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Emerging Technologies: A Suggested Design Method for Curved,
Jacked Steel Pipe
J.L. Robison, P.E.1, R.D. Hotz, II, P.E.2 and C.C. Chen,
Ph.D.3
1GeoEngineers, Inc., 3050 South Delaware Avenue, Springfield, MO
65804; PH (417) 831-9700; FAX (417) 831-9777; email:
[email protected] 2GeoEngineers, Inc., 3050 South Delaware
Avenue, Springfield, MO 65804; PH (417) 831-9700; FAX (417)
831-9777; email: [email protected] 3Missouri University of
Science and Technology, 117A Kemper Hall, 901 S. National Ave.,
Springfield, MO 65897; PH (417) 836-4912; FAX (417) 836-6260;
email: [email protected]
ABSTRACT
Proven technologies, such as straight-line, conventional
microtunneling, or curved solutions like horizontal directional
drilling (HDD) have served the pipeline industry well but have
their limitations. Less well-known (especially in the United
States) solutions such as Direct Pipe (DP) and vertical-curved
Directional Microtunnelling (DMT) are beginning to find acceptance
and application.
Existing microtunnelling and HDD engineering design methods do
not address the specific issues involved with the estimation of
jacking forces or the specific stress analyses of a curved steel
pipe loaded in compression. Building on conventional
microtunnelling theory and API Recommended Practices, the authors
developed a design method for estimating the anticipated loads
generated during a curved steel pipe drive and for assessing the
steel pipe axial, bending, and hoop stresses along with buckling
and combined stress conditions. The design method includes
calculations for estimating jacking loads and for calculating a
maximum allowable (not to exceed) axial loading for a given
geometry and pipe specifications.
This paper will introduce the DP and DMT methods, detail the
authors suggested design methodology and give example applications
of completed DP and DMT designs slated for 2013 construction in the
United States.
INTRODUCTION
The purpose of this paper is to discuss potential applications
and provide a suggested design method for curved, jacked steel
pipeline. The design method presented herein has been applied by
the authors to directional microtunnel (DMT) and Direct Pipe (DP)
designs for high-strength steel, natural gas-carrying pipelines. DP
is a trademarked process with specific patented equipment developed
by Herrenknecht AG (Herrenknecht); more than 18 DP crossings have
been completed worldwide to date, primarily in Europe. The design
method presented below uses existing,
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Figure 2. Pipe Thruster and String
established calculation procedures along with conventional steel
design code to enable the trenchless design engineer to make
evaluations on the suitability of pipe size, wall thickness,
strengths, drive length, and other parameters for a given
trenchless crossing geometry and geology. This paper does not
discuss the geotechnical explorations required for trenchless
crossings; there are several good sources on this topic. The reader
is assumed to have some familiarity with geotechnical and
structural engineering principles and the process of design of a
trenchless crossing such as horizontal directional drilling (HDD)
and/or microtunnelling.
PROCESS
A simplified description of the construction method of a DMT or
DP is to think of a combination of HDD and microtunnelling. As with
conventional microtunnelling, a microtunnel boring machine (MTBM)
head is jacked through the soil. However, to create the desired
curve, articulated joints within the machine assembly provide for
steering capability as the pipe is jacked producing a curved
alignment similar to those possible with HDD (see Figure 1). Unlike
HDD, the hole is continuously supported and during installation the
pipe is in compression, not tension. Also unlike HDD, the soil
formations in the near vicinity of the tunneling machine are not
subject to high pressures from slurry systems. Unlike traditional
microtunnelling, the entry and exit pits may be designed at or near
the ground surface, eliminating the expense of deep entry and exit
pits required for straight-line, conventional microtunnelling. Also
unlike traditional microtunneling, the use of the pipe thruster
allows the pipe to be jacked in a continuous string by clamping
around the pipe. A photograph of a pipe thruster and stringing area
is shown in Figure 2.
Figure 1. Schematic of Direct Pipe Crossing
Pipe Thruster
Microtunnel Boring Machine
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There are some obvious technical challenges to consider with the
above-described process. There are also many benefits. The obvious
advantage of this method over conventional microtunneling is the
near-surface entry and exit to a large extent reduces the
requirements of deep excavations. Compared to a conventional
microtunnel, a curved drive allows for potentially much shallower
entry and exit pits. Compared to an HDD, DP or DMT allows:
1. Potentially much shorter and shallower drives (see Figure 3
below). 2. Continuous support of drilled hole potentially for
crossing of gravels and
other collapse-prone soils. 3. Significant reduction of
hydraulic fracture and inadvertent returns risk.
