Page 255 - Trenchless Technology Piping Installation and Inspection

Pr oject Considerations for Horizontal Dir ectional Drilling 219

relationship illustrates the basic phenomenon for the idealized case
of a weightless, flexible pipe:

F = F · e υ θ (5.13)
2 1
where F represents axial tension at the entry point of a bend of mag-
1
nitude θ (radians), υ is the local coefficient of friction between the
product pipe and borehole wall surface, and F is the required axial
2
tension at the exit point of the bend. In practice, the impact of the actual
weight of the pipe may be reflected in the preceding tension, F .
1
Pipe Collapse
ASTM F1962 provides the critical (buckling) pressure, P , as given by
cr
Eq. 5.14
P = 2 E· f · f /{(1 – μ ) · (DR – 1) } (5.14)
3
2
cr o R
where E = material modulus of elasticity
μ = Poisson’s ratio
f = ovality compensation (reduction) factor
o
f = tensile stress reduction factor
R
The dimension ratio, DR, refers to the ratio of the pipe outer diameter
to its (minimum) wall thickness. In a discussion of ASTM F1962,
Petroff (2006) explains the significance of these terms. The material
properties, E and μ, for the viscoelastic HDPE pipe depend upon the
load duration, f accounts for initial or subsequent out-of-roundness,
o
and f recognizes a potential reduction in collapse strength in the
R
presence of significant tensile loads during the installation phase.
Petroff (2006) also provides an explanation of the possible sources
and nature of the pressure loads on the pipe, including that due to
hydrostatic pressure associated with drilling fluid or groundwater
pressure, and asymmetric earth pressure that cause ring deformation,
as well as the implications of their time dependent characteristics. In
general, the detailed consideration of the interaction of the various
phenomena, and the consequences for the product pipe, is relatively
complex and not within the scope of this book.

5.5.2 Simplifications for Mini-HDD Applications
The detailed application of Eq. (5.12) to determine the required ten-
sile load on the pipe during the installation phase, and Eq. (5.14) to
evaluate the possibility of pipe collapse during installation or the
post-installation (preoperational) phase, would be tedious for typical
Mini-HDD applications. On the other hand, application to potentially
problematic installations would be desirable, although not necessar-
ily feasible, for most Mini-HDD personnel, in spite of their being
otherwise well-trained in this technology. Thus, the reduction of