Earth:Directional boring

Short description: Method of installing underground utilities

Directional boring machine

Directional boring, also referred to as horizontal directional drilling (HDD), is a minimal impact trenchless method of installing underground utilities such as pipe, conduit, or cables in a relatively shallow arc or radius along a prescribed underground path using a surface-launched drilling rig. Directional boring offers significant environmental advantages over traditional cut and cover pipeline/utility installations. The technique is routinely used when conventional trenching or excavating is not practical or when minimal surface disturbance is required.

Although often used interchangeably, the terms directional boring and horizontal directional drilling are distinct in that they convey a different sense of scale. The term "directional boring" or "bore" is generally reserved for mini/small sized drilling rigs, small diameter bores, and crossing lengths in terms of hundreds of feet. Generally, the term Horizontal Directional Drilling (HDD) is intended to describe large/maxi sized drilling rigs, large diameter bores, and crossing lengths in terms of thousands of feet. Directional boring and HDD are similar in some respects to directional drilling associated with the oil industry, however, an equal comparison cannot be drawn as the procedures serve markedly different functions. Directional boring can be utilized with various pipe materials such as PVC, polyethylene, polypropylene, ductile iron, and steel provided that the pipe's properties (wall thickness and material strength) enable it to be both installed and operated (if applicable) under acceptable stress limits.

Directional boring/HDD is generally accomplished in three principal phases. First, a small diameter pilot hole is drilled along a directional path from one surface point to another. The diameter of the pilot hole is relative to the equipment being used and may range from a few inches to slightly over a foot. Next, the bore created during pilot hole drilling is enlarged to a diameter that will facilitate installation of the desired pipeline. For small diameter installations, reaming or bore enlargement may not be necessary. Lastly, the pipeline is pulled into the enlarged hole, thus creating a continuous segment of pipe underground exposed only at the two initial endpoints. Directional boring can be utilized to cross any number of surface obstacles including roadways, railroads, wetlands, and water bodies of varying sizes/depths.

The process is suitable for a variety of soil conditions including clay, silt, sand, and rock. Problematic soil conditions include large grain content in the form of coarse gravel, cobbles, and boulders. Other subsurface conditions which can impact the feasibility of directional boring include excessive rock strength and abrasivity, poor rock quality or heavily fractured/fissured rock, and rock exhibiting karst features.

Equipment

Small HDD drill rig

Maxi-sized drill rig

Typical maxi-sized HDD Rrig spread (approximately 50,000 sq ft.)

For smaller bores, proportionally smaller and more portable equipment is available. These units may be capable of anywhere from 5,000 lbs to 100,000 lbs of thrust/push force and may be used for spanning between a house basement to a nearby shared water pipe or a short road crossing. Fluid exhaustion associated with smaller bores is proportionally less as well. In many instances, smaller bores do not require the use of drilling fluid, only water, and in even less significant bores no fluids at all.

The large rig industry and small rig industry are different enough that each possesses standards and customs associated with construction and best practices. Depths of cover, radii of curvature, and general path geometry differ significantly between large rig and small rig applications. Standards associated with large rig installations are more well established and cauterized into existing literature as opposed to small rig installations, which tend to vary based on application and project specific constraints.

Technique

Directional boring is used for installing infrastructure such as telecom and power cable conduits, water lines, sewer lines, gas lines, oil lines, product pipelines, and environmental remediation casings. It is used for crossing waterways, roadways, shore approaches, congested areas, environmentally sensitive areas, and areas where other methods are costlier or not possible. It is used instead of other techniques to provide less traffic disruption, lower cost, deeper and/or longer installation, no access pit, shorter completion times, directional capabilities, and environmental safety. The cost of directional boring varies based on factors such as soil conditions, bore length, and pipe diameter. Costs can range significantly depending on project complexity and location.

Process

Starting pit with pilot hole and some drilling fluid in the pit

A mud motor (or drilling motor) may be used to steer the direction of the bore hole.