Figure 3 below graphically depicts the potential differences
between DP/DMT and HDD techniques. As discussed above, length and
depth requirements may be greatly reduced for DP/DMT versus a
traditional HDD.
The engineering analyses required for design of a DMT or DP
focus around four items:
1. Estimation of jacking forces required to accomplish the
drive. 2. Calculation of allowable jacking forces for the design
pipe size, strength, and
geometry. 3. Assessment of the difference between estimated and
allowable jacking force
and associated risk. 4. Calculation of the operating condition
stress for a given pipe geometry, size,
strength, and operating pressure.
Figure 3. Conceptual HDD and DP Layout for Small River Crossing
in Alluvial Soils
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Items 1 thru 3 are discussed in this paper, item 4 is not
discussed in this paper as it is the same evaluation used for HDD;
a good reference for calculation of item 4 is the PRCI Design Guide
(Watson, 1995).
ESTIMATION OF REQUIRED JACKING FORCE
In a general sense, the jacking force is the force required to
overcome the skin friction between the pipe wall and surrounding
soils and/or lubrication combined with the force required at the
face of the excavation to allow the tunneling machine to cut into
the soil or rock through which the machine is advancing. Another,
typically smaller force to consider for a pipe in a vertical curve
is that force caused by the alignment of the pipe in the drive,
i.e., the vector portion of the weight of the pipe as it is jacked
non-horizontally, when summed over the course of the drive this
contribution may be positive or negative depending upon the
elevations of the entry and exit locations.
Several good references are available for estimation of jacking
force for traditional, straight-line microtunneling; those
specifically used in the development of the authors design analyses
for jacking forces include Bennett and Cording (1999) and Staheli
(2006). To perform the calculations needed for the analysis, the
proposed drive length is discretized into nominal increments, such
as 5 or 10 feet, and a ground surface and proposed pipe elevation
are input into the design program. Soil parameters such as unit
weight, cohesion, and phi angle are used along with the proposed
pipe and existing ground surface geometry to estimate the normal
stress and interface friction factor to calculate the frictional
resistance and the face pressure resistance to jacking.
Additionally, an estimate is made of the effectiveness of
lubrication and the friction forces are reduced accordingly. The
estimates of skin friction and face pressure are added to the
cumulative weight of pipe contribution to develop the total jacking
load estimate.
Because the ultimate goal of the jacking force analysis is to
evaluate the suitability of a proposed pipe and jacking system in a
given geometry and geology, the end result is necessarily an
estimate of the maximum jacking force on the pipe. In our analysis,
we do not estimate incrementally the theoretical required jacking
loads during the course of the drive but rather estimate the load
experienced by the pipe along its length just prior to completion
when the highest combination of skin friction and face pressure
loading is expected. This may be thought of as a snapshot in time
load diagram just prior to tunnel completion. An example graph is
shown on the following page as Figure 4. Jacking Load Estimate Just
Prior to Drive Completion.
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Figure 4. Jacking Load Estimate Just Prior to Drive
Completion
According to Dr. Gerhard Lang, of Herrenknecht, an evaluation of
nine of the DP projects completed to date in clay, sand, and gravel
resulted in average values of 0.03-0.09 tonnes/square meter in clay
and 0.06 to 0.15 tonnes/square meter in sand and gravel (Lang,
2012). Compared to the values calculated using the more traditional
method used by the authors, those suggested by Herrenknecht
represent a significant reduction (on the order of roughly three or
more times less). The current design process (using traditional
microtunnel jacking force calculations) therefore appears
conservative. This is an area where additional refinements will
likely be possible as additional DMT and DP projects are completed
and more data is gathered.
As may be inferred from the data presented above, the estimate
of jacking forces should be considered a fairly coarse evaluation
that is highly dependent on the engineers judgment of the
subsurface conditions. Other factors that also influence the loads
ultimately incurred during construction include the procedures and
skill of the machine operator, the condition of the tunneling
equipment, and the effectiveness of the lubrication system.
