Appendix C Hypothetical Dredging Operations 



I. Hypothetical Offshore Dredging Operations in Shallow Water 



It may be useful to evaluate a hypothetical offshore dredging operation in order to bring some of the 

 problems into clearer focus. Two examples are used. One assumes an operation in Norton Sound off 

 Alaska where severe weather conditions prevail during a large part of the year. The other assumes an 

 operation in waters where less severe environmental conditions prevail, as say off the coast of Oregon.' 



In order to limit the variables, certain basic assumptions about the deposit are made: 



—The deposit is located one mile offshore and the combined depth of water and deposit thickness does 

 not exceed 1 50 feet. 



—The deposit is amenable to exploitation using standard bucket line dredging methods, giving a recovery 

 factor of 90 per cent. 



—The deposit grade averages 75 cents per cubic yard and the reserves are equal to the life of the dredge. 



—The estimates used in detailing capital expenditures are vaUd. The estimates are basically engineering 

 cost evaluations and factors such as potential pollution proTilems, royalty payments, and other possible 

 resource costs are not included. Moreover, it should be emphasized that for initial ventures with high 

 capital investments returns would have to be well above normal to compensate for the risk. 



The above assumptions are made to compensate for the lack of real data, which would normally be 

 available, and although they are for the most part artificial, they are based on sound engineering practice 

 and judgment. 



A. Operational Variables 



Analyses have been made previously for three standard sizes of bucket line dredge, of 9, 12, and 18 

 cubic feet bucket capacity which cover the general range of bucket line dredges currently employed in 

 land mining operations below 100 foot depth. These analyses showed that, working in the Bering Sea, 

 none of these dredges would produce a ''.ufficient return on investment to justify exploitation of deposits 

 valued at $1 per cubic yard. This study is therefore based on a 54 cubic foot dredge, which though never 

 previously used for mining, is quite practical from an engineering standpoint. Certain of the data relating 

 to this dredge are extrapolated graphically, other from previous work. 



B. Limitations on Operations 



In Norton Sound, ice conditions prevail which limit operations to a maximum of 185 days out of the 

 year with allowance made for 1 1 days of extreme storm condition. During the winter mohths the dredge 

 would be maintained at Anchorage in Alaska and only the salaried staff retained. Travel time between 

 Anchorage and the dredging ground would amount to some 14 days, during which time the operational 

 crew would remain aboard. 



In other more protected areas, these conditions do not prevail and cost analyses for 362 work days 

 per year are shown for comparison. These are based on a 22-hour day. 



Evaluation by the Bureau of Mines Marine Mineral Technology Center, Tiburon, California. C. M. Romanowitz, an 

 expert on dredging, served as a consultant in this study. 



VIM 78 



