Design of grabs for coarse material

Abstract

Due to the constant growth of global maritime shipping, the optimization of grabs used for unloading bulk carriers is more and more relevant. Especially for a coarse material like limestone, the average payload can be improved, by altering the grab design. It is suggested that the penetration and cutting depth of a grab can be improved, which will have a positive effect on the payload. This research is limited to the contact surface of the grab with the bulk material, the knife. Subsequently, the following research question is answered in this research:

How to enhance the penetration and cutting depth considering coarse cohesionless material?

This is done, by implementing a DEM model using the bulk calibration approach [1], several in situ
and laboratory experiments are performed to measure certain macro properties. The experiment is numerically replicated, and micro parameters are changed iteratively until the predicted macro response matches the desired results. A comprehensive study of similar papers considering the penetration resistance using a DEM model is executed, which resulted in selecting the penetration resistance and porosity as target variables. The particle shape of a DEM model has a significant influence on the bulk material behavior. The novelty of this research consists of doing a preliminary analysis before the final calibration process, in which several polyhedral and (multi)-spherical particle shapes are compared. For investigating the penetration resistance, a new test setup for in situ use is developed and constructed. The implementation of this setup proved to be successful, eleven data samples were collected. For the bulk density, an in situ and laboratory setup are compared, of which the laboratory setup proved to be more reliable. Several additional laboratory experiments are executed for the particle density, Particle Size Distribution (PSD) and wall friction. Using the results of the experiments, a DEM model is created, with which a preliminary analysis is done using extreme values of the calibration variables. Four spherical and four polyhedral shapes are compared, which resulted in the two-spherical shape begin the most optimal option. All the polyhedral shapes proved to be not fitting, especially due to their underestimation of the penetration resistance. Due to the high variance of the experimental penetration tests, two parameter sets with identical porosity are calibrated: one with a low resistance target, and one with a high resistance target. Using Design of Experiments (DoE) in combination with surrogate modelling with a linear regression, several optimal parameters sets are created, in which the uncertainty is considered as well. After selecting two optimal parameter sets, the simulations are verified by comparing the experiments and simulations qualitatively. Additionally, a quantitative verification using the stiffness of the force data of each time-step is performed as well. Using the calibrated models, an additional DEM model is created with a bigger particle bed, to investigate the penetration and cutting resistance. By altering the geometry of the knife, a decrease in resistance was achieved, mainly in the cutting phase. It is recommended to consider the flowing behavior of limestone as well, with which a complete material model can be calibrated and validated, and full-scale simulations can be performed.