Locating and guidance

Empty conduits laid by directional boring

Generally speaking, there are three types of locating equipment for locating the bore head: the walk-over locating system, the magnetic wire-line locating system and the gyro guided drilling, where a full inertial navigation system is located close to the drill head.

• Walk-over locating system — A sonde, or transmitter, behind the bore head registers angle, rotation, direction, and temperature data. This information is encoded into an electromagnetic signal and transmitted through the ground to the surface in a walk-over system. At the surface a receiver (usually a hand-held locator) is manually positioned over the sonde, the signal is decoded and steering directions are relayed to the bore machine operator. Generally speaking, the walkover locating system is wireless, meaning a wireline is not required to be ran internally through the drill pipe for power and data communication.
• Wire-line locating system — The wire-line system is a magnetic guidance system. With a magnetic guidance system (MGS), the tool registers directional inclination and azimuth values and such data is transmitted through a wire-line ran through the drill pipe leading from the drill head to the surface. The MGS also typically has a secondary means of location verification utilizing wire grids laid on the ground surface. The wire grid's positioning is surveyed and when current is transmitted through the grid it creates an electromagnetic field of known orientation. Downhole tooling senses the orientation and strength of the magnetic field associated with the surface wire allowing for a secondary determination of positioning. The MGS, even without the use of the wire grid, has been accurate to over 2 km with an accuracy of 2% at depth. The operator of the MGS communicates with the driller and guides him towards a predetermined engineered drill path.
• Gyro-based locating system — The gyro based system is fully autonomous and therefore one of the most accurate systems where sufficient diameter (200 mm) is available and where long distances (up to 2 km) have to be traversed with minimal deviation (less than 1 m position error). Currently the actual depth is not verifiable without the use of surface coils, a near surface transponder or sonde used in walkover systems. Like the MGS wire-line system, most gyro tools require a wire-line to be ran through the drill pipe to communicate data to operators at the surface.

See also

• Trenchless technology
• Underground pneumatic boring

References

• PR-277-144507-Z01 Installation of Pipelines by Horizontal Directional Drilling, An Engineering Design Guide, prepared under the sponsorship of the Pipeline Research Committee at the American Gas Association, April 15, 1995, Revised under the sponsorship of the Pipeline Research Council International, Inc., 2015.
• HDD Design Guideline Task Committee of the Technical Committee on Trenchless Installation of Pipelines (TIPS) of the Pipeline Division of the American Society of Civil Engineers. Pipeline design for installation by horizontal directional drilling - ASCE Manuals and Reports on Engineering Practice (MOP) No. 108 : ASCE manual of practice. American Society of Civil Engineers, 2005. Reston, VA. ISBN 978-0-7844-0804-9
• Skonberg, Eric R., and Tennyson M. Muindi. Pipeline Design for Installation by Horizontal Directional Drilling - ASCE Manuals and Reports on Engineering Practice (MOP) No. 108 (2nd Edition). Reston, Virginia: American Society of Civil Engineers, 2014. ISBN 978-0-784413-50-0
• ASME, Pipeline Geohazards, Planning, Design, Construction and Operations, Second Edition of Pipeline Geo-Environmental Design and Geohazard management, NY: American Society of Mechanical Engineers, 2019. ISBN 978-0791861790
• v. Hinueber, Edgar (iMAR Navigation) (2006). Most accurate drilling guidance by dead-reckoning using high precision optical gyroscopes, Proceedings NoDig Conference of Horizontal Directional Drilling, Brisbane 2006.
• Rizkalla, Moness. Pipeline geo-environmental design and geohazard management. New York, NY: ASME, 2008. ISBN 978-0-791802-81-6

External links

• The Directional Boring Advantage describes techniques used, with diagrams.
• Directional Boring provides examples of the six types of utilities installed via the directional boring method.
• Directional Boring provides details about the various types of Horizontal directional drill machines available.

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