Keying Distance . Keying distance is the vertical distance (upward) 

 required to rotate the anchor to an approximately horizontal orientation 

 (Figure 10). A rule of thumb is that the keying distance equals twice 

 the fluke length. For example, suppose that a 6 m wide fluke must be 

 embedded to 20 m to mobilize soil resistance. To account for keying 

 distance, the fluke length, i.e., required penetration = 20 _m + 2 x. 6 m = 

 32 m. For typical OTEC relative embedments" (d/Z = 5) this distance 

 becomes excessive. The large plates must be driven ^jery deep to account 

 for keying distance. 



Single Plate . Design anchor loadings (Table 8) were used with 

 Figures 7, 8, and 9 to specify required OTEC plate anchor sizes. The 

 results are listed in Table 9. Table 9 suggests that a single plate anchor 

 could be used to moor OTEC. The basic advantage of these single fluke 

 designs is their simplicity. Note, however, the size and embedment depth 

 required. These sizes represent an enormous extrapolation of current 

 plate anchor installation technology. The means to embed and key such 

 anchors neither exists today nor appears possible in the foreseeable 

 future. 



Multiple Plate. As shown in Table 9, required holding capacities 

 for OTEC could be obtained by bridling several flukes. This approach 

 introduces the problem of load equalization among anchors. A simple 

 sheave and cable system (Figure 11) has been used to solve this problem 

 for two anchors. But designing, installing, and maintaining a bridling 

 system for up to six anchors would be considerably more intricate and 

 costly. Note also that the driven depth for 3 m flukes in category A 

 soil is 37 m. The Civil Engineering Laboratory has explosively embedded 

 its 100 kip anchor to depths of 15 m in soft seafloors using a 254 mm 

 (10 in.) inside diameter gun. By increasing gun diameter to 406 mm 

 (16 in.), the 3 m plate could probably be embedded to only about 15 m. 

 Although other means of embedment are possible (vibrating, jetting, etc.). 

 experience indicates that they would be even less desirable. 



The load equalization and embedment problems associated with multi- 

 ple plate anchor installations would require a concentrated and expensive 

 design and development program with no guarantee of success. Such a pro- 

 gram is not recommended in view of the alternative anchor types available. 



Structural Design 



Method . The simple model shown in Figure 12 was used to compute 

 stresses in plate anchors. Mooring force was assumed to act vertically. 

 It was balances by a uniform distribution of soil pressure on the hori- 

 zontal plate. Maximum bending moments and resulting fiber stresses were 

 calculated and used to compute required steel thickness. Under the 

 assumptions, required plate thickness is independent of plate area. A 

 plot of mooring force versus required plate thickness (HY-80) steel is 

 shown in Figure 13. From the plot it is clear that steel of 76 mm to 

 101 mm (3 to 4 in.) thickness is required. 



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