58 



2.2.3 Construction 



There is no general configuration which bottom-crawling vehicles tend to follow. 

 The bulldozers are much like their land counterparts, but the remaining vehicles 

 are uniquely configured and cannot be described generally. Undoubtedly, the 

 most varied configurations are found within the pipe trenching and cable burial 

 vehicles which virtually defy a geometrical analogy. One aspect in common 

 with the trenching and bulldozing vehicles is their size: all are large and 

 massive. The average dr^' weight of these vehicles is 49t (54 tons) , and the 

 range is from 9 to 19 3t (10 to 213 tons) . Consequently, surface support 

 platforms are much larger and the launch/retrieval capabilities required to 

 handle these massive loads are more stringent. 



Cable burial vehicles and general purpo'se vehicles are much smaller, of the 

 active vehicles, the average weight is 1.9t (2.1 tons) and ranges from 1.25 

 to 2.5t (1.4 to 2.8 tons). 



The size of the pipe trenching and bulldozing vehicles limits the sea state 

 in which they can be deployed to about 4. In the event that inclement weather 

 moves into the operating area once the vehicle has been deployed, several 

 operators have made provisions to simply buoy-off the umbilical and re-engage 

 it after the weather has passed. 



2.2.4 Speed 



Vehicle speed varies in accordance with the nature of the work task and the 

 environment. In one instance, bulldozing, speed is not a particularly significant 

 factor since it is the capability to move quantities of sediment, rather than 

 the vehicle's speed over the bottom, which is important. In two tasks, pipe 

 trenching and cable burial, a quantitative appreciation for the vehicle's 

 average rate of advance is obtainable. For pipe trenching vehicles an average 

 advance rate range from 48m (57 f t) /hr to 500m (1,540 ft)/hr is quoted to 

 dig a trench 1.5m (4.9 ft) to 2.5m (8.2 ft) deep, respectively. The average 

 rate of advance of six trenching vehicles is 157m (515 ft)/hr for an average 

 trench depth of 2.0m (7 ft). These values are based on trenching a 122cm 

 (48 in.) diameter pipe. They do not take into account bottom sediment types 

 which will have a substantial effect on rate of advance. No trencher is 

 known to operate in hard rock or in boulder- sized material. 



Cable burying devices operate under the same environmental constraints as 

 do pipe trenching vehicles. The average rate of advance to excavate a Im 

 (39 in.) deep trench is 111m (364 ft)/hr. The range is from 60m (197 ft) 

 to 152m (499 ft)/hr. 



General purpose vehicles can obtain a maximum speed of 3 knots (6km/hr) , but 

 this, too, is dependent on the type of bottom (smooth vs. rough) and the bottom 

 slope gradient and the ability of the bottom to support the vehicle's weight 

 (trafficability) . The nature of the bottom can determine whether or not this 

 type of vehicle can work at all. An example was provided in the summer of 

 1978 when one general purpose vehicle literally sunk beneath the bottom of 

 the Gulf of Mexico and was forced to abandon its pipeline inspection task. 



