Several authors reported bendinq moment increases due to repetitive 

 loadinq. Davisson and Salley (1968) noted a 20 to 50 percent increase in 

 bendina moments on 1.2 m diameter drilled piers. In model and field 

 tests, Matlock (1970) observed moment increases of almost 100 percent 

 during cyclic loading. Several of Matlock's tests were nerformed on a 

 0.33 m diameter instrumented pile in soft, sliahtly over-consolidated 

 clay. These conditions are fairly representative of marine sediment 

 deposits. Matlock suaqested that a reduction in pile lateral capacity 

 due to cycling might be estimated on the basis of a ratio of pile deflec- 

 tion to diameter. The rouqh guidelines in Table 12 were estimated from 

 the results of his model tests. 



A typical deflection ratio expected for the 0TEC anchor is about 

 0.1. Thus, design capacity for repetitive loadinq is about 0.58 times 

 static capacity. Please note these reductions have not been applied to 

 the data presented herein. 



The effect of reDetitive axial loading on axial oullout capacity 

 should be minimal (Bea, 1975). Likewise the effect of repetitive 

 lateral load on axial canacity has not been a serious problem in present 

 offshore structures. 



Preliminary Structural Design of Required Piles 



As indicated in the lateral load capacity analysis section, the 

 pile sections were desioned to resist the maximum load conditions for 

 piles with lenoths eaual to 10 diameters in the category C soil. This 

 course of action was taken early in the study to enable an evaluation of 

 the materials available for fabrication, the wall thicknesses and weights 

 necessary, and the overall practicality of the pile anchor concent aoolied 

 to OTEC. Table 13 presents a summary of results for two, potential, 

 sinqle-oile anchor desiqns, one a 4.9 m diameter, 31 m lona pile, the 

 other a 7.6 m diameter, 7.6 m long pile. These piles in a categorv C 

 soil are capable of mobilizing soil resistance to supply the noted axial 

 and lateral load capacities, with no reductions applied for repetitive 

 loadino. In all cases, the large bending moments resulting from the hioh 

 lateral loads controlled structural desiqn of the piles. 



Reinforced concrete was eliminated from consideration because the^ 

 high bending moment combined with axial tension would result in signifi- 

 cant cracking on the tension side of the pile, an unacceptable condition 

 in the marine environment. Either prestressed concrete or steel could be 

 used for the 4.9 m diameter pile resisting the 9.8 MN (2.2xl0 6 lbs) 

 lateral load. The prestressed concrete pile would, however, require 13.3 

 MN (3xl0 6 lbs) of steel and concrete to resist the 9.8 MN lateral load. 

 Thus, the concrete pile anchor efficiency (lateral load capacity to weiaht 

 ratio) would be 0.7— little better than a good deadweight anchor. The steel 

 pile appears much more desirable with a 50 mm wall thickness and a lateral 

 load efficiency of 5.4. 



An adequate design using prestressed concrete is not possible for 

 the 7.6 m diameter pile resisting the 97 MN (22xl0 6 lbs) lateral load. 

 One simply cannot fit the required steel and concrete into the cross- 

 section available. An idealized steel pile section is suggested for 



59 



