Ch. 5— Mining and At-Sea Processing Technologies • 173 



to the seabed. This placement is more costly, since 

 the pump is either a long distance from the power 

 source or the pump motors must be submerged. 

 Such components are very heavy for large pump 

 capacities. An alternative applicable for deep dredg- 

 ing is to use several pumps in series and boost the 

 flow in the suction line by means of water jets. This 

 technique has been tested and proven but is not 

 in widespread use because it is inefficient. 



The configuration of the suction head plays an 

 important role by allowing the passage of the solids 

 and water mixture up the suction line. In harder, 

 more compact material, the action of the suction 

 head may be augmented by rotary mechanical cut- 

 ters, by bucket wheels, and/or by water jets, de- 

 pending on the specific applications. When the ma- 

 terial to be dredged is unhomogeneous, such as 

 sand and gravel, the entrance of the suction line 

 is restricted to prevent foreign objects (e.g, large 

 boulders) from entering the suction line. The main 

 technological constraints in suction and discharge 

 systems are wear and reliability due to corrosion, 

 abrasion, and metal fatigue. 



The platform or vessel that supports suction 

 dredging components must be able to lift and move 

 the suction head from one location to another. Since 

 most dredgeable underwater mineral deposits are 

 more broad than thick, the dredge must have the 

 capability to sweep large areas of the seabed. This 

 is achieved by moving the platform, generally a 

 floating vessel; although experimental, bottom- 

 supported suction dredges have been built and 

 tested. 



The main types of suction dredges currently 

 applicable to offshore mining in the EEZ are hop- 

 per, cutter head, and bucket wheel dredges. 



Hopper Dredges 



Hopper dredges usually are self-propelled, sea- 

 going suction dredges equipped with a special hold 

 or hopper in which dredged material is stored (fig- 

 ure 5-5). Dredging is done using one or two dredge 

 pumps connected to trailing drag arms and suction 

 heads. As the dredge moves forward, material is 

 sucked from the seabed through the drag arms and 

 emptied into the hopper. Alternatively, the dredge 

 may be anchored and used to excavate a pit in the 

 deposit. 



Hopper dredges are used mainly to clear and 

 maintain navigational channels and harbor en- 

 trances and to replenish sand-depleted beaches. In 

 the United Kingdom and Japan, they are also used 

 to mine sand and gravel offshore. Hopper dredges 

 are configured to handle unconsolidated, free- 

 flowing sedimentary material. The suction heads 

 are usually passive, although some are equipped 

 with high-pressure water jets to loosen seabed ma- 

 terial. The trailing drag arms are usually equipped 

 with motion compensation devices and gimbal 

 joints. These devices allow the drag arms to be 

 decoupled from vessel motion and enable the drag- 

 heads to remain in constant contact with the 

 seafloor while dredging. 



The dredged material is dewatered for transport 

 after entering the hopper. Hopper dredges may dis- 

 charge material through bottom doors, conveyor 

 belts, or discharge pumps. Some models are emp- 

 tied by swinging apart the two halves of an axially 

 hinged hull. 



Capacities of sea-going suction hopper dredges 

 currently range from 650 to 33,000 cubic yards. 

 Although the theoretically maximum-sized hopper 

 dredge has not been built, the maximum capacity 

 of present dredges is a compromise between the 

 higher capital investment required for greater hop- 

 per capacity and the higher operating costs that 

 would result from more trips with smaller hoppers. 

 Typical operating depths for hopper dredges are 



Photo credit: J. Williams, U.S. Geological Survey 



Trailing suction hopper dredge Sugar /s/and with drag 

 arms stowed and hopper space visible. 