ALLOWABLE JACKING FORCE AND STRESS ANALYSES
As opposed to the anticipated jacking load, the allowable
jacking load may be computed fairly precisely. This is due to the
relatively low level (compared to geotechnical conditions) of
variability in manufactured steel pipe. Given the proposed
geometry, pipe specifications (strength, modulus of elasticity,
diameter, and wall thickness) and the allowable factor of safety,
the allowable load calculation is possible.
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Figure 5. Direct Pipe Launch Pit with Pipe Thruster and Launch
Seal
Following the guidance provided in Chapter 3 of API Recommended
Practice 2A-WSD (API), the following stress conditions should be
considered:
1. Axial Compression Stress 2. Bending Stress 3. Hoop Stress 4.
Combined StressAxial and Bending 5. Combined StressAxial and
Hoop
Additionally, buckling must be considered between the jacking
frame or pipe thruster and the launch seal (or where the pipe
enters the ground and is assumed to be laterally supported against
buckling).
Buckling For jacked, curved pipe, the buckling analysis takes
one of three forms, depending on the diameter to wall thickness
ratio (D/t) of the jacking pipe. Assuming that D/t is less than 60,
and we do not recommend that it be greater than 60 for steel
trenchless installations, then the allowable axial compressive
stress (Fa) is calculated using the methods described in API 3.2.2.
The length used in the buckling calculation is the distance between
the pipe thruster clamp and the entry seal. (See Figure 5. The pipe
thruster clamp is in the foreground, and the launch seal is
incorporated in the far sheet pipe wall.) Once the pipe has passed
the entry seal it is assumed to be essentially fully supported as
the pipe overcut is on the order of one inch, and it is partially
filled with slurry lubrication fluid.
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Axial Compressive Stress To calculate the design factor of
safety for axial compressive stress, first the applied axial stress
(fa) is calculated from the estimated maximum load by dividing the
load by the cross sectional area:
=
Where P is the applied load and A is the cross-sectional area of
the pipe. The allowable axial stress (Fa) is then calculated using
the equations in API section 3.2.2, depending upon the pipe D/t,
the steel design strength (Fy), the unbraced length, and Youngs
Modulus of elasticity for steel. A factor of safety is built in to
the Fa calculations. Therefore, to check that the design is
acceptable, the applied compressive stress must simply be less than
the allowable.
Bending Stress To calculate the design factor of safety for
bending stress, first the applied stress (fb) is calculated from
the estimated maximum load by the following equation derived from
beam mechanics from the PRCI design guide:
=24
Where E is the steel modulus of elasticity, D is the pipe
diameter (in inches) and R is the radius of pipe curvature (in
feet).
The allowable bending stress (Fb) is then calculated using one
of three equations in API section 3.2.3, depending upon the pipe
D/t and the steel design strength (Fy). A factor of safety is built
in to the Fb calculations. Therefore, to check that the design is
acceptable, the applied bending stress must simply be less than the
allowable.
Hoop Stress To calculate the design factor of safety for hoop
stress, first the applied stress (fh) is calculated from the
estimated external and internal pressures by the following equation
from the PRCI design guide:
=
2
Where is the difference in pressure inside the pipe (assumed to
be at atmospheric pressure) and outside the pipe from groundwater
and drilling fluid, D is the pipe diameter (in inches) and t is the
pipe wall thickness.
The allowable hoop stress (Fh) is then calculated using one of
three equations in the API section 3.2.5, which checks both elastic
and inelastic hoop buckling stress. The length of cylinder between
stiffening rings is set equal to the length of the crossing. A
factor of safety is built in to the Fh calculations. Therefore, to
check that the design is acceptable, the anticipated hoop stress
must simply be less than the allowable.
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Figure 6. Estimated Stresses in Pipe Just Prior to Drive
Completion
Stress Analyses Summary The graph below (Figure 6. Estimated
Stresses in Pipe Just Prior to Drive Completion) illustrates all of
the stresses calculated for a crossing with proposed pipe alignment
and ground surface profile. As with the jacking force graph
presented in Figure 4, from which this information is partially
derived, this graph is a snapshot in timejust prior to drive
completion. Note that for this design the pipe enters and exits on
straight tangents, and the pipe is curved between the point of
curvature (PC) and point of tangency (PT).
After the initial stress calculations are completed, the pipe
must be checked for combination loading or combined stress to
evaluate its behavior under anticipated, interactive combined
loading.
