Excavation of rock materials such as Seafloor Massive Sulfide deposits at great depths is greatly influenced by ambient water pressure, but little is known about the processes involved. Dry rock specimens are known to exhibit a large increase of apparent material properties in high-pressure environments. However, in deep-sea mining processes rock material is fully saturated and it is unknown how the hyperbaric effect changes apparent material strengths and involved physical processes under these conditions. This paper discusses the influence of hyperbaric effect on fully saturated brittle rock specimens during deep-sea excavation using a grab, and the resulting changes in apparent material properties. Computations are described which are used to predict the outcome of this effect, and the validating experiments that were carried out. A new theorem is stated based on elastic deformation of the grain matrix, which causes a pressure difference in the matrix and reinforces the material. This is based on the low cutting speed of a grab, allowing the material to deform elastically and enabling water ingress into the deformed material, resulting in less cutting energy. Moreover the cutting mechanism of a grab has a very low ratio of cutting energy over excavated rock volume. The theoretical model was developed in collaboration with Delft University of Technology and experiments were carried out with Seatools BV. Experiments were conducted to investigate the phenomenon and to validate the stated theorem for saturated rock material. An experimental setup was developed in collaboration with Seatools BV, to test rock material properties at different hyperbaric conditions with a low rate of loading. The experiments were designed to carry out standard material tests of the American Society for Testing and Materials to determine the compressive and tensile strength, by crushing the material specimens up to their breaking point in different hyperbaric conditions. The experiments were used to validate the theoretical computations that predicted differences in tensile strength between saturated and dried specimen, due to ingress of water during deformation causing not the full increase of the apparent material strength. The results were consistent and a correlation between the environment pressure and the added apparent material strength was found.
Recent developments for deep-sea mining have shown multiple scenarios of gaining mineral deposits of Seafloor Massive Sulfides (SMS). One of the problems for these scenarios is the overall large energy consumption of processing rock material which are a technological challenge and are increasing production costs. This paper compares two methods for deep-sea rock excavation on their energy consumption, based on rudimentary calculations. The best known scenario for gaining mineral deposits from the seabed is to excavate rock materials with a crown or drum cutter and pump the fluidized crushed materials to the vessel at the surface. This process requires high cutting forces deep-sea due to the hyperbaric effect at large water depths, when cutting with full cavitation. This high energy consuming process therefore requires a considerable amount of subsea installed power. An alternative scenario is to use a hydraulic grab for excavating mineral deposits and not crush all the materials entirely subsea. Using a grab would be very beneficial in rough terrains and unstable seafloor conditions, compared to track-driven vehicles typically used for crown or drum cutters. Also specific cutting forces are much lower when using a grab, because it is not cutting at full cavitation in hyperbaric conditions. However the main advantage is to keep most of the rock intact which allows the material to be crushed at the surface. Mechanically uplifting large pieces of rock therefore could have the advantage that most of the required power can be installed at the surface, rather than subsea for the traditionally proposed hydraulic pumping systems. The rock can then be further crushed under atmospheric pressure at the surface, avoiding the hyperbaric effect. The combination of using a grab and further crushing at atmospheric conditions is more energy efficient and therefore requires substantially less installed subsea power. Using rudimentary calculations, a great reduction of energy consumption is found for using a grab compared to typically used crown or drum cutters. Substantially less subsea installed power is required for excavating the mineral deposits with a grab. Although additional crushing needs to be done at the surface, the overall required installed power for using a grab still can be much less than fully subsea excavating and crushing.
The presentation will describe a short outline of important historical developments in the Dredging and Mining Technology. Also the link of Dredging Technology with existing On and Offshore Mining Installations will be highlighted. New developments and business drivers (for instance: SMS deposits) in the Deep Sea Mining market are most interesting for the Dredging Industry. Extrapolation of existing know how is very useful to develop frontier technology in this new area. Especially the existing technology for Seafloor Excavation and Vertical Hydraulic Transport of the excavated materials is very important for the development of effective and profitable future Seafloor Mining Equipment. Although the Dredging Industry is approaching deep water projects like Rock Dumping etc., Sea Floor Excavation in remote areas for more then 300 m water depth is not possible with conventional Dredging equipment. New concepts have to be developed to facilitate future Deep Sea Mining. IHC Merwede and Seatools are working on new challenging developments to be launched shortly. For the development of a total deep sea mining concept it is very important to join forces in the Offshore Market. Riser technology is available. Processing on board of the Mining Vessels is also proven Technology and combined with transshipment experience in the Dredging Market efficient concepts can be developed. To develop Deep Water Excavation and Vertical Hydraulic Transport it is very important to team up with the mining company. To realize the mining company's expectations it is essential to join during the exploration phase to specify exactly the parameters for a suitable mining installation. For an effective and profitable Mining process it is also very important to set up a mining plan with the mining company including the issues of availability and utilization of the installation. Important issues are: adaptability to seafloor conditions (profile, rock characteristics vs excavation process, distribution of the minerals, water depth, environment & sustainability, etc.), utilization and high availability of the system (weather conditions, maintenance, mining preparation, versatility, reliable production rates, etc.), material handling on board of the vessel and further transportation to land based installations (hydraulic transport, process monitoring, ore conservation, etc.) The presentation will highlight various excavation tools such as a ROV-grabber, developed by Seatools and a Seafloor Mining Crawler with suitable excavation tools, developed by IHC Merwede. Specific challenges will be explained. Both principals can be used in all stages of the mining process, from sampling of soil to full-scale production. The grabber can be used for terrain preparation prior to the deployment of crawling excavators and other mining tools. For both systems the deployment systems will be explained. Besides Deep Sea Mining is this new technology also very interesting for Sea Floor Intervention which is related to the Oil and Gas Industry in deep water areas like the Gulf of Mexico. The paper will conclude with a brief comparison of the capabilities and limitations of the various excavation methods such as the Grabber and Crawler excavation compared to other methods of sub sea mining.
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