umbilical cable, which is at maximum when the current profile adds to 

 the system speed at maximum operating depth. The trench dimension is 

 typical for most of the burying operations. The system must slow down 

 or consume more power for repeater burial or deeper burial. Finally, 

 the soil characteristics have a large impact on power and force. For 

 tougher materials, the system must slow down or consume more power, and 

 for weaker soils the speed can be increased or the power reduced. 



Several excavation techniques that have been used or suggested for 

 burial of objects in the seafloor are inappropriate for deep-ocean cable 

 burial because of their inability to meet some of the basic operating 

 requirements. Therefore, they could be eliminated without performing a 

 detailed power analysis. 



FVu'ld'lzat'ion . The fluidization process is not applicable since it is 

 intended for cohesionless materials (i.e., sands) and depends on the ab- 

 sence of intergranular attractive forces for successful operation. Recent 

 tests with Shell's fluidizer showed that the system stalls when clay 

 is encountered [34]. Many of the seafloor soils which will be encountered 

 are cohesive, and switching burial equipment in mid-operation is not 

 an acceptable solution to the deep-ocean cable burial problem. 



Blasting. Blasting is not an appropriate method of burying long cable 

 runs in sand and clay since it is best used for fracturable materials such 

 as rock and coral, is a batch (rather than continous) process, and to 

 date, requires divers to prepare blast holes and set the charges. 



Table 2. Design Parameters 



Burial machine weight 1 tn (max) 



Burial machine size envelope 12 ft wide x 25 ft long 



x 10 ft tall (max) 



Motion resistance force 5 tn (max) 



Power 500 hp (max) 



Speed 1 kt (101 ft/min) 



Current 2 kt at surface 



at 300 ft 



Trench dimensions 36 in. deep x 4 in. wide 



Umbilical cable 3-in. diameter 



6,500 ft long 



Soil characteristics Clay 



Undrained shear strength, 



S = 4 psi 

 u 



Bulk density, 



p = 100 Ib/ft^ 



14 



