170 • Marine Minerals: Exploring Our New Ocean Frontier 



Table 5-2.— Currently Available Offshore Dredging Technology 



Type 



Description 



Present max 

 dredging depth 



Capacity 



Bucketline and 

 bucket ladder 



Suction 



Cutter head 

 Trailing hopper 



Airlifts 



Grab: 



Backhoe/dipper 



Clamshell/ 

 dragline 



"Continuous" line of buckets looped 

 around digging ladder mechanically 

 digs out the seabed and carries 

 excavated material to floating 

 platform. 



Pump creates vacuum that draws 

 mixture of water and seabed material 

 up the suction line. 



Mechanical cutters or high pressure 

 water jets disaggregated the seabed 

 material; suction continuously lifts to 

 floating platform. 



Suction is created by injecting air in 

 the suction line. 



Mechanical digging action and lifting to 

 surface by a stiff arm. 



Mechanical digging action and lifting to 

 surface on flexible cables. 



164 feet Largest buckets currently made are 



about 1.3 yd' and lifting rates 25 

 buckets per minute (1,950 yd'/hour 

 with full buckets). 



300 feet Restricted by the suction distance 



unless the pump is submerged. 



50-300 feet Many possible arrangements all based 



on using a dredge pump; the largest 

 dredge pumps currently made have 

 48" diameter Intakes and flow rates of 

 130 to 260 yd'/min of mixture (10 to 

 20% solids). 



10,000 feet Airlifts are not efficient in shallow 



water. There may be limitations In 

 suction line diameter when lifting 

 large fragments. 



100 feet Restricted by the duration of the cycle 



and by the size of the bucket; 

 currently largest buckets made are 

 27 yd=. 



3,000 feet The largest dragline buckets made are 



about 200 to 260 yd'/hr; power 

 requirements and cycle time increase 

 with depth. 



SOURCE: Office of Teofinology Assessment, 1987. 



Figure 5-1.— Bucket Ladder Mining Dredge 



The bucket ladder dredge is a proven and widely used dredge for offshore mining; however, its use to date has been limited 

 to calm, shallow water. 



SOURCE: M.J. Crulckshank, U.S. Geological Survey. 



