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Maxi-HDD Pull Loads in Non-Level Grade for Polyethylene Pipe Including
Ballast
Dr. Lawrence M. Slavin1, Dr. Mohammad Najafi, P.E.2, Eric R. Skonberg, P.E.3
1
Outside Plant Consulting Services, Inc., 15 Lenape Avenue, Rockaway, NJ, 078661019, PH (973) 983-0813; FAX (973) 983-0813; email lslavin@ieee.org
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2
The University of Texas at Arlington, 428 Nedderman Hall, Arlington, TX 760190308, PH (817) 272-0507; FAX (817) 272-2630, email najafi@uta.edu
3
Trenchless Engineering Corp., 15015 Inverrary Drive, Houston, TX 77095, PH
(713) 303 3319, e-mail skonberg@trenchlessengineering.com
Abstract
ASTM F 1962, Standard Guide for Use of Maxi-Horizontal Directional Drilling for
Placement of Polyethylene Pipe or Conduit Under Obstacles, Including River
Crossings, provides a procedure for estimating pull loads and stresses on
polyethylene pipe as a function of the drill path and buoyant weight of the pipe in the
drill hole. The original equations were developed assuming a level grade, or that the
HDD entry and exit elevations are the same. A previous paper has therefore extended
the equations and methodology of ASTM F 1962 to address the installation of
polyethylene pipe for the case of a non-level grade, but restricted the analysis to the
installation of pipe in the absence of anti-buoyancy measures. The present paper
further extends the analysis to include the use of ballast for a non-level grade and
helps provide a better understanding of the factors influencing the pull loads in the
maxi-HDD installation of pipelines.
1. Introduction
ASTM F 1962, Standard Guide for Use of Maxi-Horizontal Directional Drilling for
Placement of Polyethylene Pipe or Conduit Under Obstacles, Including River
Crossings, provides overall guidelines for a maxi-horizontal directional drilling
(maxi-HDD) operation. This document addresses preliminary site investigations,
safety and environmental considerations, regulations, damage prevention, bore path
design, project implementation, inspection and site cleanup. One of the more
significant contributions of ASTM F 1962 is the provision of a rational, analytical
method for selecting the polyethylene pipe strength requirements based upon the
estimated installation loads on the polyethylene (PE) pipe. The original equations in
this document for determining required pull loads were developed assuming a level
grade, or that the HDD entry and exit elevations are the same. In actual installations,
however, there may be a finite grade between the entry and exit points of the drilling
operation. A previous paper has therefore extended the equations and methodology
of ASTM F 1962 to address the installation of polyethylene pipe for the case of a
non-level grade, but restricted the analysis to the installation of pipe in the absence of
anti-buoyancy measures (Slavin etal, 2011). However, the use of ballast, typically
water introduced into the interior, is a common practice employed to significantly
reduce the buoyant weight and correspondingly increase possible placement
distances. The present paper extends the previous analysis to include the use of
internal water ballast for a non-level grade.
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802
2. Level Grade
Figure 1 illustrates a typical geometry for a maxi-HDD operation, corresponding to a
river crossing, similar to that shown in ASTM F 1962. The geometry specifically
shows a level grade with respect to the pipe entry and exit points, although the
theoretical model presented would also be valid for variations in elevation between
these two points located at the same grade. The horizontal projection of the pipe path
comprises four segments, including those spanning the pipe entry to exit point (L2,
L3, L4) and the additional length L1. The quantity L1 allows for handling at both ends
and possible other effects (path curvature, thermal contraction, stretching, etc.). The
projected bore length, Lbore, is given by Equation 1.
Lbore
= L2 + L3 + L4
[1]
Figure 1 Typical maxi-HDD route (river crossing)
(Source: Outside Plant Consulting Services, Inc.)
The entry segment (A – B) and exit segment (C – D) may each be of uniform
curvature, but not necessarily. The intermediate horizontal segment, L3, may be of
zero length depending on project geometry. The term H represents the depth of the
installation relative to the elevation at the pipe entry and exit points.
Using the terminology presented in Figure 1 and Equation 1, ASTM F 1962 provides
a set of relations to predict the required pull force -- TA, TB, TC, and TD -corresponding to the leading end of the pipe reaching point A, B, C and D, as
presented in Equations 2a through 2d.
