Once the pilot hole is successfully drilled, the hole is often enlarged to a suitable diameter for the product pipeline. For instance, if the pipeline to be installed is 8 inches in diameter, the hole may be enlarged to 12 inches or more. This is accomplished by reaming the hole to successively larger diameters. Generally the reamer is attached to the drill string on the bank opposite the drilling rig, rotated, and pulled (pushed in some instances) back through the pilot hole. Joints of drill pipe are added as the reamer makes its way back to the drilling rig. Large quantities of slurry are pumped into the hole to maintain its integrity and to flush out cuttings. While soil conditions do have an impact, the required number of reaming runs is mainly dependent on the diameter of the product pipe and the diameter of the pilot hole. It may vary from no reaming runs to several for large-diameter product pipes.
One method, typically called a continuous bore hole, involves enlarging the pilot hole to the desired diameter while simultaneously pulling back the line product behind the reamer. In some situations with small-diameter product pipe or conduit, the pipe can be pulled straight into the pilot hole after the drill is completed. However, in most HDD operations the bore hole has to be reamed to enlarge the hole to accommodate pulling in the product pipe. Generally the bore hole is reamed to 1V2 times the outside diameter of the product pipe. This provides an annular void between the product pipe and the drill hole for the drilling fluids and spoils and for the bending radius of the product pipe.
Sometimes it is necessary to ream the bore hole without pulling back the pipe. After the drill hole is reamed, the product pipe or conduit is pulled back through the hole filled with the drilling fluids. It is best to fabricate the product pipe on the exit side in one section so it can be tested and pulled in one continuous pullback.
The drill pipe is connected to the product pipe or conduit using a pull head or swivel. The swivel is used to prevent rotational torque from spinning the product pipe. A reamer is placed between the pull head and the drill string to keep the drill hole open. A schematic of the HDD reaming process is shown in Figure 1-6.
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