Authors: Watanabe Keisuke (Tokai University) | Watanabe Yoshiyasu (Tokai University) | Yuasa Fumio (Furukawa Co. Ltd.) | Nazuka Tatsuki (Furukawa Co. Ltd.)
In the concept of subsea mining field development, a subsea crawler generally has a big drill and scrapes the mineral deposit horizontally. In this “Horizontal Mining System (HMS)”, ores vary in size and large chunks should be crushed into transportable particles. So the Subsea Crushing and Feeding Unit (SCFU) must be required for efficient slurry transportation to use the traditional riser system. In this paper, we introduce and discuss our “Vertical Mining System (VMS)” concept, which consists of a Down-The-Hole Drill (DTH) unit mounted on a jack up platform. This vertical mining system can be used for both exploration and production of the subsea mineral deposit.
The DTH unit makes the mineral deposit into powder-like particles while it digs into the hole. We executed drilling experiments on land with a granite rock using a DTH unit. The result showed the drilled particle size were less than 1mm, which indicates that this method allows us to transport the ore slurry directly through the traditional riser/pump system.
Because the ore particle is even and small enough, the Subsea Crushing Unit (SCU) can be eliminated. So we believe the total project cost and period can be reduced compared to the HMS concept. The maximum drill length can be from 30 to 50 m. The mining speed per a DTH is determined by the drill diameter and the drilling speed depending on the rock strength. At present, in our granite experiments, the DTH drilling speed was around 3.0 cm/sec. The desired production amount is achieved by installing plural drilling units.
We deploy the jack up platform instead of a crawler considering several reasons. The priority is we need to keep the xy coordinate of the drilling unit precisely controlled for efficient field layout. The second reason is we should keep the drill stem vertically even though the seabed has a slope or it is uneven. It is a gravity type jack up with 4 legs and has a set of xy horizontally sliding actuators which enables us to position the drill units automatically on the seabed. The horizontal size of the jack up is around 15 m times 15 m. The position of the platform will be changed continuously by suspending it with the support vessel's crane after it digs the 10 meters square seabed area. A subsea ore separator will be settled near the platform to select high-grade ore from low-grade ore on the seabed, which reduces not only the transportation cost/time but also the amount of marine disposal from the surface facility.