Page 234 - Trenchless Technology Piping Installation and Inspection

198 Cha pte r F i v e

3. External pressure: The pipe is subjected to external pressure
from drilling fluid in the annulus, surrounding the pipe, due
to the hydrostatic pressure head, as well as the pressure incre-
ment due to the drilling fluid introduced at the reamer. This
pressure may lead to instability or collapse of the pipe during
the installation stage, possibly aggravated by the simultane-
ous tensile stresses acting on the pipe. It is also possible that
collapse may occur following installation, prior to the opera-
tional stage and corresponding internal pressurization. The
drilling fluid/slurry is typically assumed to solidify over a
period of weeks or months, prior to which the relatively dense
slurry continues to apply external hydrostatic pressure to the
pipe. Polyethylene pipe, a commonly used pipeline material
for HDD installations, is characterized by reduced collapse
strength over extended load duration, and this phenomenon
must be considered during this preoperational stage.
In order to reduce the required tensile force, as well as the effec-
tive external hydrostatic pressure on the pipe, it is a common practice
for Maxi-HDD installations to add ballast fluid (water or drilling
fluid) to the interior of the pipe, particularly for plastic products.
The ballast fluid will significantly reduce the buoyant weight and
corresponding frictional drag forces, as well as provide internal
pressure to eliminate or reduce likelihood of collapse during the
preoperational stage.
Operating (Service) Loads
With one exception, the operating loads and stresses in a pipeline
installed by HDD are not materially different from those experienced
by pipelines installed by open-cut techniques; therefore, past procedures
for calculating and limiting stresses can be applied. One exception
relates to elastic bending. A pipeline installed by HDD will contain
elastic bends corresponding to the route curvature. Bending stresses
imposed by the HDD installation method should be checked in com-
bination with other longitudinal and hoop stresses to ensure that
acceptable limits are not exceeded. The operating loads imposed on a
pipeline installed by HDD include:
• Internal pressure: With the exception of cable conduit, or grav-
ity sewers, the pipeline is deliberately pressurized to transport
the fluid through the interior. The pressure causes circumfer-
ential tensile stresses, as well as possible longitudinally
induced tensile stresses due to Poisson’s ratio effects.
• Elastic bending: The pipeline is subjected to elastic bending
resulting from the pipe conforming to the shape of the drilled
borehole, similar to that of the installation stage described
above.