Feature ArticleMeeting the Challenges Of Deep-Sea Mining
By Jort van Wijk
Cees van Rhee
Delft University of Technology
At the bottom of the ocean, large amounts of valuable raw material can be found—polymetallic and phosphorous nodules, sulfurous deposits and metallic muds. These deposits are found in depths ranging from several hundred to several thousand meters. In coastal waters, population growth and increasing demands of industrial and consumer goods result in tight supplies of raw material from terrestrial mines. Therefore, moving mining activities to the deep seas is the next step for the Dutch dredging industry.
Concept employing a single seafloor mining tool connected to the mining support vessel by the vertical transport system.
A typical Dutch deep-sea mining operation employs a mining support vessel (MSV), a vertical transport system (VTS) and a subsea mining tool (SMT). The SMT cuts or collects ore from the seafloor, which is pumped into the VTS. The VTS consists of a riser through which the mixture of solids and seawater is vertically transported from the seafloor to the MSV. Several booster stations with centrifugal pumps are placed along the riser to maintain pressure. Furthermore, a return flow line and several umbilicals are installed.
Besides explorative operations, no full-scale deep-sea mining operation exists yet. Before arriving at a full-scale operation, several challenges have to be overcome, including the fundamental physics of the hyperbaric cutting of rock (excavation at the very large hydrostatic pressures at the seafloor), the physics of vertical two-phase flows containing large solid (settling) particles, flow assurance and positioning, and control of the SMT and VTS.
Fundamental research is key in optimization of deep-sea activities. At Delft University of Technology and IHC Merwede (Sliedrecht, Netherlands), the physics of hyperbaric cutting are studied in depth. A second research program aims at fully understanding the fundamentals of the vertical transport process in order to assess flow assurance.
In dredging, mining, drilling, trenching and tunnel boring different kinds of materials are excavated. To predict the required power, torques and forces, analytical models based on the physical properties of the materials involved have been developed. In general, these models assume the soil or rock is homogeneous and the progress of excavation is steady or continuous.
From experiments, it is known that these models give reasonable predictions if soft soils like sand and clay are considered, but an accurate prediction in harder coherent materials is difficult mainly due to the anisotropy of the material and the discontinuous behavior during failing. A second complication is the fact that most experiments were carried out under low (near atmospheric) pressures, which are sufficient for mining at shallow-water depths, but in deeper waters, hydrostatic pressure plays an important role. The failure mechanisms of hard materials are based on their compressive, shear and tensile strength. A high ratio between the tensile strength and the compressive strength (such as in soft clay) usually tends to result in a ductile cutting process, while a small ratio tends to result in a brittle cutting process.
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Jort van Wijk graduated from Delft University of Technology in 2009 with a master's in offshore and dredging engineering. After graduation, he started working at MTI Holland BV. In January 2011, he started a Ph.D. project on vertical hydraulic transport for deep-sea mining applications at Delft University of Technology.
Sape Miedema obtained his master's in mechanical engineering with honors at the Delft University of Technology in 1983. He obtained his Ph.D. degree on research into the basics of soil cutting in relation to ship motions in 1987. He is an associate professor of dredging engineering at Delft, where he researches the mathematical modeling of dredging systems.
Cees van Rhee is a professor of dredging engineering at Delft University of Technology. He has 27 years of experience as a researcher for the dredging industry and was head of the dredging research department at Van Oord Dredging and Marine Contractors. In 2002, van Rhee obtained his Ph.D. His main achievements are modeling the hopper sedimentation process and the high-velocity erosion of granular sediments.