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Using DEM to simulate iron ore pellet loading in double-hatch hold ships

The loading of ships in bulk solids terminals is carried out by equipment called shiploader, composed of belt conveyors that displace their discharge point in three degrees of freedom: shuttle, luffing, and slewing. Such mobility allows the operators to load the cargo homogeneously and distribute it well in the holds, maintaining the balance of the ship and the ballast control. To make the most of the useful space in the hold, engineers from Vale conducted a study applying Rocky DEM particle simulation software and the Discrete Element Method (DEM) to simulate iron ore pellets loading in double-hatch hold ships. 

Initially, pellets with a bulk density of 2130 kg/m³, angle of repose of 28 degrees, and an average size of 12 mm were considered. Due to the high computational cost of simulating a digital twin of the hold with particles in real size, it was necessary to use scaled particle sizes, without any need to scale the volume or mass of the cargo. After some preliminary simulations, engineers observed that 700 mm would be a good particle size for simulations with the available hardware. The scaled particles allowed for a relatively high value-to-integration time step, still ensuring that small overlapping between particles/boarding was taken in the calculation step. 

It was also necessary to calibrate the contact parameters to obtain the same behavior of real-size particles. Contact parameters obtained experimentally by BARRIOS et al. (2013) were used as a starting point for preliminary simulations with the staggered particles, aiming at reaching the angle of repose values of 28 degrees and 2130 kg/m³ of bulk density, according to pellet specification.

Example of simulation of the angle of repose test in double-hatch hold ships using DEM
Example of the simulation of the angle of repose test.

A larger spherical particle needs a higher friction coefficient to have the same behavior as a smaller one. With the increase of the particle size, the effect of the roughness between the surfaces is reduced. This effect, combined with the difference in contact models, resulted in the adoption of higher friction coefficients than those used by BARRIOS et al. (2013) for both static friction and rolling friction.

Parameters adjusted for the scale.
Parameters adjusted for the scale.

Results of the simulations

A 1000 m³ virtual cube filled with material at a density of 2130 kg/m³ was generated to check compaction rate calculations, corroborating the specific mass of the virtual pellets used in the simulations. By using the standard procedure for loading a single hold, the DEM simulation showed that it is possible to perform the stowage factor of 0,47 m³/t, which indicates that it is possible to stow the entire load within the admissible parts of the hold. 

Procedure for a single hatch hold with a stowage factor of 0,47 m³/t.

Commonly in these operations, some pellets may have a density lower than 2130 kg/m³. A simulation with specification pellets with 1800 kg/m³ of bulk density showed that it is not possible to perform the stowage factor of 0,55m³/t in this vessel. It was verified that small variations in the stowage factors are enough to make a loading plan infeasible, plus each loading turn can impact the others due to flowing pellets.

The Discrete Element Method proved to be very useful for pellet loading in ships with a double hatch hold, allowing engineers to simulate loading procedures with different stowage factors relatively quickly and at a low cost.

Future studies could carry out simulations with different methods of loading a double hatch hold, making sure that the characterization will also be consistent with that loading method.

Guilherme Pereira de Oliveira

Senior Engineer, Vale

Guilherme de Oliveira has more than ten years of experience in the design and maintenance of belt conveyors and their components, including problem solving of belt mistrackings and blockages in transfer chutes, failure analysis, preparation of maintenance plans, and simulation of flow of solid bulk in transfer chutes through the Discrete Elements Method (DEM) - area in which he is currently concluding a master's degree. De Oliveira also provided training in the Teluk Rubiah Maritime Terminal Project in Malaysia, worked in the expansion of the Moatize Coal Mine in Mozambique , and participated in the preparation and review of technical standards for the Commission for the Study of Continuous Conveyors, Belt Conveyors of ABNT, Brazil.

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