Ch. 5— Mining and At-Sea Processing Technologies • 179 



NEW DIRECTIONS AND TRENDS IN DREDGING TECHNOLOGY 



Dredge technology for offshore mining falls into 

 two distinct categories: technology for mining near- 

 shore in shallow, protected water; and technology 

 for further offshore in deeper water subject to 

 winds, currents, and ocean swell. Dredging systems 

 for a shallow environment can be readily adapted 

 from the various types of dredges currently used 

 onshore. Dredges for mining in a deep-water envi- 

 ronment must be designed with special character- 

 istics. They must be self-powered, seaworthy plat- 

 forms equipped with motion compensation systems, 

 onboard processing plants, and mineral storage ca- 

 pabilities. 



Design and construction of offshore dredge min- 

 ing systems for almost any kind of unconsolidated 

 mineral deposit or environment on the continen- 

 tal shelf are possible without major new technologi- 

 cal developments. However, for some environ- 

 ments, there may be operating limitations due to 

 seasonal wave and storm conditions. No break- 

 throughs comparable to the change from the pis- 

 ton to the jet engine in the aircraft industry, for 

 instance, are needed. If deposits of sufficient size 

 and richness are found, incremental improvements 

 in dredging technology can be expected. Costs to 

 design, build, and operate dredging equipment for 

 offshore mining are the most significant constraints. 



Several new design concepts have been developed 

 to help solve some of the problems of dredging at 

 sea. The motion of platforms floating on the ocean 

 generally make dredging difficult, but there are 

 three ways to alleviate this movement other than 

 those described previously. In one approach for 

 shallow water, one firm has designed and built an 

 eight-leg "walking and dredging self-elevating plat- 

 form" (WADSEP) to support a cutter head suc- 

 tion dredging system (figure 5-10). By raising and 

 translating one set of legs at a time the platform 

 creeps slowly across the seafloor. Since the platform 

 is firmly grounded, the problem of operating in 

 rough, open water is reduced. The dredge ladder 

 and cutter head sweep sideways by pulling against 

 anchors. This self-elevating platform could equally 

 well support a bucket ladder dredging operation. 

 The practical limit for dredging using a WADSEP 

 is probably about 300 feet. Although the concept 

 and technology are sound, the WADSEP is not cur- 

 rently cost-effective to use. 



Figure 5-10.— Cutter Head Suction Dredge 

 on Self-Elevating Walking Platform 



Although the technology is proven, mining operations with 

 a self-elevating walking platform are currently very expensive. 



SOURCE: Dredge Technology Corp. 



A second technological approach to the problem 

 of dredge motion in offshore environments is to use 

 a semi-submersible platform, such as those in wide- 

 spread use in the petroleum industry. This would 

 enable a dredge to continue mining or to stay on- 

 station rather than having to be demobilized dur- 

 ing rough weather. A design for a suction dredge 

 that incorporates a seaworthy semi-submersible hull 

 is shown in figure 5-11. A disadvantage of the semi- 

 submersible platform would be its sensitivity to 

 large changes in deadweight if dredged material is 

 stored on board. 



A third approach to eliminating platform motion 

 in shallow water is to develop a submerged dredge. 

 This project has proved to be complex and diffi- 

 cult in systems tested to date. Although a proto- 

 type of a submerged cutter head suction dredge was 



