Horizontal directional drilling rigs have transformed the landscape of underground utility installation. This innovative technology allows for precise, trenchless methods to install pipes and cables, minimizing surface disruption and environmental impact. By employing advanced techniques, directional drilling not only enhances efficiency but also reduces costs in urban and challenging terrains.

What is a horizontal directional drill?

A horizontal directional drill is a type of drill used in the underground sector. The drills come with different attachments, including rods and fluid reamers, and are used as a guided, trenchless method of installing underground pipes, cables and duct. The trenchless nature of their use means that horizontal directional drills use only a very small footprint compared to other traditional installation methods. Operations using HDDs also tend to be more cost-effective too.

How does a horizontal directional drill work?

The process involved in horizontal directional in three main steps:

1.Digging a pilot hole

To begin with, a borehole is drilled along a predetermined path in the ground by the horizontal directional drill. The drilling head is hollow and is pushed through the ground, steered by the driller who uses a transmitter to determine the precise location of the drilling head.

2.Pre-reaming

The pre-reaming stage involves using the horizontal directional drill to enlarge the pilot borehole. It is completed in stages and ensures that pullback pressure will be lower when the pipe is eventually installed. Completing it in stages also means that the structural integrity of the hole is maintained.

Drilling fluid is also pumped in to remove solids and enlarge the hole. The fluid also helps maintain the structural integrity of the hole. Learn more about the differences between drilling mud and drilling fluid.

3.Pipe pullback

The final stage of pipe installation using a horizontal directional drill is pipe pullback. This is where the drill string is pulled back along the path of entry and the pipe is inserted in its place. Pipe pullback involves calculating the force required when pulling the drill string back as an incorrect force could damage the structure of the hole or product being installed.

Design Challenges of horizontal directional drill

Entry and Exit Angles

One of the most critical design parameters in HDD is the entry and exit angles. The entry angle refers to the angle at which the borehole is drilled from the surface location, while the exit angle refers to the angle at which the pipeline or conduit emerges from the borehole. These angles are crucial in determining the curvature of the borehole and must be carefully evaluated to ensure the pipeline or conduit can be pulled through without excessive bending or deformation.

The angle of entry and exit also affects the stress levels exerted on the pipeline during the installation and operation phases. The selection of these angles in HDD designs depends on several factors, including the soil conditions, the length of the borehole, the diameter of the pipeline or conduit, and the equipment used.

Soil Conditions

Soil conditions significantly impact the entry and exit angle selection in HDD designs. Soil type and properties affect the resistance during drilling and pulling and the curvature of the borehole. Hard and dense soils, such as rock formations, may require higher entry and exit angles to achieve the desired curvature. On the other hand, soft and loose soils, such as sands and clays, allow for lower entry and exit angles, as they offer less resistance to drilling and pulling forces. However, entry and exit angles that are too low may produce issues with fracking.

Length and Diameter

The length of the borehole and the diameter of the pipeline or conduit also influence the selection of entry and exit angles in HDD designs. Longer boreholes and larger diameter pipelines or conduits may allow lower entry and exit angles to achieve the desired curvature.

Equipment

The equipment used in an HDD project greatly affects the selection of entry and exit angles. Some equipment allows greater control over the drilling angle and can achieve tighter curves with lower entry and exit angles. For example, entry angles should generally be designed between 8° and 20°; however, drilling rigs are typically manufactured to operate at 10° to 12°. Similarly, exit angles generally range from 5° to 12° and should be designed to provide ease in the breakover support of the pull section.

Minimum Curvature Radius

Minimum curvature radius refers to the smallest radius achieved in the borehole without causing excessive stress or damage to the pipeline or conduit. While the minimum curvature radius is influenced by several factors, including the pipe diameter, wall thickness, and material properties, it is usually directly determined by the diameter and type of drill rods utilized for the initial bore. The design engineer must carefully evaluate these factors to ensure the minimum curvature radius is within acceptable limits.

Material Selection

The material of the pipeline or conduit also affects the minimum curvature radius due to the difference in bending stiffness and allowable bending stress. For example, polyethylene pipes are more flexible and have larger allowable bending stress than steel pipes. The soil conditions also affect the minimum curvature radius in HDD designs. The soil’s stiffness and resistance to deformation influence the maximum allowable curvature that can be achieved without causing excessive stress or deformation of the pipeline or conduit. Hard and dense soils, such as rock formations, require larger minimum curvature radii, while soft and loose soils, such as sands and clays, allow for smaller minimum curvature radii. The design engineer must carefully evaluate the minimum curvature radius in HDD designs to ensure the safe and efficient installation of pipelines or conduits.

