Underground Infrastructure - March 2023 - 26

Directional Drilling
combination with the corresponding frictional characteristics,
is the major factor in determining the required pull force.
ASTM F 1962 provides formulae for determining the
buoyant weight under various conditions. Th is weight is a
function of the drilling fl uid/slurry's density, for which a conservatively
high value is suggested for design purposes.
Capstan eff ect at bends
Although pipe stiff ness eff ects may generally be ignored
FIGURE 5. Example of " Capstan Effect " (Courtesy of Outside Plant
Consulting Services, Inc.)
first be allowed to reach mechanical and thermal equilibrium,
to avoid shrink back onto the bore hole. The exposed
leading end of the pipe should be inspected for possible damage
and a pressure or leakage test may be required, for fluid
transport applications.
Appendices
Supporting the 10 main sections, several appendices
provide the physical properties of selected PE material, as
well as a means of determining the post-installation loads
and pipe deflection. In particular, Table X1.1 provides the
modulus of elasticity and safe pull tensile stress for HDPE
(PE4710) and MDPE (PE2406) materials, as a function of
load duration. Table X3.1 provides the critical collapse pressure
for HDPE/PE4710 pipes as a function of load duration
and wall thickness.
Theoretical basis for load estimation
Th e theoretical model used to develop the formulae for estimating
the peak required tension (Section 8) assumes that
the local frictional drag forces on the pipe are proportional to
the local normal bearing forces applied at the pipe surface.
For fl exible PE pipe, with minimal bending stiff ness, the
considered bearing forces are those due to the:
* Dead (empty) weight of the pipe above ground
* Buoyant weight of the submerged pipe, possibly reduced
by use of ballast)
* Bearing forces resulting from (previously induced)
tension tending to pull the pipe snugly against any
curved surfaces ( " capstan eff ect " ).
In addition, the drilling fl uid/slurry fl owing along the
length of the pipe also contributes, but is relatively low, based
on the present model.
Frictional drag due to weight, buoyancy
In the absence of anti-buoyancy techniques, such as internal
water ballast, the frictional drag developed within the
borehole is generally much greater than that developed outside,
because of the high buoyant weight for an empty PE pipe.
For such cases, the buoyant weight of the submerged pipe, in
26 MARCH 2023 | UndergroundInfrastructure.com
for fl exible PE pipe, there is a potentially important eff ect can
be due to route bends or any path curvature of signifi cance.
Tensions induced in the PE pipe, as it passes any curve, become
amplifi ed because the tensile forces tend to pull the pipe
against the curved surface.
Such eff ects are independent of the pipe stiff ness, pipe diameter,
borehole clearance, radius of curvature or direction
of curvature, and, in some cases become a major consideration
due to their compounding eff ect. Th is phenomenon is
referred to as the " capstan eff ect, " as it is the principle of the
capstan winch, as illustrated in FIGURE 5.
For the geometry shown in FIGURE 2, with relatively shallow
entry and exit angles, the associated load amplifi cation
due to this eff ect is not major; although for more complex
paths, the eff ect could be very important. Mini-HDD applications,
for instance, tend to contain bore paths with additional
curvature because of the need to avoid known obstacles or
follow a curved right-of-way, as well as more subtle curvature
due to path corrections characteristic or typically less precisely
controlled installations.
Hydrokinetic surface drag (fl uidic drag)
Th e eff ect of shear forces directly imparted on the surface
of the pipe by the drilling fl uid ( " fl uidic drag " ) has been handled
in a widely disparate manner within the industry and is
sometimes a major consideration. In contrast, the convenient
model employed in ASTM F1962 results in a very low magnitude
eff ect, which is directly added to the estimated pulling
forces, due to frictional drag, including the capstan eff ect.
Summary
Th e recent (2022) edition of ASTM F1962 has revised the
relevant physical characteristics of the PE materials, providing
the physical properties for the most recent high density
polyethylene (HDPE) material, PE4710. Th is material is signifi
cantly tougher than older, previously used PE products, facilitating
successful completion of more complicated, diffi cult
maxi-HDD projects. Th e physical properties of the PE4710
material allow greater pulling forces and also provide greater
resistance to collapse. UI
ABOUT THE AUTHOR: Lawrence Slavin, Ph.D., provides outside
plant consulting services as a member of IEEE, the world's largest
technical professional organization dedicated to advancing technology
for the benefi t of humanity. He can be reached at lslavin@ieee.org.
http://www.UndergroundInfrastructure.com
Underground Infrastructure - March 2023

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