A Suggested Design Method for Curved, Jacked Steel Pipe

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,

864Pipelines 2013 ASCE 2013

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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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Documents

A Suggested Design Method for Curved, Jacked Steel Pipe