The Multi-Spreader Crane Scheduling Problem: Partitions and supersequences

In the Multi-Spreader Crane Scheduling Problem (MSCSP), containers with identical dimensions but variable weights are arranged along a grid. A multi-spreader crane is used to lift all containers. The has m > 1 modes. When it in p th mode, can remove adjacent same row at time as long total weight of does not exceed loading capacity ? . Such takes h minutes. It also c , q minutes for switch from mode when ? goal find sequence so that minimized. This paper investigates computational complexity MSCSP. First, we establish connection between greedy sequences and supersequences. We then prove MSCSP NP-hard three or more modes by reduction version Shortest Common Supersequence problem. Lastly, investigate two problems arise naturally heuristics solve show one be solved using dynamic programming while other remains computationally hard. provide an approximation algorithm behaves nicely changeover times much larger than lifting crane.