First, the assumption was made that the entire lateral load on a 

 pile is absorbed by the first 0.3 m of rock adjacent to the pile below 

 the seafloor. Using a conservative limestone compressive strength of 

 29 MPa (4200 psi), and assuming a 0.25 m diameter pile, lateral resistance 

 per pile was found to be 1.43 MN (320,000 lbs). Roughly 125 such piles 

 are required to resist a lateral load of 180 MN. For simplicity a 12 by 

 12 pile array (144 piles) was assumed. 



Next, the adequacy of the group design against vertical pullout was 

 treated. Failure mode three (Figure 58c) was found to be more stringent 

 than mode two (Figure 58b) for the pile spacing and rock condition assumed. 

 Therefore, the pile group in Table 18 was designed to resist uplift failure 

 of the entire block. In other words, pile spacing and length were designed 

 so that the weight of entrained limestone (11 kn/m 3 ) plus friction developed 

 along the vertical sliding surface balanced uplift force. 



Analysis of other cases in the literature suggests that the procedure 

 used is conservative. Even with this conservativeness, a significant 

 anchor size and weight reduction, as compared to a pure deadweight was 

 indicated. 



The cost of drilling and grouting the required 144 piles would be 

 high. However, by increasing pile size or perhaps by using a deadweight 

 and battered pile combination, this cost might be reduced. Selection of 

 an optimum design requires a refined analysis procedure based on actual 

 rock properties and loading conditions. 



Pre-stressed Tendons. A similar reduction in anchor size might be 

 achieved by using pre-stressed tendons. First, holes would be drilled 

 through a template into the rock seafloor. High tensile strength steel 

 tendons would then be inserted and grouted in place. Finally, each tendon 

 would be tensioned by jacking down against a template on the seafloor. 

 The template would be forced against the seafloor with a normal force 

 large enough to provide the required frictional resistance to sliding. 

 In effect, the tensionong force acts as an equivalent deadweight force. 



For example, using the same coefficient of friction as for the 

 simple deadweight, a tensioning force of 780 MN is required. The total _ 

 cross-sectional area of steel tendons required is 0.5m^ assuming 250 ksi 

 (1725 MPa) steel. As was the case for piles, the uplift failure mode 

 shown by Figure 58c governs. Therefore, a template of the same size (20 m), 

 and steel tendons of the same length (20 m as for piles is needed to 

 entrain the required limestone mass. 



Examination of Table 18 shows that little is gained by using post- 

 tensioned tendons rather than piles. The intricacy of installation and 

 resulting high cost would reinforce this conclusion. 



Plate Anchor-Sinkhole Combination. Another anchor concept which was 

 considered again asssumed the availability of a suitable sinkhole near 

 the 0TEC site. A large plate is placed into the sinkhole and covered with 

 a fill material. This would remove the major problem of embedment of the 

 large plate size required for OTEC. Approximate plate size was calculated 

 as before and is repeated in Table 18. 



120 



