illustrative purposes. The 7.6 m diameter pile would require pile walls 

 150 mm (6 in.) thick. HY-80 steel can be obtained in such thickness, 

 is weldable, and would serve well in seawater at the ambient 4°C; it is 

 not known if such plates can be formed to the necessary shell curvature. 

 Of course, the real pile structure would probably look considerably 

 different from the idealized simDle shell. One fact is known, the pile 

 strucutre will be steel and will not be reinforced or prestressed concrete. 



Summary 



Probable anchor pile ultimate capacities are shown in Fiqure 29 for 

 the deep ocean environment and in Figure 30 for the Gulf Stream environ- 

 ment. In reviewing and using this ultimate caoacity data, the reader 

 must remember that safety factors on the order of 2 must be aoolied to 

 this data when determining the safe working load. Working loadings 

 higher than this will cause excessive pile deflections leading to pullout 

 or failure in bending. Capacities for axial and lateral load were 

 computed independently. Pile lengths plotted in Figure 29 were chosen 

 to: (1) minimize deflection and (2) reflect practical constraints 

 (installation, fabrication, anticipated costs). Figure 29 suqgests that 

 single piles used to anchor OTEC power plants in the deep ocean will have 

 to be about 4.9 m in diameter to resist the lateral load component. The 

 pile length required would vary with the vertical load component, from 

 about 50 m long for zero vertical load to about 120 m long for a 100 MN 

 vertical load comoonent (see Figure 25 for source of pile length data). 



Fiqure 30 indicates the difficulty of obtaining the very high 

 lateral load capacities necessary for the Gulf Stream environment. 



Pile anchors of conventional diameter, say 2.4 m, linked in grouDS, 

 do not appear economical for resisting the large lateral forces encount- 

 ered in locations like the Gulf Stream. To resist repetitive lateral 

 load with piles alone would reauire as many as 118 2.4 m diameter, 

 49 m long piles. 



In the less severe deep ocean environment piles could prove to be 

 effective and economical. They have one distinct advantage over dead- 

 weights. Pile lateral capacity is not severely reduced as axial pullout 

 forces increase. Thus piles are better able to cone with high moorinq 

 line angles than are deadweight or drag embedment anchor systems. 



DEADWEIGHT ANCHORS 



Introduction 



Horizontal Load Resistance. Deadweiqht anchors derive their lateral 

 load caoacity by developing friction between their bottom surface anH the 

 seafloor (Figure 31). The maximum horizontal resistance, R|_, between the 

 seafloor and the deadweight is a function of the gravitational force 

 imposed by the deadweight on the seaflnnr and nf the coefficient of fric- 

 tion between the deadweiqht and the seafloor. The coefficient of fric- 

 tion varies widely with seafloor material type from 0.1 to 0.5 with the 

 type of seafloor material and the nature of the deadweight anchor 



62 



