The emerging deep-sea mining industry aims to retrieve mineral resources from the ocean floor but poses significant environmental and engineering challenges. Fluid mechanics, the physics of fluids in motion, underlies many of these issues and will be vital to developing sustainable mining practices.
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The Fundamentals of Fluid Mechanics
Fluid mechanics studies the behavior of liquids, gases, and other fluids at rest and in motion. A significant difference compared to systems that operate in the air is that water is approximately 800 times denser than air and is considered incompressible. In addition, water's high density and incompressibility have profound implications for subsea technologies. As a result, fluid mechanics principles are critical for the design and operation of any underwater engineering.
One of the most important principles in fluid mechanics is buoyancy – the upward force exerted by a fluid on an immersed object. In deep waters, the high density of seawater results in considerable buoyant forces acting on vessels, equipment, and structures. Carefully controlling an object's buoyancy, achieved by fine-tuning its volume, weight, and ballast systems, is crucial for the safe descent, hovering, and ascent of vessels and equipment.
Another vital fluid mechanics concept is hydrostatic pressure, which refers to the pressure a fluid exerts due to its weight. Hydrostatic pressure in water increases quickly with depth, at about one atmosphere per every 10 meters. This poses a significant engineering challenge for equipment operating at great depths, such as in seabed mining, where pressures exceed 400 atmospheres. Adapting vessels and tools to withstand these crushing pressures and perform precise tasks relies heavily on fluid mechanics principles.
Characterizing and analyzing fluid flows is critical for underwater designs. Unlike solids, fluid flows are governed by viscosity, turbulence, boundary layers, vorticity, and compressibility principles. Expertise in fluid flow physics enables submarine designers to profile and optimize the hydrodynamic shapes of vessels to reduce drag forces. It also helps model and predict flows around complex underwater structures and formations.
Control of fluid mechanics fundamentals is imperative for achieving efficient, controllable, and safe operation of equipment in the high-pressure, dark depths of the ocean.
Environmental Challenges of Deep-Sea Mining
While deep seabed mining offers a potential new source of scarce metals, it also poses risks of environmental harm that must be carefully studied and mitigated.
One major environmental challenge in mining operations arises from sediment plumes. Disturbing seabed sediments during the collection of deposits, such as polymetallic nodules, massive sulfides, and cobalt crusts, results in suspended particulate matter.
With slow settling velocities, ocean currents can disperse fine sediments hundreds to thousands of kilometers from mining sites, potentially causing ecological impacts across extensive ocean regions. However, the full scope and effects of these sediment plumes remain unclear.
This is where targeted fluid mechanics research can provide pivotal insights. By studying the fundamental dynamics of plume formation, evolution and settlement, researchers can develop physics-based models to simulate the dispersion of emissions from deep-sea mining.
Factors including the mining vehicle design, sediment particle sizes, ocean stratification, currents, and seafloor topology all influence plumes in complex ways best captured through fluid mechanics principles.
Mining equipment's disturbance of seafloor sediments creates turbid plumes that can propagate over large areas via ocean currents. Modeling the formation and evolution of these plumes using fluid mechanics principles is crucial for predicting their spatial footprint and impact, which involves examining seabed slopes, particle settling rates, and sediment rheology.
Engineering Challenges of Deep-Sea Mining
A primary engineering challenge is developing seafloor crawlers, pumps and collection systems that can efficiently harvest mineral deposits with minimal sediment disturbance. Polymetallic nodule mining should include innovative systems that carefully pick or suction individual nodules without scouring large swathes of the substrate.
Various prototype designs are based on fluid mechanics principles, such as the Coanda effect – the tendency of a fluid jet to follow a curved surface – to achieve targeted nodule lift-off while limiting resuspended sediments. In addition, computational fluid dynamics (CFD) simulations enable iterative optimization of collector vehicle shapes, intake flows, and suction pressures to balance nodule collection efficiency and environmental disturbance.
Some collector designs allow above-seabed maneuvering using buoyancy control or vertically oriented propellers. While this reduces direct seabed impact, it raises concerns about induced resuspension, especially when jets interact with the top sediment layer. There is ongoing research in this area, but accurately predicting sediment resuspension remains challenging.
Mining vehicle trajectory is often overlooked but substantially impacts sediment plume size and shape. Collector speed, especially when it exceeds background currents, can significantly affect plume dynamics due to current direction and mining patterns.
Therefore, researchers employ computational modeling, flow visualization, and fluid-structure interaction analysis to investigate how a collector's motion interacts with the surrounding fluid and sediment to optimize collector designs and evaluate their environmental consequences.
Vertical transport of the mined slurries to surface ships poses another fluid mechanics challenge. The two-phase slurries of solid mineral particles mixed in water must be pumped up long vertical risers against gravity, viscosity, and turbulence. In addition, maintaining optimal flow velocities is crucial to prevent settling and clogging within the pipe networks.
The complex physics of particle-laden shear flows, density stratification, and flow instability are active research areas that enhance vertical transport efficiency and reliability.
The kilometers-long vertical pipes employed for transport are prone to vortex-induced vibrations that can damage the system over time. Advanced control of turbulence using fluid mechanics principles has been proposed to minimize these vibration effects in deep-sea risers.
Sedimentation and Environmental Impact
One of the most severe environmental risks posed by deep-sea mining is the potential for discharged sediments to irritate seafloor organisms near mining sites.
Recent investigations in Norwegian fjords, where excessive sedimentation occurs due to mine tailing deposition, suggest that octocorals, such as Duva florida and Primnoa resedaeformis, may be more sensitive to suspended sediments than scleractinian corals. These studies revealed tissue damage caused by the sharp morphology of mine tailings and increased energy demands associated with the active removal of sediments from the polyps.