TA
TB
TC
TD
=
=
=
=
eνa α · νa · wa · (L1 + L2 + L3 + L4)
eνb α · (TA + νb · |wb| · L2 + wb · H - νa · wa · L2 · eνa α )
TB + νb · |wb| · L3 - eνb α · (νa · wa · L3 · eνa α )
eνb β · (TC + νb · |wb| · L4 - wb · H - eνb α · [νa · wa · L4 · eνa α] )
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[2a]
[2b]
[2c]
[2d]
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803
where wa represents the empty aboveground weight (downward positive) of the pipe
and wb denotes the net buoyant weight (upward positive) of the pipe as submerged in
slurry belowground; νa and νb. are the corresponding aboveground and belowground
Coulomb “coefficients of friction”. The buoyant weight may reflect the use of antibuoyancy techniques, including the use of liquid ballast inside the pipe. The pipe
entry angle α and exit angle β are expressed in radians, where one radian equals 180º
/ π. For a bore path of approximately uniform curvature from the entry and exit
points to the horizontal segment (B – C), the lengths L2 and L4 may be estimated as
the following:
L2 = 2H / α, and
L4 = 2H / β
[3a]
[3b]
In addition to the calculated loads as given by Equations 2a – 2d, an incremental
tensile force, ΔT, must be added to account for the drag effect of the drilling
fluid/slurry (“fluidic drag”), which is determined from the magnitude of the
“hydrokinetic pressure”, ΔP:
ΔT = ΔP · (π/8) · (Dhole2 – D2)
[4]
where Dhole is the diameter of the borehole and D is the outer diameter of the PE pipe.
ΔP is the incremental drilling fluid pressure in the borehole at the leading end of the
pipe during the pullback operation, which is in addition to the hydrostatic pressure
corresponding to the head (depth) of relatively dense slurry. The incremental tension,
ΔT, is properly added to the local tension TA , TB , TC , or TD as specified in
Equations 2a – 2d, for each of the four points, but is not cumulative; e.g., the value of
TA inserted into Equation 1b is that given by Equation 2a, as written, and not TA +
ΔT.
3. Non-Level Grade
If the pipe entry and exit points, A and D, respectively, are at different elevations, the
pulling load will be affected by gravity and buoyancy. Figure 2 illustrates such a
configuration, for an upward grade or installation. In this case, the pipe entry and exit
angles, α and β, respectively, are still considered to be measured relative to a
horizontal plane. In general, there may be a local finite grade, corresponding to the
angle φ, at the pipe entry point A, considered positive (+) as shown, which is not
necessarily equal to the average grade between points A and D given by:
Average % Grade = 100 x (H2 – H1) / Lbore
[5]
where H1 and H2 represent the depth of the horizontal segment (B – C), below point
A and point D, respectively. The surface grade at intermediate points along the route
need not be uniform, and can vary from that corresponding to Equation 5.
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804
Figure 2 Maxi-HDD route for upward grade
(Source: Outside Plant Consulting Services, Inc.)
The individual elevations, H1 and H2, must be introduced into the equations in place
of the original depth, H. The slope of the external (non-submerged) portion of pipe
protruding from the borehole, at the pipe entry point, represents an additional load on
the pipe as it is pulled upgrade, at the local grade angle, φ (radians). Furthermore, the
angle φ effectively changes the pipe entry curvature at point A, influencing the
magnitude of the “capstan effect”.
For pipe entry and exit paths of approximately uniform curvature, the lengths L2 and
L4 may be estimated as:
L2 = 2H1 / α, and
L4 = 2H2 / β
[6a]
[6b]
In general, it may be assumed that the slurry will drain from the upper elevations in
the borehole, such as that above point A in Figure 2. Indeed, it is often necessary in
such cases to subsequently fill the corresponding void at the upper portion with grout
minimize possible surface subsidence. Thus, the continuously created slurry would
typically drain from the higher elevation segment D* – D, to result a temporary void
of slurry in this section. The slurry would only remain in bore path segments A – B,
B – C, and C – D*. The point D* is located at distance L4* (see Figure 2) from the
point C, which, for a pipe exit path of approximately uniform curvature, may be
estimated as:
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L4* = L4 · {1 – [(H2 - H1) / H2] }½
805
[7]
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However, it is also of interest to consider the case for which it is assumed that the
flow of the slurry may be somewhat restricted, possibly due to a local (minor)
blockage, allowing the borehole to remain full during the pullback operation. This
situation reveals some interesting phenomena, and is therefore of academic, as well as
possible practical, interest, depending on the assumptions regarding the conditions in
the borehole.
