wrap-up of cable because both ends of the cable are fixed. To avoid this 

 cable wrap-up, the excursion of the surface unit should be beyond the center- 

 line of the load module. Thus, the surface unit should never pass above the 

 vertical centerline of the module. 



Two methods can be used to provide the cable with added strength. 

 The first is to provide armor or wire wrapping. The second is to provide a 

 separate cable to which the power cable is attached. If in each case the 

 material used for strength is the same and an equivalent area can be applied, 

 then it is advantageous to use armor. Armor can be placed on the cable 

 during manufacture. Original estimates were made for a standard, available 

 submarine cable with galvanized steel armor. Such a cable would have an 

 ultimate strength of 60,000 to 70,000 psi. A higher strength armor with an 

 ultimate strength of 250,000 pounds can be used, but the cost rises sharply. 

 The added cost of this armor is reduced for extended depths because the 

 high-strength armor reduces the amount of support which must be supplied 

 by buoyancy devices. 



A safety factor of 2, based on the ultimate strength for the standard 

 armor and for the high-strength armor, was used in determining the unsupported 

 cable length. This provides an allowable working stress of 30,000 psi for the 

 standard armor and 125,000 psi for the high-strength armor. 



There is a restriction on the weight that a surface plant can support 

 which, for the purpose of the study, was established at 50,000 pounds. 

 Therefore, even if the supporting strength of the cable can exceed this 

 loading, some system has to be employed to reduce the cable load below 

 50,000 pounds. Buoyancy devices can be added either completely throughout 

 or at discrete points in the cable system. Adding buoyancy material to the 

 cable during manufacture or during deployment is not considered a practical 

 solution to the provision of natural buoyancy for cables in the undersea 

 environment. Therefore, the use of the discrete buoyancy system was pre- 

 ferred. 



Optimizing a cable support system requires definite knowledge of the 

 actual techniques used for the installation of any given number and type of 

 buoyancy devices. The number of buoyancy units can be kept at a minimum 

 by so placing them along the cable length that the weight of the cable hanging 

 below results in a maximum allowable working stress in the cable. A factor 

 of 1.3 was used in determining the effective net buoyant force necessary to 

 maintain the cable in a generally vertical attitude, resist ocean current drag 

 forces, and account for the load component of the surface cable between the 

 surface unit and the first buoy. 



60 



