172 • Marine Minerals: Exploring Our New Ocean Frontier 



Figure 5-3.— Motion Compensation of Bucket Ladder on Offshore {Mining Dredge 



Motion compensation systems might be necessary offshore to ensure that the lower end of the dredge ladder maintains constant 

 thrust against the cutting face while the dredge hull pitches and heaves in swells. 



SOURCE: Dredge Technology Corp. 



With the availability of new materials and higher 

 strength steels, it is now possible to design bucket 

 ladder dredges capable of digging twice as deep (330 

 feet) as present dredges, but the capital and oper- 

 ating costs would be greatly increased. 



Suction Dredging 



Suction dredging systems have three principal 

 components: a suction device, a suction line, and 

 a movable platform or vessel (figure 5-4). The suc- 

 tion device can be either a mechanical pump or 

 an airlift. Pumps are most common on suction 

 dredges; airlifts have more specialized applications. 

 Pumps create a drop in pressure in the suction line. 

 This pressure drop draws or sucks in a mixture of 

 seawater and material from the vicinity of the suc- 

 tion head and up the suction line into the pump. 

 After the slurry passes through the pump, it is 

 pushed by the pump along the discharge pipe un- 

 til it reaches the delivery point. 



Pump technology is considered relatively ad- 

 vanced. Dredge pumps are a specialized applica- 

 tion. The main features required of dredge pumps 

 are large capacity, resistance to abrasion, and effi- 

 ciency. To accommodate the large volumes of ma- 

 terial dredged, the largest dredging pumps have in- 

 takes of up to 48 inches in diameter and impellers 

 up to 12 feet in diameter. These parts require large 

 steel castings that are both costly and complicated 

 to make. The flow of solids (e.g., silicate sand or 

 gravel) and water at speeds of 10 to 20 feet per sec- 

 ond through the pump and suction line causes abra- 

 sion and wear. 



Figure 5-4.— Components of a Suction Dredge 



Power 

 It 



Dredge pump Discharge 

 line 



Unconsolidated seafloor 



The main types of suction dredges currently applicable to 

 offshore mining are hopper, cutter head, and bucket wheel 

 dredges. 



SOURCE: Office of Tecfinology Assessment, 1987. 



Pumps create suction by reducing the pressure 

 in suction lines below atmospheric pressure. Only 

 80 percent of vacuum can be achieved using present 

 mechanical pumping technology. This constraint 

 means that dredge pumps cannot lift pure seawater 

 in the suction line more than about 25 feet above 

 the ocean level. This distance would be less for a 

 mixture of seawater and solids and would vary with 

 the amount of entrained solids. Greater efficiency 

 can be a'chieved by placing the pump below the 

 water line of the vessel, usually as near as possible 



