Small scale testing of crawler outlet diffusers for plume reduction of offshore mining

Abstract

Over recent years, there has been an increase in demands for rare and precious minerals worldwide. Mostly this is due to the rise of the world’s population and the drive towards a green energy transition and low carbon economy. Prices are rapidly increasing, and there is an identifiable risk of an increasing supply shortage of raw materials, including those identified as critical to Europe’s high technology sector. The development of surveying techniques and advances in new technologies in remotely operated vehicles (ROV) has allowed detecting that the most valuable and rare mineral resources are spread out in the sea-floor and international waters. Currently, the most significant setback towards exploitation licenses is not because of lack of technology but because of a lack of knowledge on deep-sea biodiversity and the impacts of mining on ecosystems. The ISA (International Seabed Authority) is responsible for the regulation, and the control of mineral-related activities in the international waters is currently working on drafting environmental regulations. This presents significant opportunities for research on the development of technologies that incurs in the least environmental impact possible. Currently, the main concerns are regarding the horizontal sediment-laden plume that is generated as a result of the mining process. Therefore, the industry is working towards the development of equipment that lessens the plumes spread. Work done in this research focuses on small scale sized laboratory experiments in which the sea-floor crawler’s outlet shape is varied, and its plume’s effects of the sediment waste and other effluents (SWOE) are measured. Besides gaining insight in the horizontal plume spreading under different conditions and geometries, the outcome of the research is to provide a set of measurement data that can be used for future numerical model validation towards determining an optimal outlet shape for the seabed crawler. For this purpose, a total of three diffusers were designed and tested based on a specific scaling factor and input parameters defined by IHC Mining. Experiments consisted of capturing visual imagery of the generated plume on both top and side views for further analysis and performing velocity and concentration profile measurements in different locations. The offset jet transition from jet to plume and the respective impingement point for all diffusers was compared. Velocity and concentration measurements were analyzed and compared to determine an ideal diffuser. Using diffusers reduces the plume’s initial momentum while maintaining the density differences and therefore reduces the transition from jet to plume, allowing for gentler deposition over the surface and resuspending less material. All experiments show similar behavior in which a free jet can be observed initially. Then, after substantial entrainment takes place, the jet impacts the bed due to the negative buoyancy. Once the discharge impinges the lower boundary, it forms a radially spreading layer along the boundary which transitions into a wall jet. Lowering the momentum of the jet by diffusing the outlet gave better results in terms of drawing the impingement point, and the deposition profiles nearer to the diffuser. Also, the measured velocity and concentration profiles considerably decreased, reducing the plumes spread.