This concept faces the same difficulties as noted for a deadweight in a 

 sinkhole, notably limited applicability and line abrasion. In addition, 

 acquiring the necessary fill material would be a formidable task. 



Conclusions and Recommendations 



Anchor design for a rock seafloor site in -the Gulf Stream requires spe- 

 cific knowledge of the local topography as well as rock engineering 

 properties. The investigation revealed that anchoring in such a location 

 will present problems not encountered on normal seafloor sediments. A 

 commensurate increase in anchor cost is probable. 



Use of the pile anchor system for the Gulf Stream rock environment is 

 very attractive at present because the pile anchor system can easily 

 accomodate much of the topographic irregularity found in the rock environ- 

 ment and because pile configuration can be altered on site to accomodate 

 variable rock properties. However, pile installation in 460 m of water on 

 an exposed or thinly covered rock seafloor will require the development of 

 some new technology. To resist uplift loads, piles in the rock environment 

 will probably be grouted into bored holes. The equipment for starting such 

 bored holes in thinly covered rock from a floating platform is not avail- 

 able. However, the necessary equipment is not overly complex when compared 

 to present offshore oil-production technology. 



Deadweight anchors (simple or grouted) would be an excellent choice 

 for OTEC if: 1) a heavy lift capability is available for placing the 

 anchor and, 2) the ability to pour large concrete masses on the seafloor 

 is demonstrated. Simplicity, economy, and reliability are the primary 

 advantages of deadweight anchors. 



SUMMARY 



Deadweight Anchors 



Deadweight anchors have been found very suitable for mooring the 

 OTEC platform, especially on unconsolidated sediments. On unconsolidated 

 sediments, the deadweight is fitted with cutting edges about its periphery 

 and at intermediate locations beneath the anchor block. The anchor weight 

 in cohesive soils is dictated by (1) the force required to embed the cutting 

 edges or (2) to prevent overturning, whichever is larger. The cutting edges 

 serve to depress the failure plane in lateral sliding into deeper and 

 stronger soil strata. Through this mechanism, the lateral holding capacity 

 to weight ratio of the deadweight (assumed concrete) is about 1/2 to 1 

 (1 to 1 in terms of submerged weight) on cohesive soils. On non-cohesive 

 soils (sands), the lateral holding capacity to weight ratio is about 1/3 

 to 1 (2/3 to 1 in terms of submerged weight). In this latter case, on 

 sand, the mass required for the deadweight may not be dictated by the 

 weight required to prevent sliding along the base shear plane. Rather, the 

 mass may be dictated by a bearing capacity problem beneath the leading 

 edge of the anchor block: on sands the anchor block is in danger of fail- 

 ing locally in bearing capacity and nosing over into the sediment. 



121 



