where d 

 Q 

 P 



trench depth 



flow rate 



jet water pressure 



constant determined by grain size 



constant determined by distance from jet to seabed surface 



It can be seen that the excavation depth increases more rapidly with 

 increasing flow rate than with increasing pressures. If the hydraulic 

 power, P = Q p, is kept constant, increasing the flow rate will produce a 

 deeper trench than increasing the pressure. Also, the trench depth 

 decreases as the speed of the jet across the seafloor increases. To 

 estimate the power required to jet a ditch into the seafloor, the power 

 and performance of two pipeline jetting devices and a planned cable 

 jetting device were used to calculate a power density function, defined as 



_ Delivered Power (hp) 



Soil Excavation Rate (ft^/min) 



Table 4 is a summary of the jetting systems' characteristics and resultant 

 power densities. The variation in power densities for the three systems 

 is not readily explained, but may be the result of several factors: 



System 3 is still on the drawing board and 

 may be underpowered . 



Systems 1 and 2 may be excavating more soil 

 than the nominal trench dimensions. 



Systems 1 and 2 may be supplying more power 

 than is required to do the job. 



Table 4. Characteristics of Three Jetting Systems 



Characteristic 



System 1 [41] 



System 2 [16,42] 



System 3 [20] 



Trench depth (ft) 

 Trench width (ft) 

 Trench shape 

 Speed (ft/min) 

 Flow (gpm) 

 Pressure (psi) 

 ?^ (hp/ft3/min) 



12 



9 



rectangular 



3.3 



36,000 



28 



1.7 



7 



9 



rectangular 



47 



16,000 



1,750 



5.5 



1.2 



2 



triangular 



50 



300 



125 



0.4 



21 