For an upward grade, there is the additional consideration corresponding to the partial
drainage of the internal liquid (water) ballast within the pipe, which depends on the
magnitude of the differential height (H2 - H1) in relation to the height of a column of
liquid (water) ballast, h, which can be supported by atmospheric pressure; i.e.,
approximately 34 ft of water. Thus, the horizontal portion of the segment Dh – D,
drained of internal ballast, is determined by the distance L4 - Lh, with Lh indicated in
the callout in Figure 2 and given by:
Lh = L4 · (1 – {(H2 - H1) – MIN [(H2 - H1), h]} / H2)½
[8]
In this case, the generalized governing equations for an upward grade, considering all
of the above effects, become:
TA
TB
TC
TD
=
=
=
=
eνa (α + φ) · (νa + φ) · wa · (L1 + L2 + L3 + L4)
eνb α · (TA + νb · |wb| · L2 + wb · H1 - (νa + φ) · wa · L2 · eνa (α + φ) )
TB + νb · |wb| · L3 - eνb α · [(νa + φ) · wa · L3 · eνa (α + φ)]
eνb β · (TC + νb · |wb| · L4* + νb · |w*| · (Lh - L4*)
+ νb · |wh| · (L4 - Lh) - wb · H1 - w* · MIN [(H2 - H1), h]
- wh · {MAX [(H2 - H1), h] – h}
- eνb α · [(νa + φ) · wa · L4 · eνa (α + φ)])
[9a]
[9b]
[9c]
[9d]
where the various “weight” terms in Equation 9d depend on the assumed borehole
conditions, as specified in Table 1. The additional terms introduced into Equations
9a – 9d have been highlighted to emphasize the difference relative to the original
basic Equations 2a – 2d. As before, the incremental tension, ΔT, must be added to
each of these terms to account for the fluidic drag component.
Table 1 Effective Weight of Pipe in Borehole (Note 1)
effective
weight
borehole “not filled”
borehole “filled”
(slurry drains freely)
(slurry does not drain)
-wa1
wb1 = wb
w*
(submerged, with any internal
(unsubmerged, with any internal
ballast)
ballast)
wb1
-wa
wh
(unsubmerged, without internal
(submerged, without internal
ballast)
ballast)
Note 1: Net submerged “buoyant” weight, wb1, in borehole, with or without ballast, is
considered to be positive if acting in upward direction (opposite gravity);
1
unsubmerged weights wa or wa, with or without internal water ballast, are
considered to be positive if acting in downward direction (same as gravity).
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It is recalled that wb is the net buoyant weight of the pipe (upward positive), with or
without internal water ballast, where submerged in slurry, and wa is the weight of the
empty pipe (downward positive), whether aboveground or in a portion of the
borehole void of slurry. The newly introduced term in Table 1, wa1, is defined as the
unsubmerged weight of the pipe (downward positive), containing water (ballast), in a
portion of the borehole drained of slurry.
Using similar principles, corresponding equations may be developed for a downward
installation. In this case, there are no terms associated with the column of liquid
ballast that may be supported by atmospheric pressure.
4. Results and Discussion
The main source of drag during the maxi-HDD operation typically relates to the
frictional drag associated with the relatively high buoyant weight wb of the otherwise
light-weight PE pipe, which also tends to pull the pipe upwards, opposite gravity.
Nonetheless, it is sometimes believed that the pull loads would be reduced if the pipe
exit point were at a lower elevation than the entry point, in order to take advantage of
gravity tending to help pull the pipe down the grade. In general, however, the correct
conclusion regarding pull loads depends on the possible deployment of anti-buoyancy
techniques (ballast), as well as assumptions regarding the draining of the slurry from
the higher portions of the borehole.
Initial results for the case in which ballast is not utilized (Figure 3) have been
provided previously, and tentatively indicate:
• There is little or no penalty in pulling upgrade.
• The tensile loads for an uphill pull are less than that of a similar installation at
a level grade.
• Pulling downhill should correspond to a lower tensile load, assuming the
borehole drains freely (“not filled”) to the lower elevation pipe exit point.
• However, in the event of inhibited free draining of the borehole (“filled”), the
tensile load for a downhill installation could exceed that of the uphill
installation by a significantly greater margin, and also exceed that of a similar
installation at a level grade.
These results suggest that the minimum risk, with respect to required tensile loads,
may correspond to pulling uphill for applications in which anti-buoyancy techniques
are not employed. However, for the present case for which anti-buoyancy techniques
are employed, the phenomena associated with the internal (water) ballast may be
anticipated to result in significantly different conclusions and guidelines. Therefore,
similar to previous investigations, it is helpful to perform calculations based upon the
above equations for specific installations, such as the nominal pipe and route
characteristics indicated in Table 2. This corresponds to an average maximum depth
of 45 ft, but a non-level grade. For example, an average nominal grade of ±2.5%
corresponds to depths H1 and H2 equal to 13.75 ft or 76.25 ft, depending on whether the
grade is uphill or downhill. (The results for the pull load for the nominal case, at a
2.5% magnitude slope, are indicated in Figures 3 and 4.) A maximum average grade
of approximately 3.5% is limited by the geometry of Figure 2, and the indicated
average depth (45 ft). In general, it is recognized that the overall quantitative results
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depend on the specific geometry and parameters considered. It is assumed, however,
that useful qualitative information and conclusions may be judiciously extracted from
such results.
