Scheduling of tanker lightering via a novel continuous-time optimization framework

The problem of scheduling a fleet of marine vessels for crude oil tanker lightering is addressed. A novel continuous-time mathematical formulation is developed on the basis of the concept of event points proposed in a formulation for short-term scheduling of chemical processes (Ierapetritou, M. G.; Floudas, C. A. Ind. Eng. Chem. Res. 1998, 37, 4341; 1998, 37, 4360. Ierapetritou, M. G.; Hené, T. S.; Floudas, C. A. Ind. Eng. Chem. Res. 1999, 38, 3446). A sequence of event points is introduced for each vessel, and binary variables are defined to determine whether the vessel is to start a task at each event point, while the task consists of mounting a tanker, pumping on oil from it, dismounting the tanker, traveling to the refinery, docking the refinery, pumping off oil, undocking, and traveling back to the anchorage. The mathematical formulation leads to a mixed-integer linear programming (MILP) problem. The model is further extended to incorporate two complicating features of the lightering process: (i) lightering in multiple stages and (ii) loading vessels with material from multiple tankers. A number of case studies are presented, and the computational results demonstrate the effectiveness and efficiency of the proposed approach.