As shown in Figure 3, the screw piles are square and are rotated 

 by a kelly drive that allows longitudinal motion of the pile while it 

 rotates. An alternative drive system was considered with which two 

 piles are driven at the same time by a drive unit attached at the 

 upper end of the piles. The piles are counter-rotated to absorb the 

 torque. It was concluded that this system is much less desirable than 

 the kelly-drive system because the weight would be concentrated so far 

 above the template, and the free-fall of the piles would be hindered. 



Comparison of Conceptual Designs 



Each conceptual design was carried only far enough to provide a 

 reasonable basis for comparison. Because of the large number of oper- 

 ating requirements that a seafloor pile emplacement system must meet, 

 and because the four conceptual designs may differ in the manner or 

 degree in which they fulfill any given requirement, a quantitative 

 method of comparing the systems is necessary. The method adopted is 

 essentially the same as was used to select the three emplacement 

 mechanisms^. Relative weights are assigned to each of the operating 

 requirements identified in Table 1 to reflect the degree of importance 

 they exert in concept selection. Then each system is rated according 

 to how well it meets each operating requirement, employing a numerical 

 rating scale, called an effectiveness number. The effectiveness number 

 and the weight are multiplied and the products are summed for each 

 system. The resulting aggregate number reflects comparative effective- 

 ness of the systems. 



It was determined that all of the systems could be designed to meet 

 the requirements for water depth, sea state capability, capacity, number 

 of piles, and ship support in an equivalent manner. Thus, these require- 

 ments can be neglected in comparing the various systems. 



The weights assigned to each of the remaining operating requirements 

 are shown in Table 2. It should be noted that the requirement concerning 

 versatility has been divided into two categories. The most important 

 factors in comparing the systems are submerged weight, development re- 

 quired and complexity. The submerged weight has a large effect upon the 

 handling system, the range of choice of surface support system, and the 

 likelihood of successful emplacement. The required development effort 

 affects both the time and overall cost of procuring a prototype emplace- 

 ment system. Systems utilizing the greater number of state-of-the-art 

 components are superior, provided of course, that all other criteria are 

 met satisfactorily. Complexity is related primarily to the reliability 

 of the system and secondarily to cost. In general, systems utilizing 

 fewer mechanical operations and control functions are more reliable and 

 less costly; such systems are superior. The other operating requirements 

 have relatively less effect upon system comparisons, as reflected by 

 the weight assigned. 