Installation and Operation Loads

The installation loads are another critical design parameter that must be considered in HDD. These loads refer to the forces exerted on the pipeline or conduit during installation, such as tension, compression, and bending. The design engineer must evaluate the installation loads to ensure the pipeline or conduit can withstand them without damage. Factors such as soil type, depth, and groundwater conditions can also affect the installation loads and must be considered during the design phase. HDD installation loads should be calculated based on pipe materials, soil conditions, and boring depth to determine if a pipe specification is adequate. Installation stresses include tensile stress and bending stress. Operating loads imposed on a pipeline installed by HDD are also important to be evaluated, including internal pressure, bending, thermal expansion, and external pressures. CHA has developed a method to calculate these loads following the latest Pipeline Research Council International (PRCI) guidance document entitled “Installation of Pipelines by Horizontal Directional Drilling, An Engineering Design Guide.”

Pipe Material Selection

The choice of pipe material is a crucial design parameter for HDD. The type of material selected for the pipeline or conduit depends on several factors, including the anticipated loads, the soil conditions, and the expected service life of the pipeline. The design engineer must evaluate these factors to select a pipe material that can withstand the anticipated loads and environmental conditions. High-density polyethylene (HDPE) material, steel, polyvinyl chloride (PVC), and even ductile iron pipe (DI) are all viable options for an HDD design.

Construction Challenges of horizontal directional drill

Staging Areas and Maintenance of Traffic (MOT)

Performing an HDD installation in congested urban areas presents unique challenges for contractors, particularly regarding staging, mobilization, and maintenance of traffic (MOT) operations. Staging and mobilization are critical components of any HDD installation project. In an urban environment, it is essential to have a clear understanding of the project site and surrounding areas to determine the best location for staging and mobilization, not just for the most efficient operation and installation but for the safety of workers, pedestrians, and vehicular traffic. The traffic management plan (TMP) should include information on the timing of road closures, detours, lane shifts, and the duration of these closures. The TMP should also consider closure impacts for businesses and local residents and provide strategies to mitigate these impacts.

Geological Conditions

Unique geological conditions, such as highly porous limestone bedrock, underground karst formations, or extremely sandy soils, can present significant construction challenges for contractors performing HDD installations. Drilling through limestone bedrock can pose challenges due to its inherent hardness and density, resulting in heightened wear and tear on drilling equipment. Additionally, limestone can contain voids and pockets of air, which can create instability and cause the drill bit to deviate from its intended path. Another challenge associated with drilling is the presence of underground karst formations. Karst formations are created when limestone bedrock is dissolved by groundwater, resulting in the formation of underground caves, sinkholes, and other structures. These formations can be unpredictable and difficult to detect, posing a safety risk to drilling operations.

Hydraulic Fracture (Frac-out)

Hydraulic fracture, or frac-out, is a challenge in HDD installations, occurring when drilling fluids exceed soil strength, causing fractures and fluid escape. Factors influencing frac-outs include soil type—softer soils like clay and sand are more susceptible—borehole size and shape, and shallow entry and exit angles that reduce cover. These conditions can lead to soil erosion, environmental damage, and project delays.

Crossing Rivers, Canals, and other Waterways

Performing an HDD installation that crosses environmentally sensitive waterways can present significant construction challenges. These waterways include lakes, rivers, wetlands, and other aquatic habitats home to various plant and animal species. A careful and measured approach to site preparation, drilling operations, and environmental management should be taken to minimize the impact of HDD installations on these environments.

What are the top applications of a horizontal directional drill?

Horizontal directional drills are used for a variety of applications where trenchless installation is preferred or required. The environments themselves include residential areas, urban areas, under roads, rivers or environments with unstable soil conditions. This is because of the lesser footprint offered by horizontal directional drills in comparison to trenching methods, which means minimized disruption to the site’s surroundings.

Horizontal directional drills are frequently used for all types of utility installation in urban and residential areas and all utility pipelines under obstructions. Horizontal directional drills have a wide range of benefits for drillers that can be summarized by their efficient nature of installing pipes and cables, with less disruption than methods that involve trenches. The process involved in horizontal drilling also requires calculating the precise tensile force needed to maintain the structural integrity of the hole before installing a pipe, and transmitters are used to determine the location of the drill head for precision.

Benefits Of Directional Drilling

Directional drilling offers a number of benefits over traditional trenching methods, including:

Minimal surface disruption:

Directional drilling allows contractors to install utilities without disrupting the surface, which can minimize traffic disruption, protect landscaping, and reduce the risk of environmental damage.

Faster installation time:

Directional drilling can be used to install utilities more quickly than traditional trenching methods, especially for projects that involve long distances or difficult-to-access areas.

Reduced costs:

Directional drilling can be more cost-effective than traditional trenching methods, especially for projects that involve minimal surface disruption or difficult-to-access areas.

Increased safety:

Directional drilling is a safer method of installing utilities than traditional trenching methods because it reduces the risk of accidents, such as cave-ins and blowouts.

Conclusion

Horizontal directional drilling rigs represent a significant advancement in construction and utility installation. Their ability to navigate complex underground conditions while maintaining safety and efficiency makes them indispensable in modern infrastructure projects. As technology continues to evolve, the benefits of directional drilling will undoubtedly expand, paving the way for sustainable development.

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