Scheduling Trans-shipment Operations in Maritime Chemical Transportation

Trans-shipment or direct ship-to-ship transfer of bulk liquid cargos for regional distribution is a routine activity in maritime chemical logistics. Efficient scheduling of trans-shipment operations is economically crucial, because of the expensive multiparcel carriers. In this work, we consider a general chemical trans-shipment problem, in which multiple donor carriers trans-ship several chemical cargos to multiple recipient carriers. We develop nine continuous-time mixed-integer linear programming (MILP) formulations of three types for determining the optimal sequences, positions, and timings for unloading various cargos. Two of these models using the big-M relaxation to solve several test problems with reasonable speed, but their performance varies with the numbers of two-sided cargos and ships. For even larger problems, we simplify our rigorous model heuristically using a cargo aggregation assumption, which reduces the formulation size tremendously and decreases model solution times by 2 orders of magnitude, yet gives near-optimal solutions.