59 



2.2.5 Power 



All bottom-crawling vehicles but one receive their power through an umbilicial 

 cable from a surface platform. The general-purpose vehicles operate generally 

 on the same power as do the tethered, free-swimming vehicles, e.g., 440 V, 

 3 phase, 50/60 Hz. The pipe trenching vehicles require higher voltages in 

 the neighborhood of 3,300 V which are transformed to lower operating voltages 

 at the vehicle. 



The exception is found in the UNDERWATER BULLDOZER (D 155W) which is powered 

 by an onboard diesel engine (KOMATSU S6D155-4) providing 270 hp at 2,000 rpm. 

 Since this vehicle has a maximum operating depth of only 7m (23 ft) , it is 

 possible to incorporate the intake and exhaust ducts into a rigid mast which 

 protrudes above the water surface. 



2.2.6 Propulsion 



Pipe trenching vehicle propulsion is by one of two means: 1) using the pipe 

 itself for traction; or 2) using the sea bed for traction. Over half of 

 the vehicles investigated fall into the first category. Initially the vehicle 

 is joined to the pipe (with diver assistance or, in the case of the KVAENER 

 MYREN system, by thrusters) , and then engages either hydraulically- powered 

 wheels or push-pull grasping arms to initiate movement. The two vehicles 

 which rely on the bottom for traction, TM 102 and the UNDERWATER TRENCHER, 

 receive propulsion from caterpillar tracks which straddle the pipe. In each 

 case the vehicle provides it's own propulsion means. 



The trenching vehicles of Land and Marine Engineering, Ltd. vary from all 

 the others, in that they are towed along the pipe by the surface support 

 ship. Coated rollers protect the concrete coating and guide the vehicle. 



Bulldozing vehicles, similar to their land counter-parts, use caterpillar 

 tracks for propulsion. 



The general purpose vehicles use either individually- suspended wheels (4 to 5) 

 or caterpillar tracks. 



2.2.7 Tools/Instrumentation 



The primary tool of the trenching vehicles is the cutting device. Two means 

 of excavation are used: water jets which fluidize the sediment, and hydraulic 

 cutters which mechanically breakdown the sediment. In both instances the 

 cuttings are removed by suction pumps (airlift excavation is sometimes used 

 in shallow water) and expelled to the side of the trench. In one instance, 

 TM 102, the cuttings are expelled over the pipe as backfill. Closed circuit 

 TV is occasionally used to monitor the excavation progress, and scanning 

 sonar may also be used to profile the trench or to assist in initially docking 

 the vehicle to the pipeline. In most instances the trencher's progress and 

 performance is monitored by divers, remotely controlled vehicles or manned 

 vehicles. Instrumentation can also include inclinometers (to monitor vehicle 

 tilt and roll) , and air pressure and hydraulic pressure gages. 



