BoreAid The first comprehensive HDD design tool

• Key Components of Project:
• the client wishes to install a 8.625” HDPE pipe (DR 9)
• the surface length of the project is 2200 feet (26400 inches)
• we must inform the client as to whether the project is possible and if so, provide detailed design features such as the bore path, projected loads, reaming sequences, project duration, drill areas, required equipment, supplies, etc.

1. Drill site
2. Geotechnical considerations
3. Bore path design
4. Load calculations
   1. Operational
   2. Installation
5. Drill fluid and reaming sequence design
6. Hydrofracture considerations
7. Equipment and supplies
8. Summary
9. BoreAid Input Summary

At the surface, this site did not pose any major difficulty. The topography was flat and surface obstacles did not hinder construction of the drilling area. 

Figure 1. Topography at site (exported from BoreAid).

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2. Geotechnical Considerations
                                                                            
The geotechnical conditions existing at this site are shown in the figure to the right. There is a layer of well graded sand from the surface to a depth of 16 meters and the water table is at the surface. The sand has the following geotechnical properties: 
 

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Figure 2. Soil conditions at site (exported from BoreAid).

The bore is constructed automatically in BoreAid using the entrance/exit angles and the minimum depth of cover. In this project entrance/exit angles were chosen as 10 and 12 degrees, respectively, and the depth of cover was taken to be 35 feet (420 inches). The resulting bore path is shown in the figure below. In determining this path, BoreAid calculates the required radius of curvature and checks to see if this value falls within the acceptable range of design.

Figure 3. Bore path calculated by BoreAid (exported from BoreAid).

BoreAid also allows the user to access the calculated bore path information. For instance, the user can print the bore path or drill rod locations along the bore path. If you require more details, BoreAid allows the user to export these drill rod locations directly into an Excel (.xls) file. The user may then include them in their own analysis or graphics.

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Load calculations are divided into two sections: operational and installation loads. Both types of load calculations have design limits/restrictions associated with them which are built directly into the software. In this project, the maximum operational loads as calculated  by BoreAid are given in Table 1 below.

In this table we observe the results based upon three different assumptions of the bore condition.  The values in this table represent the values for the various deflections and pressures at the location of maximum net pressure. The point-by-point values of each of these quantities as a function of drill rod location may be exported to an Excel file, if required. Alternatively, BoreAid contains an interface to plot and export all of the above parameters along the bore path.  Fore example, Figure 4 shows the earth pressure calculated along the bore path. 

Figure 4. Earth pressure along the bore path (exported from BoreAid).

The built-in plotting environment allows you to label axis, change axis limits, and even convert the x-axis value automatically between inches, feet, and meters, regardless of the units of the problem. In addition, BoreAid automatically labels key points of interest along the bore path which are crucial in the calculation of installation forces, as noted below. 

Installation loads are also calculated and summarized in Table 2. The four locations of interest are: the pipe entrance location (A), the location when the pipe reaches the depth of cover and is about to transverse horizontally (B), the location when the pipe is about to begin its rise to the surface (C), and the exit location (D). 

Again, all of these values and calculations are fully exportable and printable so that the user man complete further in-house analysis. Figure 5 shows a screenshot of the page where these installation loads are calculated.

Figure 5. Screenshot of installation load calculation page.

Notable on this page is that, firstly, there are four different methods (or models) that the user may use to calculate these installation loads. These are outlined in the user manual. In addition, there are checkbox below the selection of calculation methods which allows the user to see the effects of adding ballast during the installation, and/or rollers at pipe entry, and/or additional bending at pipe exit. These options may be added by checking the appropriate box and clicking calculate. Once results are calculated the “Export Results” button is enabled so that the user may export the results to an Excel file. 

In Table 3 we include the effect of using rollers on the surface during installation of the pipe. Rollers, ballasts, and additional bends are all options available to the user during the design procedure.

The effect of adding these rollers can also be viewed graphically using BoreAid (and exported or printed as before) as seen in Figure 6.

Figure 6.  Calculated pullback force without (top) and with (bottom) the use of rollers
at pipe entrance.

The use of the rollers at pipe entrance has significantly reduced the required pullback force at locations A and B. Finally, there is an interface within BoreAid which checks that these loads/deflections fall within acceptable limits for design (see Figure 7).

Figure 7.  Screenshot of BoreAid page use to check that loads/deflections fall with
acceptable limits of design.

Should one of the values result in a factor of safety less than one, a warning that the design will fail is issued to the user .

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The drilling/reaming sequence was designed in the following way:

We use an overcut ratio of 1.5 for the installation of this pipe and the soil to fluid volume ratio is 2.5. Our drill rig has a pump capacity of 600 gallons and pump rate of 60 gallons/min is operating at 80% efficiency. 

With this design criteria the pilot bore and reaming sequence is input using the user friendly interface seen in Figure 8. In addition to the size of the reamer pass, the time required is also input by the user in terms of minutes spent per drill rod.

 

BoreAid calculates total (and per rod) soil and fluid volumes required for each reamer pass as well as time required. Key calculations are summarized in Figure 9. Note that BoreAid issues warnings if the pump capacity is exceeded during any stage of the drilling/reaming. For example, if we change the drill time per rod to 1 minute for the final reamer pass we observe in Figure 10 that BoreAid highlights the calculations warning that the pump rate required exceeds the pump capacity.

Figure 10.  Drill fluid/time requirements calculated by BoreAid.

Finally, these results may be viewed via bar charts. A sample bar chart representing the drill fluid required per reamer pass (measured in number of tanks) is given in Figure 11.

Figure 11.  Drill fluid requirements per reamer pass calculated by BoreAid.

The user may navigate between pages to change various parameters until they are happy with their design and results.

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Based upon the reaming sequence designed in Section 5 above, BoreAid calculates estimates bounds on the maximum limiting pressure inside the bore to prevent hydrofracture or frac-out. The results of this calculation when using a radius of plasticity equal to 120 and then 40 inches is shown in the Figure 12 and 13 for the reaming sequence of Table 4.  

Figure 12.  Limiting pressure along the bore path with radius
of plasticity 120 inches (exported from BoreAid).

Figure 13.  Limiting pressure along the bore path with radius
of plasticity 20 inches (exported from BoreAid) - different colors
represent each reamer pass as noted by text color inside BoreAid.

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Using the equipment and supplies module (the drill rig portion is shown in Figure 14),  BoreAid lets the user see that a large mini-HDD or small size midi-HDD drill rig is required to achieve a pullback factor of safety of nearly 2 to complete the project. The factor of safety for the pump rate is also given. Finally, BoreAid does allow the addition of custom drill rigs if the user has a drill rig pump/pullback properties not given by one of those in the list.

Figure 14.  Screenshot of drill rig section of equipment supplies module.

In addition, in other sections of the equipment and supplies module, BoreAid also indicates that well-graded sand is generally suitable for drilling using a spade drill bit.  

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Using BoreAid, we were able to determine that this HDD project is feasible at a depth of cover of  420 inches. Further, we are able to provide our client with figures showing the topography, soil layers, bore path, installation and operational loads, deflections, reaming sequences, and limiting pressure estimations. We have also exported all BoreAid calculations to a spreadsheet so that we may provide details of the project to the client. To achieve a factor of safety greater than 2.0, the contractor must have a drill rig with a pullback capacity or nearly 40,000 lbs.  Any changes to the design, such as a larger depth of cover, or the addition of ballast or rollers can be incorporated and forwarded to the client upon request.

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A complete list of input parameters used in this simple design project is available upon request to BoreAid users.

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