Combined StressAxial and Bending Using the API equations for
combined axial and bending stresses, two conditions must be
satisfied as detailed in API 3.3.1:
+
1.0 And . +
1.0
If fa/Fa is less than or equal to 0.15, then the following
formula is used in lieu of the first two.
+
1.0
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Figure 7. Stress and Capacity Analysis
Combined StressAxial and Hoop Using the API equations for
combined axial and bending stresses, two conditions must be
satisfied as detailed in API 3.3.4:
.
1.0 And
1.0
If fx is greater than 0.5Fha, then the following formula is used
in lieu of the first two.
0.5 0.5
1.0
Combining all of the stress analyses factors of safety together
with the combined stress analyses, we can develop the graph below
(Figure 7. Stress and Capacity Analysis) that details the design
checks of the adequacy of the proposed pipe for the anticipated
jacking loads and geometry. Note that because the allowable
compressive, bending and hoop stress calculations (Fa, Fb, and Fh)
include required factors of safety, the indicated capacity analysis
(capacity divided by anticipated stress) must simply be greater
than 1.0. The combined stress analyses are the results of the
equations given above and must be less than 1.0; only the highest
set of combined stress calculations is presented for clarity. The
hoop stress capacity analysis does not appear on the graph because
it is much higher than the other analyses, i.e., for the example
scenario the hoop capacity is much greater than the anticipated
applied stress.
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If the completed analysis results indicate that stress
violations are likely at the as-designed geometry and pipe
specifications, then the geometry and/or pipe specifications may
need to be altered to provide for an acceptable design.
After the pipe geometry, stress conditions, and other parameters
are summarized in the design model, it is very simple to assess
higher than anticipated loads by replacing the anticipated,
calculated jacking loads with arbitrary values. This analysis
provides a maximum load that may not be exceeded without potential
stress violations for a given pipe geometry and pipe strength and
size specifications.
The maximum allowable load should be compared to the anticipated
jacking force load and a decision made whether the design is
acceptable or if pipe specifications or geometry should be changed
to provide a larger cushion between allowable and anticipated
loading. Several factors must be weighed in this evaluation,
including the confidence the designer has in the anticipated
jacking load calculations, the amount and quality of geotechnical
information available, the consequences of a failed drive, the
amount of risk of which the owner is tolerant, and many other
site-specific factors.
APPLICATIONS
The authors have provided detailed design services on four DMT
and DP crossings for 2013 construction and are currently providing
preliminary design services on several additional crossings. These
trenchless sites all have geometry or geotechnical issues that make
an auger bore, traditional microtunnel, or HDD infeasible, costly,
and/or risky.
Specifically, the following challenging conditions have been
faced:
1. Deep, granular (gravels, cobbles) soils not optimal for HDD
work. 2. Short, shallow design profiles required by right of way
constraints and
geologic conditions. 3. Continuous casing in a curved drive
beneath an Interstate highway.
SUMMARY
The DP and DMT trenchless applications offer great promise and
utility to the pipeline engineer needing to cross an area with a
minimum of impactparticularly where traditional methods such as HDD
or microtunneling are not possible or risky due to geometry and
geological conditions. DP and DMT technology is gaining acceptance
in the American pipeline design community and has been shown to
work in Europe where more than 15 DP crossings have been completed.
Engineering design and stress analyses for these crossings is
possible using the design procedures discussed above, and it is
responsible for owners to require the analysis be completed. As
data is gathered from future construction projects, additional
refinements in the design procedures will be possible.
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REFERENCES
API Recommended Practice 2A-WSD. (21st Ed., December 2000, with
errata and supplements December 2002, September 2005, and October
2007). Recommended Practice for Planning, Designing and
Constructing Fixed Offshore Platforms-Working Stress Design.
Bennett, D., Cording E.J. (1999). Jacking Loads Associated with
Microtunneling. Geo-Engineering for Underground Facilities, G.
Fernandez and R.A. Butler, eds., ASCE Geotechnical Special
Publication No. 90, 731-745.
Lang, G. (2012). Herrenknecht AG, Personal Communication.
Staheli, K. (2006). Jacking Force Prediction: An Interface Friction
Approach Based
On Pipe Surface Roughness. Ph.D. dissertation, Georgia Institute
of Technology.
Watson, D. (1995). Installation of Pipelines by Horizontal
Directional Drilling an Engineering Design Guide. Pipeline Research
Council International, Inc.
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