To reduce the steel weight (cost) imposed by large plate thicknesses, 

 a hollow plate design was also considered. Typical solid and hollow designs 

 are shown in Figure 14. 



Anchor loading was plotted against anchor weight for both solid and 

 hollow designs in Figure 15. The reader should note that this figure does 

 not indicate anchor holding capacity. It merely gives the weight (thickness) 

 required to prevent steel failure under an- applied load. Anchor capacity 

 will be a f unci ton of soil type and embedment depth as shown in Figures 7, 

 3, and 9. The nine square metre (100 ft ) anchor has a maximum holding 

 capacity of 3.3 MN (0.75xl0 6 lbs). Now Figure 15 is entered to determine 

 anchor weight. 



Cost . Table 9 shows several possible anchor designs for OTEC appli- 

 cations. The 3 m square anchor was chosen for cost comDarison to present 

 anchors. The estimated cost of fabrication and materials only for a deep 

 ocean plate anchor installation (nine 3-m anchors) is $234,000 (Table 10). 



Summary 



Plate anchors, either singly or in arrays are not reasonable choices 

 for OTEC. Load equalization for multiple fluke designs and embedment limits 

 for either multiple or single fluke design are critical. In view of the 

 alternative anchors available, further investigation of pla.te anchors is not 

 justified. 



Existing or moderately scaled up plate anchors could be useful in con- 

 junction with another primary anchoring system. 



Screw-In Plate Anchors 



Present types of screw anchors consist of small diameter steel shaft 

 with a large diameter plate, or blade, near the bottom. The plate is 

 typically a flat, single-flight helix. Rotation of the screw anchor causes 

 it to pull itself into the soil. Several blades may be spaced along the 

 shaft for greater capacity. Screw anchors have been used extensively on 

 land as guy anchors. They have also been used to anchor offshore pipelines. 

 Present offshore systems are installed in water depths to 300 m and have a 

 pullout capacity of about 110 kN (25 x 10 3 lbs). 



Relatively minor modifications are required to increase capacity by 

 a factor of 2 (Raecke, 1973). However, scaling up to the sizes required 

 for OTEC would be a monumental task. Used as the primary anchor, a screw- 

 in anchor would be limited by lateral load capacity. As discussed in the 

 section on piles, the sizes required to resist lateral loads are probably 

 unreasonable. For this reason screw-in anchors will be considered only 

 as supplementary anchors to assist in resisting vertical load. 



Proposed Use. Single helix anchors might be used to supplement the 

 vertical holding capacity and resistance to overturning of a deadweight 

 anchor (Figure 16). The screw anchor and associated embedment system 

 would be fabricated as an integral part of the deadweight. The deadweight 



35 



