Horizontal negatively buoyant jets in deep sea mining

The influence of initial concentration of discharged slurries on the spatial spread of turbidity currents

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Recently, deep-sea mining has acquired renewed interest from commercial companies due to a rise in the demand of minerals and metals. Deep-sea mining is focussed on different minerals and rare earth elements found in the oceans that are increasingly used in the manufacturing of goods and products related to renewable energy. In order to collect these minerals, a mining operation at several kilometres depth has to be undertaken. Apart from the technical challenges that mining at several kilometres depth poses, also the environmental consequences receive a lot of attention and criticism from environmental agencies. Deep-sea mining operations are believed to cause damage to the environment by the sediment plume, as well as by producing light and noise which are absent under normal conditions at the deep ocean floor. The sediment plume is generated by the movement of the miner, but more importantly by the discharging of the picked up and unwanted sediment. This happens at the back of the miner using a set of diffusers. In order to acquire a better understanding of the effect that the initial concentration of the slurry has on the spatial spread of the sediment plume and turbidity current caused by the deep sea mining vehicle, experiments have been performed in which the initial concentration has been altered so that the characteristics of the ensuing current can be compared. The main objective of this research is to determine how changing the initial volumetric concentration of the discharge mixture influences the flow parameters after impingement and sediment flux inside the turbidity current for a constant sediment flux and discharge velocity.
One of the most important assumptions for investigating the influence of initial concentration of the slurries has to do with the different phases of the discharged flow. During the initial phase, the density difference between the slurry and ambient mixture is thought to be dominant for the settling process, as the mixture behaves as one coherent fluid. After impingement on the bottom, the mixture spreads downstream as a turbidity current. During this phase it is the weight of the solids inside the slurry that causes it to settle. Because there are a lot of solids present in these slurries this process of settling is controlled by both hindered settling and flocculation. Flocculation is the aggregation of cohesive particles, forming a larger, heavier particle. Clay, which is cohesive, is expected to be present at most mining sites, resulting in flocculation to be an important factor in the process.
The research has been performed by conducting a total of three different experiments. These experiments consist of pumping a pre-mixed slurry of water and spherical glass particles into a large modular tank, where is it discharged through a diffuser. For each experiment a mixture is prepared with a different concentration, which is discharged into the modular tank with three different flow rates in order to keep the sediment flux constant between each experiment. A total of three diffusers are used, one for each concentration. The diffusers differ in cross section in order to achieve a constant discharge velocity for all experiments. Experiments have been performed using both spherical glass particles and clay. The sediment flux is kept constant between different experiments as it is assumed that the sediment flux of real deep sea mining operations will be mainly determined by the width, cutting depth and velocity of the mainly vehicle. For performing measurements during the experiments, an ADV was used. This device is capable of measuring the velocity in three directions over a profile of 30 mm with 1 mm intervals. The ADV also measures the signal-to-noise ratio, which after performing a calibration can be used for calculating the concentration inside the turbidity current. Together this data can be used for determining the spatial spread of the turbidity current.
Based on the findings of this research, it is believed that a higher concentration than the 1% volumetric concentration that is currently being assumed by deep sea mining companies could prove to be beneficial in preventing the spatial spread of the sediment plume and subsequent turbidity current. It is believed that in general, increasing the initial concentration of the discharged slurry, and thereby decreasing the momentum/buoyancy ratio of the buoyant jet, directly leads to both the velocity and concentration of the turbidity current to decrease over a shorter distance after impingement, while the impingement distance itself is also decreased for a higher initial concentration. As such, the total reach of the turbidity current resulting from the slurry discharge from a deep sea mining vehicle can be decreased by using a higher initial concentration of said slurry. It is believed that mainly the higher density difference with the ambient water plays a vital role in decreasing the spatial spread of the sediment plumes produced by deep sea mining operations. The higher density achieved by a high concentration results in the plume to deflect towards to bottom over a shorter distance and a decrease in turbidity current height. This decrease in height is beneficial for the later stage of the turbidity current in which the dying out is dominated by the settling velocity of individual particles.