Designing the layout and routing of a manufacturing facility to mitigate workflow congestion
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The first part of this work discusses the notion of workflow congestion in the context of material handling equipment interruptions in a manufacturing or warehousing facility. Development of a combination of probabilistic and physics-based models for workflow interruptions permits evaluation of the expected link travel time. The problem is then of routing in a way that minimizes total expected travel time. The re-routing problem is modeled as a multicommodity flow problem with link capacity design. A greedy upper bounding and Lagrangean relaxation algorithm are developed to solve this efficiently. To calibrate our modeling process we develop an object-oriented simulation model which explicitly considers various workflow interruptions. Our major finding is that re-routing of traffic in a congested facility can significantly alleviate congestion delays and improve the efficiency of material movement. A managerial insight derived from this work is that re-routing is most effective in medium traffic intensity situations. In the second chapter, we present a model labeled Full Assignment Problem with Congestion (FAPC), which simultaneously optimizes the layout and flow routing. FAPC is a generalization of the Quadratic Assignment Problem (QAP), a classical problem for the location of a set of indivisible economical activities. A branch-and-price algorithm is proposed and a computational study is performed to verify its effectiveness as a solution methodology for the FAPC. A numerical study confirms the benefits of simultaneous consideration of layout and routing when confronted with workflow congestion. A detailed simulation for a case problem is presented to verify the overall benefits of incorporating congestion in layout/routing. A critique of FAPC with two alternative models is also provided. Three conclusions are offered from our work. First, a combination of re-layout and re-routing is a more powerful way to mitigate the impact of workflow congestion rather than using just the re-layout or just the re-routing options. Second, it is important to model workflow congestion in a manufacturing facility--namely, ignoring it can result in a significantly poor design. Third, the QAP layout is dominated by the FAPC layout for situations of medium workflow intensity. In the third part, we present the modeling details of the simulation model that captures different instances of interruptions especially including pedestrian traffic and acceleration/deceleration factors. The importance of this work stems from the realization that we apply the statistical results from the simulation model that is of high fidelity to analytically study and measure workflow congestion in a factory.