Figure 3 Pull Load (TD + ΔT) as function of average grade -- no ballast used
(Source: ASCE Journal of Pipelines Systems Engineering and Practice; Slavin etal, 2011)
In Table 2, the term DR is the ratio of the pipe outer diameter to wall thickness, and
γa , γb and γc represent the specific gravity (weight density) of the HDPE pipe
material, slurry and ballast (e.g., water), in that order. These quantities determine the
corresponding effective weight of the pipe (wa, wa1, wb). The nominal value for the
specific gravity of the slurry γb is assumed to be 1.2, similar to that of the previous
study (Slavin etal, 2011).
For average upward grades less than approximately 2%, for which the differential
elevation H2 - H1 is less than the height h (33.9 ft) for the 2,500 ft route under
consideration, the internal ballast continues to fill the pipe. In this range, as shown in
Figure 4, for the case of inhibited draining of the slurry from the borehole (“filled”),
the pull load is less than that corresponding to a level grade. This decrease is due to a
net beneficial effect of the buoyant weight wb parameter (including as reflected in w*
via Table 1) applied to the elevation-related terms involving H1 and H2 in Equations
9b and 9d, which act to increase or decrease the incremental tension along the portion
A – B or C – D (Figure 2) respectively, as the upward directed buoyancy tends to
inhibit or assist the pipe in increasing or decreasing its depth. Thus, for the
interesting, albeit unlikely, case in which the entire borehole length remains full with
slurry, an uphill bias, for which H2 > H1, would be anticipated to be somewhat more
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beneficial for pulling back the pipe than for a downhill installation, or where both
entry and exit points are at the same elevation. For this scenario, the effective pipe
weight is equal to wb (submerged, including ballast); i.e., 42.1 lbs/ft acting upward
(opposite gravity).
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Table 2 Nominal Installation Parameters
Parameter
Nominal Value
bore length, Lbore
entry angle, α
exit angle, β
H1
H2
average depth, H
average grade
local grade, φ
L1
L2
2,500 ft
15°
15°
13.75 ft or 76.25 ft
76.25 ft or 13.75 ft
45 ft
± 2.5%
± 2.5%
125 ft
105 ft or 582 ft
L2*
L3
L4
L4*
h
Lh
D
DR
Dhole
ΔP
ΔT
γa
γb
γc
wa
wa1
wb, wb1
NA or 247 ft
1,813 ft
582 ft or 105 ft
247 ft or NA
33.9 ft
460.2 ft or NA
24.0 in.
11
36.0 in.
10 psi
2,827 lbs
0.955
1.2
1.0
61.9 lbs/ft
193.1 lbs/ft
173.4 lbs/ft
42.1 lbs/ft
0.1
0.3
νa
νb
Remarks
0.262 radians to horizontal
0.262 radians to horizontal
upward or downward grade
upward or downward grade
= (H1 + H2) / 2
= (H2 – H1) / Lbore
= average grade
= 5% Lbore
uniform curvature
= L2 · {1 – [(H1 – H2) /H1]} ½
= Lbore – L2 – L4
uniform curvature
= L4 · {1 – [(H2 - H1) /H2]} ½
column of water equal to 14.7 psi
= L4 · {1 – [(H2 - H1) - h] / H2}½
pipe outer diameter
2.182-in. wall thickness
50% clearance
incremental pressure
= ΔP · (π/8) · (Dhole2 – D2)
HDPE
realistic
water ballast
unsubmerged pipe weight (empty)
unsubmerged pipe weight (with ballast)
submerged buoyant weight (without ballast)
submerged buoyant weight (with ballast)
rollers
within borehole
For average upward grades greater than approximately 2% (e.g., the nominal 2.5%
grade) and for which the entire borehole length remains full with slurry, the internal
ballast partially recedes and drains from the pipe. This results in a significant
increase in (upward) buoyant weight wh (Table 1) and corresponding frictional drag
along the segment Dh – D, which more than overcomes the beneficial effect of the
increased buoyant lift in this segment. For this segment, the effective pipe weight
(submerged, without local ballast) of 173.4 lbs/ft, acting upward (opposite gravity), is
considerably greater than the weight if ballast was still present (42.1 lbs/ft), resulting
in the subsequent increasing trend (“filled”) with grade, indicated in Figure 4.
