Delft University of Technology
Faculty Mechanical, Maritime and Materials Engineering
Transport Technology / Logistic Engineering

J.G. Vermeer Feasibility study on the possiblities of implementing a computer based scheduling application at Scaldis Reefer Chartering.
Masters thesis, Report 96.3.LT.4793, Transport Technology, Logistic Engineering.

Scheduling of vessels is the assignment of cargoes (demand for capacity) to vessels (supply of capacity), in such a way that all technical and time constraints are met, and the pre-set goal achieved.

The demand for transportation capacity, the cargo owners wanting to ship their cargo to buyers, is a very dynamic factor in the scheduling process. The available cargo quantities are never certain, changing from year to year and from day to day. Each season though is characterised by a low and a high season, the last being roughly from February through April/May.

The supply side of the market is largely characterised by the small amount of shipping companies. Seatrade, of which Scaldis Reefer Chartering is a subsidiary, is the world's largest supplier of transportation capacity. The supply side has its own major uncertainty, being the position of the vessel at any time in the future. Because of both the uncertainties from the supply and demand side, scheduling far ahead in time is useless.

The chartering practice is the interaction between the demand and supply side. Scaldis has its own people (brokers) who try to contract cargo, both for the short and the long term.

Today, the scheduling of the vessels at Scaldis Reefer Chartering is still done by hand. Since this process is time consuming and scheduling further ahead than one voyage is too complicated to handle, this report investigates the possibilities of implementing a computer based scheduling application.

The investigation was started by creating a model of the scheduling process and one of the future situation. These models showed that there is a difference between 'soft' and 'hard' variables, each influencing the outcome of the scheduling process. The hard variables can be formalised and used by a scheduling application. The soft variables can either be represented in the application by other hard variables, or the user of the application has to make sure they are accounted for in any final solution. Therefore, it is made clear that the user will always have to interact with any future application, always being the one taking the final decisions about the produced solution.

Based on the findings of the model formulation, a set of requirements has been drawn up, with which any scheduling application should apply. These requirements consist of three groups. The optimiser requirements demand a quick calculation of large problems, solving as close to optimality as possible. The system requirements mostly focus on user friendliness and flexibility in the creation of solutions, while the user requirements call for experienced schedulers who understand the methods underlying the application.

In finding a possible application, a mathematical form of the scheduling model had to be defined. It was then found that the branch and bound algorithms are the most widely used solution methods for the kind of models as defined. A general solution method cannot be used though, since too much computing time would be needed. Specific algorithms have appeared in literature each making use of a specific characteristic of the problem formulation. The Elastic Set Partitioning algorithm, or X-System, is one of them. The X-System has been found to be the only algorithm that is probably capable of solving the scheduling problem, and has at the same time been used on a commercial basis. Besides this, algorithms developed for crew scheduling problems are thought to be useful, but more study is needed to find out whether any specific application is availabie. The development of a custom made application, based on algorithms available in literature, is not practical, since too much time and money will be involved.

Based on the layout of the so-called 'MOS' application, in which the X-System is incorporated, a future layout of the Scaldis scheduling software has been designed. Besides a central database, the set up will consist of three main modules:
  1. ship schedule generator;
  2. cost calculator;
  3. optimiser.
The ship schedule generator calculates all possible combinations (ship schedules) between vessels and voyages. The cost calculator calculates the costs of all ship schedules and the optimiser finally calculates the optimum set of ship schedules.

The design of the cost calculator proved to be one of the most important factors of success to the program. The schedule costs, all costs for 'idle time' and final position of the vessel, are based on stochastic, so uncertain, variables, and will play a central role in the production of practically acceptable schedules. More research should be carried out to make good results possible.

The user will at last make the difference between a useful or useless program. He will be the one who is abie to pre-assign variabies and thus manipulate the results, in such a way that the produced schedules comply with the wishes of the management.

Testing the MOS application proved to be impossible for most requirements, except for the calculation speed, which is acceptable. Other requirements can be complied with based on reasonable assumptions.

Only a very small fleet result improvement is necessary to pay off initial investments. It is therefore reasonabie to expect good results from an implementation of the X-System. Before implementing the X-System though, it is recommended to check the companies references, since reasons for doubt did appear.

Reports on Logistic Engineering (in Dutch)
Modified: 2001.01.22; , TU Delft / 3mE / TT / LT.