Page 44 - Trenchless Technology Piping Installation and Inspection

18 Cha pte r O n e

cutterhead. The first stage of installation consists of drilling a small-
diameter pilot hole along the desired centerline of a proposed profile.
The subsequent stages of installation consist of enlarging the pilot
hole to the desired diameter to accommodate the product pipe and
the eventual pull of the product pipe through the enlarged borehole.
To avoid heaving of the surface and reduce possibility of damaging
adjacent utilities, the reaming or enlargement process should be
achieved by pulling only and not by pushing the reamer. This can be
achieved by adding drill rods from the product pipe side and con-
tinuing reaming operation by pulling a slightly bigger-size reamer
(step-up) until the desired borehole size is achieved.
As the name horizontal directional drilling implies, this method
has a unique capability to track the location of the cutterhead and
steer it during the drilling process. The result is a greater capability in
placing the utilities in difficult underground conditions. The direc-
tional drilling methods can be classified into three broad categories of
small (Mini-HDD), medium (Midi-HDD), and large (Maxi-HDD).
Table 1.3 presents comparison and main features of HDD methods.
Drilling Procedures Trailer-mounted drill rig is brought to one side of
the obstacle (river, lake, road, and so on). Sections of the product pipe
to be used for crossing, and other ancillary equipment, are brought to
the opposite side of the obstacle. Drilling and pipe installation are
done in two or three steps. In the first step, a small pilot hole of 2 to
6 in. diameter is drilled along the desired path (i.e., a near-horizontal
curved path beneath the obstacle to be crossed) of the pipeline. As
drilling proceeds, segments of the drill pipe are added to form the
pilot string. Through the pilot string, drilling fluids (also called drill-
ing mud, usually a bentonite and/or polymer slurry) is pumped
through the nozzles in the drill bit to lubricate the drill, and to carry
the cuttings (spoil) back to the rig side. Step 1 ends when the drill bit
has emerged from the ground of the pipe side.
In step 2, called prereaming, the drill bit is removed from the
product pipe side, and a reamer assembly is attached to the pilot
string to enlarge the borehole. Prior to pulling in large pipes, it is
often desirable to pull the larger reamer through the borehole with-
out the pipe attached to swab the borehole. By reversing the direc-
tion of the rotation of the pilot string, the rig is now used to pull
the reamer into the pilot hole. Segments of the drill pipe are
added on the pipe side to the pilot string as they are being pulled
back. This step is specifically required to prevent the contractor from
pushing the reamer through the borehole, which may cause heaving
of ground surface and damage to nearby utilities. Step 2 ends when
the reamer covers the entire path, and starts to emerge from ground
on the rig side. Several passes of step 2 may be needed for large-
diameter pipes which require large boreholes that cannot be created
in a single pass.