Aspectos generales de la quitina y del quitosano

The arrival of exports in the terminal is a random process and because of storage space constraints, terminals are forced to stack containers of different groups in the same stacks. It follows that the final layout of exports for a vessel will not suit the loading

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sequence. It becomes necessary to re-marshall (generating rehandles) the containers to suit the loading sequence and this is done by allocating space close to where the vessel will berth to minimise the travel distance of the container handling equipment. The problem becomes finding the most efficient way of re-marshalling, as it needs to be done without slowing vessel operations.

Upon confirmation of a vessel’s load list, Kim and Bae (1998) propose a methodology for re-marshalling export containers in a yard block with two yard cranes. This method focuses on minimising the travel distance of the yard cranes as well as performing the re-marshalling process with the least amount of moves. The re-marshalling problem is addressed in two stages by mathematical programming. In the first stage, the bay matching and move planning problems are addressed by dynamic programming and the transportation problem technique respectively. The current stacking layout of exports is matched to the ideal layout, and then the number of moves required is solved by an iterative process considering the constraint imposed by the minimum distance allowed between yard cranes. The second stage, the task sequencing part, then uses these results to find the sequence that minimises the distance travelled by the yard cranes. The authors draw attention to the long computational times demanded by this method. Kang, Oh, Ahn, Ryu and Kim (2006) put forward an approach to solve the problem of developing an intra-block re-marshalling plan in the least possible time, using multiple cranes. This approach assumes the vessel loading plan is known prior to sorting, the objective being to create a re-marshalling plan that generates no rehandles during sorting and vessel loading, and minimises crane interference. As expected, there are numerous possible crane schedules so to pick the optimal one the simulated annealing process is employed to search for good partial orders. These good partial orders are then expressed in the form of partial order graphs. An evaluation heuristic is then employed to construct full crane schedules by crane simulation, whose quality is judged by the length of time taken to complete each schedule. Kang et al., point out that in practice rehandles are inevitable given storage space constraints and incorrect container weights given.

Lee and Hsu (2007) propose a integer programming model with constraints reflecting movement rules containers follow in practice, that seeks to optimise the re-marshalling

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of exports. The research is based on the sorting of containers in one bay with one yard crane. In practice moving containers between bays in a block is avoided for safety reasons, yard trucks are used to perform the horizontal movements. The model is intended to reposition exports so vessel loading generates no rehandles, and more importantly, the remarshalling process occurs with the least amount of moves to minimise the time taken. The model’s computational time is significant so a heuristic is suggested to simplify the computational process.

In a similar study Lee and Chao (2009) propose a heuristic model to address the re- marshalling problem. The methodology is less computationally demanding than the integer programming model proposed by Lee and Hsu (2007). The model sought to have a final layout that produced little or no rehandles during loading and that required the least amount of moves during the re-marshalling process. The model had two major subroutines, the first a neighbourhood search process that seeks to minimise rehandles in the target bay layout. The second, a binary integer programming formulation, that reduced the length of the movement sequence. Three minor subroutines worked in the background to enhance the work of the major subroutines by ensuring at least one stack was completely emptied, stream lining the movement process and reducing the

rehandles in the final bay layout.

Park, Park and Ryu (2009) propose a cooperative co-evolutionary algorithm to develop a remarshalling plan for an automated container terminal. Their work is based on a yard block employing two automated transfer cranes, one that works on the seaside and the other on the hinterland side. The vessel operations (seaside) and receiving and delivery (hinterland) operations are kept separate so when time permits, exports stacked on the hinterland side of the blocks are moved to the seaside and imports from the seaside to the hinterland side. The problem is subdivided into two parts, the first, target stacks are chosen heuristically and secondly, specific slots are chosen and the movement sequence is determined by the cooperative co-evolutionary algorithm. The ideal candidate is one that eliminates rehandles during the remarshalling and vessel loading operations, and consumes the least time. Travel distances between yard cranes in different blocks are kept the same to keep the quay cranes even and minimise vessel turnaround time. Simulation results showed the plans generated by this method were more efficient than those generated by algorithms that don’t decompose the problem.

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Choe, Park, Oh, Kang and Ryu (2009) employ simulated annealing and heuristic algorithms to propose an intra-block re-marshalling plan that generates no rehandles during the sorting and vessel loading, and one that minimises yard crane interference in an automated terminal. The problem is decomposed into two parts, the first identifying the possible target slots. The second part schedules the yard cranes to move the boxes in each possible configuration, the ideal one being the one consuming the least time. This section dealt with the rehandling of exports. What is evident in the literature is that re-marshalling is standard practice as terminals seek to optimise the use of scarce storage space. The problem is addressed by focusing on generating yard crane schedules that minimise the time spent on moving the containers and on producing final bay layouts that expedite vessel loading. It stands out that the literature reviewed in this section is all based on high throughput terminals that utilise yard cranes (rail mounted or rubber tired gantries). A search of the literature revealed very little work on small terminals, particularly those employing reachstakers/forklifts systems.

It is worth mentioning that re-marshalling is carried out differently in

forklift/reachstacker terminals. As access is only from the side of bays, ideally bays have to be homogenous unlike straddle or yard crane operations where only the rows (single stacks) have to be homogenous (as access is from the top). As a result re- marshalling will be inter-bay and inter-block unlike the research covered here that is based on intra-bay and inter-bay moves. Despite the difference, the motivations remain the same.