Page 225 - Trenchless Technology Piping Installation and Inspection

Pr oject Considerations for Horizontal Dir ectional Drilling 189

nonrotating aspect of the drill string allows the steering bias to be
held in a specific position while drilling. If a change in direction is
required, the drill string is rotated such that the direction of bias
agrees with the desired change. Leading edge asymmetry is typically
accomplished with a bent sub or bent motor housing located several
feet behind the bit (see Fig. 5.2). A straight path is maintained by
applying simultaneous thrust and torque (rotation) to the drill head.
In soft soils, drilling progress is typically achieved by hydraulic cut-
ting with a jet nozzle. If hard areas are encountered, the drill string may
be rotated to drill without directional control until the hard spot has
been penetrated. Mechanical cutting action required for harder soils or
rock may be provided by a positive displacement mud motor, which
converts hydraulic energy from drilling fluid to mechanical energy at
the drill bit. This allows for bit rotation without drill string rotation.
The actual path of the pilot borehole is monitored during drilling
using a steering tool positioned near the bit as shown in Fig. 5.5. The
steering tool provides continuous readings of the inclination and
azimuth at the leading edge of the drill string. These readings, in con-
junction with measurements of the distance drilled, are used to calcu-
late the horizontal and vertical coordinates of the steering tool rela-
tive to the initial entry point on the surface. When the drill bit
penetrates the surface at the exit point opposite the horizontal drill-
ing rig, the pilot borehole is complete.

Preream Phase
Enlarging the initial pilot borehole is typically accomplished using
a single backreaming operation, simultaneous with pullback of the
product pipe, or––for larger pipes and/or harder soils––by means
of several prereaming passes prior to product pipe installation.
Reaming tools consist of a circular array of cutters and drilling fluid
jets, and are often custom made by contractors for a particular bore-
hole size or type of soil (see Fig. 5.6). For a prereaming pass, a reamer
attached to the drill string at the exit point is rotated and drawn back
toward the drilling rig, thus enlarging the pilot borehole. Drill rod is
added behind the reamer as it progresses toward the drill rig to ensure
that a string of pipe is always maintained in the borehole. It is also
possible to ream in the opposite direction, away from the drill rig, in
which case a reamer fitted onto the drill string at the rig is rotated and
thrust forward, however, in certain soil and project conditions this
may cause ground disturbance and surface heave.

Pullback Phase
Pipe installation is accomplished by attaching the prefabricated prod-
uct pipeline section behind a reaming assembly at the bore exit point
and pulling the reaming assembly and pipeline section back toward
the drilling rig. This is undertaken after completion of the preream-
ing phase, or, for smaller diameter lines in relatively soft soils, directly