Figure 2. Trencher modified for shallow-water coral trenching. 



The disadvantages associated with plowing center on the high force 

 required to penetrate the soil, both vertically and horizontally. In order 

 to effect initial plow penetration, and to keep the plow in the soil, 

 ocean plow machinery has been very heavy (19 to 23 tons). To support the 

 plow on the seafloor, large skids are used. The skid drag coupled 

 with the force required to plow can be as high as 100,000 pounds. Since 

 deep -ocean plowing systems are towed from the surface, the high towing 

 force, high weight, and slow speed of operation impose requirements on 

 the surface support ship that are not easily met . The high drawbar pull 

 requirement was recognized as a problem for land cable plows when the 

 undergrounding of services for older residential communities was increasing 

 Tractors, required because of their high drawbar pull capability, caused 

 surface damage which had to be repaired. Analytical and experimental in- 

 vestigation of vibrating the plowshare showed that up to a 99% reduction 

 in drawbar pull could be achieved [6-14], allowing the use of smaller, 

 rubber-tired machines. Roughly half of the total power requirement is 

 supplied to vibrate the plow, and the other half for running the machine. 

 This approach worked well on experimental plows, and now most major equip- 

 ment manufacturers supply vibratory plow equipment. To date, vibration 

 has not been employed for deep -ocean plows. 



Water Jetting 



Water jetting is used mainly for burying offshore oil pipelines. 

 The jetting machine straddles the pipeline and extends into the seafloor 



10 



