(8h) Simultaneous Front-end Crude Scheduling and Unit Maintenance Management for Inland Refineries

The front-end crude scheduling (FECS) is a well-studied method, which has been convincingly proven to be very effective for the scheduling of crude supply process. It can effectively tackle crude movement problems from crude importing by vessels, unloading through berths, temporarily storing at port-side storage tanks, transferring via long-distance pipeline, blending at refinery-side charging tanks, to continuously feeding refineries. Through FECS, optimal crude transfer schedules with the lowest cost and the highest operability can be determined. In previous studies, almost all exclusively were conducted with an implicit assumption that all units were at normal working status. However, the performance of every processing unit is inevitably decaying along with the processing time and load. Unit maintenance operations have to be proactively considered, so that unit working performance could be recovered. Meanwhile, the risks for serious unit failure and/or incidents could be minimized. Apparently, such maintenance operations will affect the crude transfer schedules, resulting in higher cost and infeasible operations. Some studies dealt with this challenge in a reactive manner, exploring remedial strategies after the reveal of unit shut-down or failure. However, the reactive response would unavoidably and stiffly disrupt the original schedule, causing a gap between the real optimal crude transfer schedule and the actual one. To address this issue, unit maintenance schedule should be proactively integrated with normal crude movement schedule and simultaneously optimized, so that the optimality of operability and cost can be ensured.

In this study, a novel continuous-time FECS model combined with proactive unit maintenance management has been developed. It involves a large-scale mixed-integer nonlinear programming problem, covering all crude movements from crude shipping to refinery processing. Through this model, the optimal schedule with the minimal operating cost for both crude transfer operations and unit maintenance operations will be simultaneously identified. It can not only determine when and how to isolate designated unit(s) from the process system for conducting the maintenance operation (i.e., how to discharge the leftover inventory in units), but also identify the best opportunity for the maintenance-completed unit(s) to rejoin the process system to pick up production tasks during the scheduling time horizon. In addition, real process requirements, such as tank inventory, transfer flowrate, blending specifications are all taken into account to guarantee the obtained schedule is practical. The efficacy and performance of the proposed model has been tested and demonstrated through various unit(s) and maintenance scenarios.