For soil conditions from 5Kpa to 100kpa, the optimal method of burying a cable is to jet water into the sea bottom to “cut” a trench for the cable to settle into. Because the sea floor can be studied from the pre-project surveys, this process should be pre-planned to utilize the best jetting swords and nozzles for the soil conditions. Fluid modeling should be analyzed to determine optimal “jetter” design and angle of attack. Small changes in jetter design can have huge impact on trenching speeds, with even greater impact on project duration and vessel costs. A small investment up front can means big savings on the back end.
Mechanical cutting (as discussed above) with a chain or wheel is another method utilized when the sea bottom conditions are too hard for jetting. However, this mechanical cutting has other intrinsic problems ranging from clogging, fouling and maintenance down time, to a higher risk of accidental damage to the product cable itself.
The ability to control the jetting force is very important. Some jetting systems are either “on” or “off,” and in the case of jetter power, bigger isn’t always better. Too much power can result in cable trenches that are too wide and therefore self-defeating. If the resulting trench ends up being simply a large ditch, the product cable can still be exposed to anchors, fishing nets and other hazards.
One method that has proven effective in gaining control over the jetting force is to employ a variable speed motor that drives the water pumps supplying the jetting force. Varying the driver (and therefore the jetting force) allows the operators to “fine-tune” the process to compensate for variances in soil conditions.