High-pressure water jetting, or ''water cannon,'' is a technique that 

 has particular application for fracturing rock. To generate high pressures, 

 a rapidly moving piston impacts on a slug of water and extrudes it through 

 a nozzle, producing a very high velocity water impulse. The water impulse 

 jet of a prototype underwater unit used for cleaning scale from steel is 

 about 1/2 cm in diameter, and the device requires 250 hp [32]. The applica" 

 tion of high-pressure water jetting or cavitation cutting for high -volume 

 excavation in soft materials has not been reported. 



Directional drilling is a technique reported on by Valent [33] for 

 installing the nearshore end of a cable system. Its attractive feature 

 is that a shore -based drilling rig can drill under the surf zone and rocky 

 areas to a distance offshore where nearshore effects have dissipated. 

 Piercing tools, such as the Pneuma Gopher, have been developed to dig 

 their way from one point to another when trenching is undesirable, such 

 as under a busy highway. Both of these techniques are suitable for producing 

 a relatively short path through which cabling can be led after the hole is 

 made. 



CONCEPT DEVELOPMENT 



The major problem areas associated with existing ocean cable burial 

 systems are the machine/soil interface, and the machine/surface support 

 control and propulsion interface. These problem areas are quite closely 

 related; for example, the large forces experienced in the machine /soil 

 interface cause problems in propulsion and control for the support ship. 

 To approach a solution to these general problem areas, three major catego- 

 ries were analyzed, and the resulting information combined in various 

 ways to formulate concepts. These categories are: 



1 . Excavation Subsystem 



2. Propulsion Subsystem 



3. Running Gear Subsystem 



To provide a common basis for comparision of the various techniques 

 of burying cables, a set of parameters was selected from the specific 

 operational requirements which represent maximum normal operating condi- 

 tions. Each concept was analyzed to determine the power required and 

 the resistance force produced by operating under these conditions. Maximum 

 allowable target values for size, weight, and force required were also 

 assigned as they must be used for some of the power and force calculations. 

 The values are shown in Table 2, and power conversion efficencies are 

 shown in Table 3. 



The burial machine weight and size were selected as desirable maximum 

 values to allow convenient handling from ships of opportunity. The size 

 affects water drag on the system and bottom stability. Machine weight 

 impacts on the running gear /soil interface forces and allowable ground 

 pressure. The machine speed and current profile create a drag force on the 



13 



