The HDD Process

The basic components of a horizontal directional drilling system include:

  • Drill unit
  • Guidance system
  • Drilling fluid system
  • Drill pipe and downhole tools, including bits and back reamers
  • Drilling fluid mixing or recycling system

The HDD rig is connected to the cutting bit by the drill string, which is made up individual joints of pipe. Back reamers are used to increase the diameter of the pilot hole to the required size to accommodate the diameter of the pipe to be installed. The drilling fluid, commonly known as mud, plays an essential role in drilling, back reaming, and product pullback. The fluid mixing system is separate from the drilling rig. Fluid recirculating systems often are employed on long bores to install large-diameter pipe.

After offloading the HDD rig, it is positioned over the bore path centerline at an adequate distance from the drill entry point to allow the drill bit to enter the ground at the desired location and angle. The HDD rig is often tied down using the powered rotating screws located on the front of the drill rig. The project area, including the

10 Chapter 1 ■ Introduction to Horizontal Directional Drilling

HDD entry and exit points, should well marked in accordance with the project plans and specifications. Ensure that all the equipment and material required for the HDD project are on hand and in good condition. This includes ensuring that appropriate types and quantities of drilling fluids and additives are on hand.

HDD systems are defined by:

  • Thrust and pullback force, stated in pounds
  • Spindle torque, stated in foot pounds
  • Maximum volume of drilling fluid a machine can pump per minute, and spindle revolutions per minute

A typical HDD rig is illustrated in Figure 1-4.

The HDD drill rig is used to drill and ream the pilot hole and pull the product pipe back through the hole. HDD drill rigs provide torque, thrust, and pullback to the drill string. The drill drive assembly resides on a carriage that travels under hydraulic power along the frame of the drill rig. The thrust mechanism for the carriage can be a cable, chain, screw, or rack-and-pinion system. Table 1-3 lists the three general categories of drilling rigs used in the industry.

Mini rigs are mounted on a trailer, truck, or self-propelled track vehicle. The self-propelled units are self-contained, with the engine, hydraulic power, and drilling fluid pump all part of the unit. The lower end of this class of drill rigs (less than 20,000 pounds thrust/pullback) is designed for drilling in relatively soft semiconsol-idated formations and is used primarily for the installation of utility conduits and small-diameter pipelines in congested urban areas. They are not suitable for drilling gravel, cobble, or other formations where bore-hole stability is difficult to maintain. The higher end of this class of drill rigs is suitable for drilling in gravel and cobbles as long as the bore lengths are not excessive.

Medium-sized drilling rigs are used to install larger conduits and pipelines, normally up to 16-inches in diameter, with drill lengths ranging up to 2000 feet. They are particularly suitable for the installation of municipal pipelines, as they are sufficiently compacted to be used in urban areas while at the same time they have the capacity to install large-diameter products beneath highways, subdivisions, and

Drill Rack

Drilling Install Pipelines

FIGURE 1-4 Typical HDD Rig

Drill Rack

FIGURE 1-4 Typical HDD Rig

TABLE 1-3 Typical Characteristics of HDD Rigs

Mini Rigs

Midi Rigs

Maxi Rigs


<40,000 lbs.

40,000-100,000 lbs.

>100,000 lbs.

Maximum Torque

<4,000 ft.lbs.

4,000-20,000 ft.lbs.

>20,000 ft. lbs.

Rotational Speed

>130 RPM

130-210 RPM

<210 RPM

Carriage Speed

>100 ft./min.

90-100 ft./min.

<90 ft./min.

Carriage Drive

Chain, Cylinder, or Rack & Pinion

Chain or Rack & Pinion

Rack & Pinion

Drill Pipe Length

5-15 ft.

10-30 ft.

30-40 ft.

Drilling Distance

<700 ft.

<2000 ft.

<6000 ft.

Power Source

<150 HP

150-250 HP

>250 HP

Mud Pump

<75 gpm

50-200 gpm

>200 gpm

Drill Rig Area Required (width x length)

3 ft. x 10 ft.-7 ft. x 20 ft.

7 ft. x 20 ft.->8 ft. x 45 ft.

8 ft. x 45 ft.

Recommended Work

20 ft. x 60 ft.

100 ft. x 150 ft.

150 ft. x 250 ft.

Area Requirements (width x length)

Area Requirements (width x length)

rivers. Bores can be installed in unconsolidated to consolidated sediments. Many of the drill rigs in this class are self-contained units.

Maxi rigs typically involve a large operation with multiple trailer-mounted support equipment and substantial mobilization and demobilization periods. High operating costs make their use somewhat prohibitive in the utility installation market, and they are employed primarily in the pipeline industry. These large units may be used in the installation of large diameter pipes (16 to 48-inches) and/or exceptionally long bores.

In addition to the drilling rig, a variety of support equipment may be required. Depending on the HDD project, a drilling fluid or mud cleaning and recirculation unit, drill-pipe trailer, water truck, and pump and hoses may be required. An excavator is needed to dig the entry, exit, and recirculation pits. In urban or environmentally sensitive areas a vacuum truck may be required to handle the fluid in the return pits or inadvertent returns.

The physical size of the HDD equipment is also important because the available setup space at many project locations is often limited. HDD units come in a wide range of sizes, with the units most often employed for utility work yielding between 5,000 and 90,000 pounds of pullback. Models with half a million pounds of pullback and more are available. The smallest models are designed to make installations up to 150 feet in length. Units in the 70,000- to 90,000-pound range can

12 Chapter 1 ■ Introduction to Horizontal Directional Drilling make pilot bores and pull in pipe to distances of 6,000 feet and more, depending on soil conditions.

Pipe Installation

Installation of a pipe by HDD is usually accomplished in three stages. The first stage involves directionally drilling a small-diameter pilot hole along a designed directional path. The second stage consists of enlarging (reaming) the pilot hole to a diameter that will support the pipeline, and the third stage consists of pulling the pipeline back into the enlarged hole.

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