The deep sea mining horizontal discharge plume

Abstract

The rising demand for minerals and metals and the depletion of land-based resources has created a renewed interest in the extraction of resources from the deep ocean, i.g., deep sea mining. This research focuses on the mining of manganese nodules, these are potato-shaped objects, 5-10cm in size, composed of metals half buried in the seafloor. A Seafloor Mining Tool (SMT) is used to mine the nodules, however, during mining besides the nodules, also the sediment around the nodules is collected. This sediment is separated in the SMT and discharged through a horizontal diffuser located at the rear of the SMT, thereby creating a sediment plume. The sediment plume needs to be reduced for environmental, economic and legal reasons. The sediment plume causes changes in the sediment characteristics, clogging of pores of suspension feeding organisms and burying of the benthic fauna. The plume will spread in all directions, thereby burying still to be collected nodules and reducing the pick-up efficiency, making the process inefficient from an economical perspective. Legal issues will arise when the discharged sediment plume spreads to other exploration areas owned by other parties or to environmentally protected areas. The aim of this research is to investigate the influence of the choice of diffuser height on the reduction of the sediment plume. To achieve this goal a Matlab model was made and small-scale experiments in the laboratory of Dredging Engineering were done. The Matlab model is based on the JETLAG model, described in 'Jets and Plumes' by Lee and Chu (2003), and is a Lagrangian model predicting the centerline trajectory of the plume. The small-scale experiments provide insight in the behaviour of the horizontal discharge plume at different diffuser heights and generate measurements to validate the model. A diffuser designed specifically for these experiments is produced and the height and flow velocity are differed. The experiments consist of visualization experiments, capturing top view and side view images of the buoyant jet, and experiments measuring the velocity and bottom concentration at different locations. The velocity profiles, sediment concentration, deposition pattern and impingement range are compared and analysed. The combined results from the model and the experiments show that the start of the impingement range increases with increasing diffuser height. Furthermore, a lower diffuser height increases the amount of bottom deposition near the diffuser outlet. The measured velocity profiles, near bottom concentration and impingement range are in agreement with this observation. The measured near bottom velocities at 15D from the diffuser outlet are approximately 0.1 and 0.2m/s, depending on the discharge velocity, both have the potential of resuspending settled sediment. This research suggests that the spreading of the plume can be reduced by the choice of diffuser height above the seafloor. Based on the diffuser heights tested in this research a diffuser height of 100mm is recommended. Since a higher diffuser decreases the amount of particles settling close to the source and a lower diffuser increases the potential for erosion. This research has shown that up to a distance of 15D from the diffuser, the near bottom velocities are large enough to cause erosion. Research into the near bottom velocity decay and magnitude further away from the source, inside the turbidity current, is necessary as well to understand the full spreading behaviour and the potential for erosion. Furthermore it is recommended to do research into spreading behaviour of finer particles, especially the behaviour of the actual deep sea sediment in combination with salt water causing flocculation. Moreover, research into the influence of the forward motion of the SMT is recommended since the SMT will create a wake that will influence the spreading behaviour of the plume. Research into the potential creation of a recirculation region caused by the limited entrainment at the rear of the SMT is recommended as well, this can be done by aligning the diffuser outlet with a screen.