenance intervention is planned for the monitored component in order to prevent a failure. Both thresholds are optimised to minimise the total expected maintenance costs of the monitored component, or to minimise the downtime of the machine due to maintenance on the monitored component. We perform an extensive numerical experiment to demonstrate the potential gains of this hybrid policy with two thresholds compared to using a traditional PM policy, and we identify its key drivers of performance. We also benchmark our results when only one threshold is implemented. Our model is validated and applied at an OEM in the compressed air and generator industry.
    • A win-win solution for the bullwhip problem

      Boute, Robert; Disney, Stephen M.; Lambrecht, Marc; Van Houdt, Benny (Production Planning & Control, 2008)
    • Altruistic behavior in supply chain management

      Boute, Robert; Lambrecht, Marc (Tijdschrift voor Economie en Management, 2007)
    • An integrated production and inventory model to dampen upstream demand variability in the supply chain

      Boute, Robert; Disney, Stephen M.; Lambrecht, Marc; Van Houdt, Benny (European Journal of Operational Research, 2007)
    • Characterizing order processes of continuous review (s,S) and (r,nQ) policies

      Noblesse, Ann; Boute, Robert; Lambrecht, Marc; Van Houdt, Benny (European Journal of Operational Research, 2014)
      We provide a novel approach to characterize the order process of continuous review and inventory policies, and study the impact of the batching parameter (the value of Q or ) on the variability in the order process. First, we characterize the distribution of the time between orders, as well as the distribution of order sizes. We find that the coefficient of variation (cv) of the time between orders is smaller than the cv of the time between demands. The size of the orders can exhibit either variance amplification or dampening, compared to the demand sizes, depending on the demand size distribution and the value of the batching parameter. This may motivate a supplier to adjust his imposed fixed order cost to influence the batching size. Second, we look at the compound order process, defined by the number of units ordered during an arbitrary interval. The compound order process always exhibits variance amplification compared to the compound demand, which increases linearly in the batching parameter for large values of Q or ; for small values, the variance amplification is fluctuating. We point out that the time interval, during which the number of units ordered/demanded is observed, also impacts the level of variance amplification, and we show to what extent larger time intervals (resulting in more aggregation of the data) lead to lower values of variance amplification. Both perspectives (looking at time between orders and order quantities, or observing the compound order process) provide useful information for the upstream supplier.
    • Coordinating lead times and safety stocks under autocorrelated demand

      Boute, Robert; Disney, Stephen M.; Lambrecht, Marc; Van Houdt, Benny (European Journal of Operational Research, 2014)
      We consider a supply chain in which orders and lead times are linked endogenously, as opposed to assuming lead times are exogenous. This assumption is relevant when a retailer’s orders are produced by a supplier with finite capacity and replenished when the order is completed. The retailer faces demands that are correlated over time – either positively or negatively – which may, for example, be induced by a pricing or promotion policy. The auto-correlation in demand affects the order stream placed by the retailer onto the supplier, and this in turn influences the resulting lead times seen by the retailer. Since these lead times also determine the retailer’s orders and its safety stocks (which the retailer must set to cover lead time demand), there is a mutual dependency between orders and lead times. The inclusion of endogenous lead times and autocorrelated demand represents a better fit with real-life situations. However, it poses some additional methodological issues, compared to assuming exogenous lead times or stationary demand processes that are independent over time. By means of a Markov chain analysis and matrix analytic methods, we develop a procedure to determine the distribution of lead times and inventories, that takes into account the correlation between orders and lead times. Our analysis shows that negative autocorrelation in demand, although more erratic, improves both lead time and inventory performance relative to IID demand. Positive correlation makes matters worse than IID demand. Due to the endogeneity of lead times, these effects are much more pronounced and substantial error may be incurred if this endogeneity is ignored.
    • Designing replenishment rules in a two-echelon supply chain with a flexible or an inflexible capacity strategy

      Boute, Robert; Disney, Stephen M.; Lambrecht, Marc; Van Houdt, Benny (International Journal of Production Economics, 2009)
    • Did just-in-time management effectively decrease inventory ratios in Belgium?

      Boute, Robert; Lambrecht, Marc; Lambrechts, O. (Tijdschrift voor Economie en Management, 2004)
    • Exploring the bullwhip effect by means of spreadsheet simulation

      Boute, Robert; Lambrecht, Marc (INFORMS Transactions on Education, 2009)
    • Extending the Production Dice Game

      Lambrecht, Marc; Creemers, Stefan; Boute, Robert; Leus, Roel (International Journal of Operations and Production Management, 2012)
    • Lot sizing and lead time decisions in production/inventory systems

      Noblesse, Ann; Boute, Robert; Lambrecht, Marc; Van Houdt, Benny (International Journal of Production Economics, 2014)
      Traditionally, lot sizing decisions in inventory management trade-off the cost of placing orders against the cost of holding inventory. However, when these lot sizes are to be produced in a finite capacity production/inventory system, the lot size has an important impact on the lead times, which in turn determine inventory levels (and costs). In this paper we study the lot sizing decision in a production/inventory setting, where lead times are determined by a queueing model that is linked endogenously to the orders placed by the inventory model. Assuming a continuous review (s, S) inventory policy, we develop a procedure to obtain the distribution of lead times and the distribution of inventory levels, when lead times are endogenously determined by the inventory model. This procedure allows to determine the optimal inventory parameters within the class of (s, S) policies that minimize the expected ordering and inventory related costs over time. We numerically show that ignoring the endogeneity of lead times may lead to inappropriate lot sizing decisions and significantly higher costs. This cost discrepancy is very outspoken if the lot size based on the economic order quantity deviates significantly from desirable production lot sizes. In these cases, the endogenous treatment of lead times is of particular importance.
    • Numerical study of inventory management under various maintenance policies

      Poppe, Joeri; Basten, Rob; Boute, Robert; Lambrecht, Marc (Reliability Engineering and System Safety, 2017)
      Capital assets, such as manufacturing equipment, require maintenance to remain functioning. Maintenance can be performed when a component breaks down and needs replacement (i.e., corrective maintenance), or the maintenance and part replacement can be performed preventively. Preventive maintenance can be planned on a periodic basis (periodic maintenance), or it can be triggered by a certain monitored condition (condition-based maintenance). Preventive maintenance policies are gaining traction in the business world, but for many companies it is unclear what their impact is on the resulting inventory requirements for the spare parts that are used for the maintenance interventions. We study the impact of the maintenance policy on the inventory requirements and the corresponding costs for a setting that is realistic at an OEM in the compressed air industry. Preventive policies increase the total demand for spare parts compared to corrective maintenance, since the former do not exploit the entire useful life of the components. This leads to higher inventory requirements. At the same time, the preventive policies inhibit advance demand information, as the interventions, and correspondingly the spare parts demands, are planned in advance. Using a simulation study, we show that by using this advance demand information in managing the spare part inventory, the increase in inventory requirements of preventive maintenance policies can to a large extent be offset, for condition-based maintenance, we find that inventories can even be lower compared to corrective maintenance, provided that the advance demand information is used correctly when managing inventories. Our analysis sheds light on the behaviour of the inventory related costs under various maintenance policies.
    • Performance evaluation of a production/inventory system with periodic review and endogenous lead times

      Boute, Robert; Lambrecht, Marc; Van Houdt, Benny (Naval Research Logistics, 2007)
    • Sectoriële prestaties en financiële impact van het voorraadbeheer in België

      Boute, Robert; Lambrecht, Marc; Lambrechts, O.; Sterckx, P. (Kwartaalschrift Economie, 2007)