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Figure 4 Pull Load (TD + ΔT) as function of average grade – ballast used
For the more typical case in which the slurry will freely drain to the level of point D*
(Figure 2), the results (“not filled”) in Figure 4 indicate a relatively high increase in
required tension due to the upward pull. The increase in pull load is primarily due to
the very high effective weight of 193.1 lbs/ft of the unsubmerged water-filled pipe in
the absence of slurry within the segment D* – Dh (or D* – D for grades < 2%). This
causes significantly higher frictional drag, further aggravated by the accompanying
downward (gravity oriented) force on the pipe as opposed to the otherwise buoyant
lift in this segment. Furthermore, as described above, for an average grade greater
than approximately 2%, the internal ballast also partially recedes and drains from
within the pipe within the segment Dh – D. The frictional drag within this portion is
due to the increased effective weight wh (Table 1) of the unsubmerged empty pipe
weight of 61.9 lbs/ft (in comparison to the buoyant weight of 42.1 lbs/ft of the
submerged pipe if internal ballast was still present, as in a level grade installation).
Although considerably less than the 193.1 lbs/ft of the unsubmerged water-filled
pipe, this effect contributes to an increase in the required pull load.
Thus, relative to a similar installation for a level grade application (i.e., entry and exit
points at same elevation), for the nominal average 2.5% grade indicated in Table 2,
there would likely be a penalty of more than 50% increase in tensile load for the
upward pull (borehole “not filled”), in comparison to the possibility of only a 5%
decrease in the event the borehole does not drain freely (“filled”). The net results for
a downhill installation are also shown in Figure 4, for which the pipe will remain
filled with ballast for all magnitude grades, since the leading (pulling) end of the pipe
is assumed closed.
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For the likely case of a freely draining borehole, the decreasing trend of the pull load
with grade magnitude when not deploying ballast (Figure 3), in comparison to the
generally increasing trend of the pull load with magnitude grade when utilizing
ballast (Figure 4), suggests that, for applications characterized by a relatively large
average grade, it may be advisable to forego the use of the ballast, depending on the
bore path details (Slavin etal, 2011a).
5. Summary
The present analysis extends the model originally used in the development of ASTM
F 1962 to reflect a finite net grade or difference in elevation between the entry and
exit points of the drilling operation, including the possible deployment of antibuoyancy measures (i.e., water ballast added to pipe interior). This analysis therefore
supplements a previous analysis addressing the installation of pipe in the absence of
anti-buoyancy measures. In both cases, the effect on the pull load depends upon the
degree to which the cavity is free to drain of slurry to the lowest access (entry or exit)
point during the pullback operation.
In contrast to the previous investigation (no ballast) which indicated that it may be
more desirable (i.e., less risky) to pull upgrade, the present results (water ballast),
based on a sample geometry and application, show that a downgrade installation is
preferable. Furthermore, in the latter case, the tensile loads for an uphill or downhill
installation are typically greater than that of a similar installation at a level grade.
However, for applications characterized by a relatively large average grade, it may be
advisable to forego the use of the ballast, depending on the bore path details.
In general, it is recognized that specific quantitative results depend on the details and
parametric values considered. Thus, for a particular application of interest, the
formulas for the generic geometry as provided herein, or those for a similarly
considered alternate geometry (Slavin etal, 2011a), may be used, as appropriate, to
estimate the required pull force for various installation parameters and conditions.
For other generic geometries similar principles may be used to develop corresponding
formulas, or other available HDD planning and design tools may be applied to
investigate the issues addressed in this investigation. It is also noted that the
recommendations and considerations regarding the direction of pulling are based
entirely on anticipated tensile loads, independent of the potential advantages or
disadvantages with respect to facilitating the removal of spoils during the initial
boring or back-reaming operations. These issues have not been not directly
considered in the investigations.
References
Slavin, L.M., Najafi, M., and Skonberg. E.R. (2011). “Maxi-HDD Pull Loads for
Polyethylene Pipe for Non-Level Grade”, accepted for publication, ASCE Journal of
Pipeline Systems Engineering and Practice, 2011.
Slavin, L.M. and Najafi, M., (2011a). “Maxi-HDD Pull Loads for Entry and Exit
Points at Different Elevations”, accepted for publication, Proc., International
Conference on Pipelines and Trenchless Technology 2011, Beijing, China.
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