base the design on the minimum number of piles because the operation 

 and utility of the screw-pile emplacement method could be demonstrated 

 adequately and at minimum cost with a three-pile system. An equilateral 

 triangle is the obvious choice for shape. A pile spacing of ten feet, 

 center-to-center, was chosen because the resulting foundation/anchorage 

 is at once large enough to be representative of a prototype system and 

 small enough to be handled over the side of a moderate-sized ship. The 

 overall length of one of the sides of the triangle formed by the template 

 is about 14 feet. 



The length of the piles and the size and number of the helical 

 blades can be varied within limits to suit the expected soil conditions 

 at the proposed site. It is believed that piles longer than 50 feet 

 will be too difficult to handle, and a minimum length of about 25 feet 

 will be necessary to provide adequate embedment of the blades. The 

 blade diameter can be varied from about 16 inches to 30 inches. The 

 minimum diameter is limited by the cross-sectional dimensions of the 

 pile shaft. The diagonal dimension of the planned pile cross section 

 (10 inches by 10 inches) is approximately 14 inches. If a smaller pile 

 cross section is desired, bushings would be necessary to reduce the 

 size of the hole in the kelly drive. (A smaller pile cross section 

 would reduce considerably the lateral load capacity of the foundation/ 

 anchorage.) The maximum diameter of the helical blades is limited by 

 the installation torque available. The use of multiple blades to aid 

 penetration is common in commercial screw anchor installations^. 



The required installation torque was estimated from a theoretical 

 analysis, and from results of installation and load tests of actual 

 screw anchor installations. The theoretical analysis suggests required 

 torque of 10,000 to 15,000 ft-lb for adequate penetration of all expected 

 seafloor soils. Industrial users of screw anchors commonly utilize 

 power equipment with a stalling torque of 8,000 to 10,000 ft-lb to 

 emplace high-capacity anchorages^. The larger torque value of 15,000 

 ft-lb was adopted for the preliminary design to ensure full pile pene- 

 tration. In order to estimate the required power, a minimum pile rota- 

 tion speed of 10 rpm was established. At 10 rpm the 15,000 ft-lb 

 torque requires 28.6 horsepower to be delivered to the pile. Assuming 

 reasonable efficiency of the electrohydraulic power system, approximately 

 60 horsepower must be delivered at the surface to emplace each pile. 



The general operation sequence for the screw-pile emplacement 

 system is shown in Figure 4. As indicated, the piles and template 

 will be lowered together as a unit. Figure 4a. As the unit nears 

 the seafloor, a bottom-sensing trip mechanism releases the piles to 

 free-fall a short distance for initial embedment in an approximately 

 vertical attitude. The template is supported on the piles by one-way 

 grips that permit the piles to move only downward with respect 

 to the template. Figure 4b. The one-way grips are designed to be 



14 



