Page 233 - Trenchless Technology Piping Installation and Inspection

Pr oject Considerations for Horizontal Dir ectional Drilling 197

stage should be reviewed and analyzed in addition to the subsequent
operating stresses to ensure that acceptable limits are not exceeded.
ASCE 108, and ASTM F1962, are useful references for such purposes
(ASCE, 2005; ASTM, 2005). Such procedures would be further refined
by competent engineering expertise, including an analysis of pipe
and soil characteristics and interactions, often including the use of
relatively sophisticated software tools.
A brief discussion of the installation and operating loads which
affect an HDD installation is presented in the following section and
Sec. 5.5.1.

Installation Loads
During installation, a pipeline installed by HDD is subjected to the
following loads as it is pulled into a properly prereamed borehole.
1. Tension: The product pipe is subjected to tension as it is pulled
through the borehole by the drill rig. This tensile load is
required to compensate for:

• Frictional drag due to contact between the outer surface of
the pipe and the wall of the borehole.
• Fluidic drag on the pipe due to the drilling fluid in the
borehole.
• Effective (submerged) weight of the pipe as it is pulled
through elevation changes in the borehole.
The frictional drag is due to a combination of effects leading
to normal (perpendicular) pressure between the pipe and the
borehole walls, including the submerged (buoyant) weight of
the pipe as it rests (or is pushed) against the borehole surface
in the relatively dense drilling fluid, lateral bending forces
imposed on a relatively stiff product pipeline (e.g., steel) at
bends along the route, and increased bearing pressure at
route bends due to local tension in the pipe. Fluidic drag is
due to the drilling fluid flow relative to the pipe surface.
Depending upon the position of the pipe along the submerged
path, the longitudinal component of the buoyant weight of
the pipe locally increases or decreases the required tension.
2. Bending: The pipe is subjected to bending as it is forced to
negotiate the curves in the borehole. Local bending stresses
are proportional to the ratio of the pipe diameter to the radius
of curvature. These stresses must be added to those due to the
pulling tension to determine the peak tensile stress along the
installed path. In general, the bending stresses will be more
significant for a pipe with significant stiffness (e.g., steel pipe)
than a relatively flexible product (e.g., polyethylene), depend-
ing on the pipe diameter.