For cohesionless soils, s_= and N = 0. For cohesive soils under 



short term leading, N = 1 and N ranges from a value of zero to nine, 

 q c 3 



Long Term Holding Capacity . Long term static holding capacity refers 



to the situation in which an anchor pulls out after a constant upward force 



has been applied over a long period of time. Such a situation might occur 



for a submerged buoy moored to the seafloor (Taylor et al., 1975)! 



Equation (2) with s and TT set to zero was used to find long term capacity. 



Ng ranged from two to twelve. Under long term loading, the excess pore 



pressure in cohesive material escapes (drained behavior). As a result, 



clays under long term loading act as frictional materials. Cohesionless 



materials were assumed to exhibit drained behavior only. 



Design Holding Capacity . Short and long term holding capacities were 

 plotted against aeptn and plate size in Figures 7, 8, and 9. For cohesive 

 soils the short term capacity was much less than predicted long term capac- 

 ities. Therefore, short term capacity was taken as the critical or design 

 holding capacity. The curves show the large plate sizes and deep embedment 

 depths required to achieve OTEC holding capacities. The single plate size 

 required in the deep ocean environment, soft clay (categeory A), is 12.2 m 

 wide. Compare this with the largest existing embedment anchor plate which 

 measures 1.5 m wide to 2.5 m long. 



Effect of Inclined Load . Another consideration that enters into a 

 single point mooring (single or multiple flukes) is the effect of inclined 

 or non-vertical loads. Several studies (Meyerhof, 1973; and Colp and 

 Herbich, 1972) on the effects of inclined loads have been conducted. For 

 almost all soil types and relative depths of embedment Meyerhof found that 

 holding capacity under an inclined load equalled or exceeded that under a 

 vertical load. Tests by Colp and Herbich in a saturated sand showed that 

 capacity increased up to an angle of .44 rad (25 deg) and then decreased 

 to near the vertical capacity at an inclination of 0.79 rad (45 deg) 

 vertical. For this report the effect of load inclination changes was 

 assumed to be negligible. 



Operational Factors 



Mooring Line Angle . For a given horizontal load at the surface, the 

 load felt at the anchor is a function of the mooring line angle (angle 

 between seafloor and mooring line). For a taut line the load at the anchor 

 increases approximately as the secant of the line angle at the seafloor. 

 Thus, for high line angles mooring loads ace increased significantly. For 

 example, a horizontal force of 10 MN (4x10 lbs) would produce a force in 

 the mooring line about 25 MN (5.7x10° lbs) at a line angle of tt/4 rad 

 (45 deg). At an angle of 1.4 rad (30 deg) the force in the mooring line 

 would be roughly 102 MN (23x10^ lbs). The advantage of maintaining a low 

 mooring line angle is apparent. 



24 



