180 • Marine Minerals: Exploring Our New Ocean Frontier 



Figure 5-11. —Conceptual Design for Suction Dredge 

 Mounted on Semi-Submersible Platform 



Side view 



fl,' Maximum dredging 

 draft 72 ft 



Survival draft 54 ft 



successfully built and operated offshore for several 

 months, it was not an economic success and its de- 

 velopment was discontinued. 



Greater dredging depths can be attained by sub- 

 merging pumping systems or by employing airlift 

 or water jet lift systems. While submerged pump 

 technology can be readily adapted from military 

 submarine technology or from deep-water petro- 

 leum technology, the development costs are high. 



No breakthroughs are foreseen that could vastly 

 increase the capacities of offshore dredging systems 

 and bring substantial cost reductions. However, ex- 

 isting technology is largely based on steel construc- 

 tion, and the use of new, lighter materials with 

 higher strength-to-weight ratios has not been widely 

 investigated. 



Semi-submersible platforms have been developed for 

 offsfiore oil drilling. TUe semi-submersible platform offers a 

 stable platform from v>/fiicfi to operate, but is very expensive. 



SOURCE: Dredge Technology Corp. 



MINING CONSOLIDATED MATERIALS OFFSHORE 



Two principal types of consolidated deposits that 

 are known to occur in the U.S. EEZ are massive 

 polymetallic sulfides and cobalt-rich ferroman- 

 ganese crusts. Alternatives for mining manganese 

 nodules, where present in the EEZ, have much in 

 common with dredging techniques used in shallow 

 water, although the deep water in which nodules 

 are found presents special problems. However, 



techniques for mining polymetallic sulfides and co- 

 balt crusts are likely to be very different than the 

 dredging techniques used to mine placers and other 

 unconsolidated deposits. Unlike unconsolidated de- 

 posits, these deposits must be broken up (using ei- 

 ther some type of mechanical device or blasting) 

 and possibly must be crushed prior to transport to 

 the surface. Moreover, all known cobalt crust and 



