The average value of these three systems is taken as an estimate for the 



power density function for burying cables. To jet a 4-inch-wide, 36-inch- 



deep trench at a speed of 1 knot » the soil excavation rate is 101 



ft^/min. Using Pq = 2.5 hp/ft^/min, the total power required is 256 hp. 

 avg 



Auger Trenching. Auger trenching and cutter head dredging are 

 basically very similar mechanisms in that a rotating surface fails the 

 soil. The basic difference is that augers physically remove the spoil 

 where dredges crumb the soil and a dredge pump removes the spoil. A dredge 

 cutter head is basically spherical which requires that the trench depth 

 equal the trench width on a single pass. Thus, to attain a 3-foot-deep 

 trench, the trench must be 3 feet wide. Since the trench need only be 

 4 inches wide, more work than necessary is being done, which leads to 

 higher power requirements. 



Double vertical counter -rotating augers have been used with some 

 success for burying pipelines. Very little information was available in 

 the literature, so a power analysis parallel to that for trenching was 

 performed (Appendix C) . The power level for one 4 -inch-diameter auger 

 was 75 hp , somewhat higher than for vibratory plowing and about the same 

 as for trenching. The soil removal rate, however, requires an auger speed 

 of 5,000 rpm. This high rotary speed required by the soil removal rate 

 associated with a 1 -knot speed of advance appears unrealistic for soil 

 cutting. Encountering a rock or other unyielding surface at that speed 

 would most likely damage the auger considerably. The force necessary to 

 move counter-rotating augers through the soil is expected to be near that 

 for vibratory upward soil cutting (2,200 pounds) in that the soil is being 

 lifted and failed at a rapid rate. 



Propulsion Subsystems 



Towing. Virtually all of the cable and pipeline burial systems 

 rely on towing as their primary means of propulsion. Most use the 

 kedging anchor technique with power winches on the support barge. Since 

 this process is slow and may necessitate stopping while anchors are reset, 

 it is not suitable for deep -ocean cable burial. The Bell and Japanese 

 cable plows are towed from a ship, which lays cable simultaneously. For 

 the Bell system, 2,000 shp is required to tow with an average tension 

 of 33,000 pounds at an average speed of 1 km/hr [37]. Taking the ratio of 

 the delivered power to the supplied power results in the overall 

 efficiency (exclusive of the prime mover) 



^del ^ (33,000 lb) (0.911 ft/sec) / hp 



^ P 2,000 hp 1550 ft-lb/sec 



sup V 



n TT = 0.06 

 overall 



22 



