Experiments and 3D CFD simulations of Deep-Sea Mining plume dispersion and seabed interactions

Abstract

Growing markets for sustainable technologies such as solar panels, wind turbines, and electric cars require an enormous amount of raw materials that might not be available from sources on land. Exploitation of seafloor mineral resources could secure the supply of raw materials for the future. However, proposed deep-sea mining (DSM) systems will return wastewater containing fine sediments and other effluents back to the sea via a pipe discharging plumes close to the seabed. Concerns are that the DSM plumes will have significant environmental impacts on the mostly uncharted deep-sea ecosystem. Until today, the environmental licensing procedure for DSM operations is incomplete. Accurate numerical models to predict the behavior of returned DSM plumes are still lacking.

This research aims to support environmental impact assessments for DSM operations. Laboratory tests were carried out to provide validation data for numerical models that can be used to predict the spreading of DSM plumes in the near-field domain. Additionally, Computational Fluid Dynamics (CFD) technique was used to predict the same laboratory simulations. Wide ranges of initial parameters such are the plume release elevations relative to the bed, source initial Suspended Sediment Concentrations (SSC) and different bed slopes were considered.

Experiments and CFD results agreed well. Negatively buoyant plumes fall vertically towards the bed and impinge generating turbidity currents. Excess buoyancy gained by increasing the initial SSC at the source leads to a considerable increase in the plume dispersion rate. Plume release height is found to be an important parameter. Discharging close to the bed leads to the formation of a circular levee around the impingement point at a distance 10 times the orifice diameter. The sedimentation ring acts as a barrier against the turbidity current leading to the deposition of sediments right outside the ring. As a consequence, the plume dispersion rate reduces and DSM wastes stay closer to the disposal location. In contrast, when plumes are released at a greater height from the bed, due to long falling distance and settling of particles, generated turbidity currents accelerate and the sediments particles are transported in suspension to large distances from the disposal location.

The main goal of this research, to provide a unique set of validation data for numerical models was achieved and a Large Eddy Simulation CFD model was successfully used, tested and approved for its capability to accurately predict the near-field spreading of DSM plumes. Based on obtained results, in order to minimise the environmental impacts, the flow velocity at the bed should be minimised to prevent re-suspension of already deposited sediments. It is recommended that sediment wastes and other effluents should be released close to the seabed. CFD model setup considered only sedimentation of particles and ignored erosion. Based on observed seabed morphological changes during the laboratory experiments, it is recommended that erosion should be considered within numerical models. To fully predict the environmental impacts of DSM operations, the near-field model used in this research should be coupled to a far-field model but also considering multiple environmental pressures caused by DSM tailings.