A MIT study conducted experiments off the coast of Southern California to investigate the environmental impact of sediment plumes generated by deep-sea mining vehicles. The researchers developed fluid dynamic predictive models by considering factors like ocean turbulence and sediment discharge volume to estimate plume reach and its potential impacts.
They found that the turbulence caused by the sediment discharge prevents it from aggregating into larger particles, leading to rapid dilution in the surrounding water. However, the ecological consequences of such seabed deposition remain unknown.
To fully assess such risks and develop appropriate impact thresholds, advanced modeling of the pathways, inventories, and timescales of all emitted sediments from deep-sea mining activities is needed.
Computational Fluid Dynamics (CFD)
Computational fluid dynamics (CFD) models play an important role in simulating deep-sea mining processes to predict environmental impacts and optimize operations. They serve as a valuable tool for assessing the environmental consequences of mining activities.
In a Delft University of Technology study, CFD was employed to simulate and analyze the behavior of deep-sea mining plumes generated during the extraction process. The research provided validation data for numerical models and used laboratory-scale experiments and CFD simulations to gain insights into plume dynamics.
CFD models also support mining equipment engineering, optimizing stability, efficiency, and durability through simulations of fluid-structure interactions. CFD models enable virtual testing of new technologies and operational strategies, offering insights into flow dynamics. This is especially advantageous given the challenges of field testing in deep ocean environments.
Promising technologies that aim to extract seabed resources with minimal environmental harm are emerging.
One example is precision robotic crawlers, such as Eureka from Impossible Metals, designed to pick individual polymetallic nodules delicately without disturbing surrounding sediments. These nimble robots utilize advanced machine learning, real-time sensor feedback, and fluid mechanics principles to optimize collection while minimizing sediment plumes.
At the processing stage, some pilot plants, such as IHC Mining, have demonstrated all-electric mineral processing. Their solution involves a unique permanent magnet (PM) motor that remains open to the surrounding seawater environment, eliminating the need for fossil fuel-based components. This design ensures environmental friendliness, even in extreme depths, setting it apart in the market.
Some proposed offshore platforms would also pipe mining slurries directly to shore using subsea pipelines. This circumvents the need for ships to shuttle back and forth, improving efficiency.
Such innovations aim to push the deep-seabed mining industry toward a minimal-impact, circular economy model.
Future Prospects of Deep-Sea Mining
The future of sustainable deep-sea mining relies on the active engagement and leadership of the fluid mechanics research community. They are essential for informing international governance contractors and educating the public about this emerging industry.
Applying fluid mechanics principles at each stage, from resource assessment to extraction to processing, will lead to engineering innovations that retrieve seabed resources while safeguarding fragile marine ecosystems.
The future of deep-sea mining hangs in the balance, but insights from fluid mechanics may tip the scales toward sustainability.
Deep-sea mining holds prospects for supplying growing mineral demands but also poses substantial environmental and engineering obstacles that fluid mechanics research can help overcome.
By elucidating the complex flows, forces and transport dynamics inherent to deep ocean systems, fluid mechanics principles provide the foundation for developing best-practice minerals collection, processing and impact management strategies. Collaboration between researchers across disciplines will be key to balancing resource utilization goals with ecological stewardship as this new blue economy industry takes shape.
The sustainable future of deep-sea mining rests critically upon advancing fundamental fluid mechanics knowledge and integrating those insights into transformative engineering solutions.
More on the Topic: Can Deep-Sea Mining Ever Be Sustainable?
References and Further Reading
Peacock, T., & Ouillon, R. (2023). The fluid mechanics of deep-sea mining. Annual Review of Fluid Mechanics, 55, 403-430. https://doi.org/10.1146/annurev-fluid-031822-010257
Boomsma, W. (2023). Sustainable power for deep-sea mining. [Online]. Available at: https://www.royalihc.com/mining/mining-innovations/sustainable-power-deep-sea-mining
Byishimo, P. (2018). Experiments and 3D CFD simulations of Deep-Sea Mining plume dispersion and seabed interactions. [Online] Available at: https://repository.tudelft.nl/islandora/object/uuid:75293f58-9a4d-46b0-9b7b-d13fcdc17d34?collection=education
Guo, X., Fan, N., Liu, Y., Liu, X., Wang, Z., Xie, X., & Jia, Y. (2023). Deep seabed mining: Frontiers in engineering geology and environment. International Journal of Coal Science & Technology, 10(1), 23. https://doi.org/10.1007/s40789-023-00580-x
Impossible Metals. (2023). Eureka 1 Autonomous Underwater Vehicle: A Landmark In Sustainable Harvesting. [Online]. Available at: https://impossiblemetals.com/technology/eureka-1-autonomous-underwater-vehicle-a-landmark-in-sustainable-harvesting/
Kang, Y., & Liu, S. (2021). The development history and latest progress of deep-sea polymetallic nodule mining technology. Minerals, 11(10), 1132. https://doi.org/10.3390/min11101132
Muñoz-Royo, C., Peacock, T., Alford, M. H., Smith, J. A., Le Boyer, A., Kulkarni, C. S., ... & Ju, S. J. (2021). Extent of impact of deep-sea nodule mining midwater plumes is influenced by sediment loading, turbulence and thresholds. Communications Earth & Environment, 2(1), 148. https://doi.org/10.1038/s43247-021-00213-8
Carreiro-Silva, M., Martins, I., Riou, V., Raimundo, J., Caetano, M., Bettencourt, R., ... & Colaço, A. (2022). Mechanical and toxicological effects of deep-sea mining sediment plumes on a habitat-forming cold-water octocoral. Frontiers in Marine Science, 9, 915650. https://doi.org/10.3389/fmars.2022.915650