to the desired depth of burial. Water or an air-water mixture is supplied 

 to the machine from the surface. These systems are generally high-flow, 

 medium-pressure systems (20,000 to 30,000 gpm at 1,000 to 2,500 psi) that 

 are towed along the pipeline [15]. The jets break up the soil, and 

 then air or water eductors lift the soil/water mixture out of the trench. 

 The pipeline settles into the trench after the machine passes, and natural 

 action eventually backfills the trench. The main disadvantage to jetting 

 a trench for the cable to settle in is the large amounts of power required. 

 Comparing four operational pipeline jetting systems working at capacity, 

 the average power supplied per unit excavation rate is [17,18,19,16]: 



p ^ 11.6 hp* 

 avg ft /min 



Using this power -excavation rate density figure for a deep -ocean cable 

 burial system would require over 1,000 hp. These systems normally operate 

 at 5 to 30 ft /min (1 kt = 101 ft /min) and work best when guided by a stiff 

 pipeline. They also are constrained to work in a relatively firm soil so 

 that the excavation will not fill in before the pipeline settles into place. 



Analytical studies for pure jetting (i.e., where no equipment pene- 

 trates the seafloor) have shown that the power-excavation rate density 

 can be as low as 0.4 hp/ft^/min [20]. Using this figure, 40 hydraulic 

 horsepower would have to be supplied to jet a 3-foot-deep, 4-inch-wide 

 trench at a speed of 1 knot. No information was encountered which discussed 

 the effect of depth of cut and speed of advance on power requirements. 

 Pure jetting is a simple technique that has been used with some success 

 by the Pisces Submersible and the Alcoa Seaprobe. Disadvantages of pure 

 jetting are that (1) the amount of material which must be excavated depends 

 on the angle of repose of the soil, (2) there is no positive means of en- 

 suring the desired burial depth, and (3) backfill depends on re- 

 sedimentation of the excavated soil. 



Jet Plowing 



As the name implies, jet plowing combines the features of both water- 

 jetting and conventional plowing. This technique has been used quite 

 successfully by the Harmstorf Hydro jet and Aquatech cable plow for 

 shallow water and river crossing [17-25]. In essence, the water jets 

 loosen the soil in front of the plowshare, reducing the frontal resistance 

 on the plowshare. The soil is kept in suspension until the plowshare and 

 cable guide pass, whereupon it settles. Very little soil is actually remov- 

 ed from the ditch, and no backfill is required. The Harmstorf unit is 

 equipped with a vibration means to help break up competent soil. Jet plows 

 are usually pulled with winches from a barge or from shore. Total power 

 required ranges up to 1,500 hp. These systems historically have required 

 supervision and inspection by divers, which is not to say they cannot 

 be redesigned to operate without first-hand supervision. 



In Reference 26, this function is referred to as Nominal Overall 

 Specific Energy /in /lb 

 in. 3 



11 



