Horizontal Directional drilling systems were originally developed by the oil industry for river crossings of small diameter where no high degree of accuracy was required. They are now widely used for installing conduits and pressure pipes under major obstacles such as motorway intersections, large rivers and airport runways.

Mini-HDD methods are used typically for the installation of small diameter lines (2 to 10in. diam), up to 600 ft. in length and up to 30ft. deep. Some systems can be used for installations as deep as 50ft. without hardwired transmitters and as deep as 75ft. or greater with hardwired transmitters. The feasibility of the mini-HDD techniquedepends on the ground conditions, the accuracy required and that achievable for both alignment and grade. Mini-HDd has been used for gravity sewer liners but must be approached with great caution when there are precise line and grade tolerences.

The mini-HDD process involves the creation of a small-diameter (2 to 4in.) pilot bore hole using steerable mechanical fluid-jet cutting tools, followed by pullback of the utility line through the borehole. The borehole is often enlarged with the help of a reaming assembly to accommodate the utility line. Although a few dry systems exist, most mini-HDD systems use a slurry to stabilize the walls of the borehole and to reduce the frictional drag on the cable line or pipeliine being installed. Most mini-HDD systems manufactured in the United States use fluid-assisted mechanical cutting technology. Survey systems locate the drill head position so that steering corrections can be made as the boring progresses. Steering correction are made in soils by rotating the slanted cutter shoe to the desired orientation and then advancing the string with out rotating. In weak rock and very stiff soils, a bent sub may be use in conjunction with a down hole mud motor to make steering corrections.

In the pullback operation, the pipe is attached behind the reaming assembly, and a gripper that holds the product pipe is connected to the swivel while the pipeline is pulled into the borehole. Among the most common product line or pipe materials used are HDPE, PVC, steel(for small diameter pipes), copper (for service lines), and cables. Potentially vulnerable (e.g., fiber-optic) cable should not be pulled directly, but may be contained inside the product pipe (e.g., HDPE pipe) during the pulling operations or may be carefully placed after installing the host pipe. The product pipe or utility line should be installed in one continuous operation, preferably without pause, and sould prodtrude a sufficient distance beyond the exit and entey points to allow proper termination or connection at a later time, as required.

Courtesy of TT Technologies, Inc.

To ensure that an accurate path is maintained throughout the drilling operation, a transmitter is located just behind the cutting head. User-friendly computer software is available to support HDD applications. This software is used to plan the bore, monitor the real-time progress of the bore, and print out a complete survey or as-built map upon completion of the pilot bore. If necessary, the drill string can be drawn back and the bore re-drilled to improve accuracy or negotiate any small boulders or obstacles encountered.

Drill heads, Cutting Bits and Backreamers (Courtesy of Vermeer Manufacturing Company)

The drilling rig and control rooms are positioned at an appropriate drill launch site. The rotating and steerable hollow drill is launched from the surface or shallow excavation at an angle of 8-15° and is used to drill a pilot hole to the exit or endpoint of the bore with an advance rate of about 2 feet per minute.

A variety of cutting heads are used depending on ground conditions. To ensure that the correct cutting head or method is used it is essential to carry out a series of ground investigations. Typically, for mini-HDD applications, a 2 to 4in. drill head is used. A wider steering surface is used in soft soils because of the relatively low resistance provided by the soil to induce directional changes. Conversely, for hard soils, a narrower steering surface should be used because of the high resistance provided by the soil. Furthermore, a wide drill head in hard soil would require greater torque to rotate the drill head and may reduce productivity. Therefore, care should be taken to select the appropriate drill head for anticipated soil conditions. In general, a 4-in. drill head would be appropriate for sand, and a 3-in. drill head may be used for clay or average soil conditions. Water, a drilling fluid, or drilling mud is used for lubricating the drilling head and the hole. The drilling fluid also helps remove the soil cuttings, stabilize the borehole, and cool the drill head and transmitter.

From the exit point, the pilot drill string is retracted and the bore is enlarged by a rotating back reamer that is pulled back by the drill string. Subsequent reaming continues until the required hole diameter is achieved. The reaming assembly consists of circular cutters, compactors, and swivels. Typically, circular cutters (reamers) are attached to the drill pipe by replacing the drill head. A swivel mechanism is usually provided between the reamer and the product pipe gripper. This swivel prevents the rotation of the drill string from being transmitted to the product pipe. However, since the swivels are not completely efficient, torsion may be tansmitted to the pulled section, but this is usually not a problem. The swivel sould be in good condition and sized to be compatible with the tensile and torsional resistance of the product pipe. Reamers have fluid jet opening through which drilling fluid flows to facilitate cutting, stabilize the enlarged hole, lubricate the trailing pipe and transport spoils. Different types of reamers are available including cutting, tri-action, compaction(or packer), barrel, blade reamers or combinations. Sometimes cutting and compacting action is done by one assembly. Many reamers have carbide cutter bits that assist cutting in a wide variety of soil conditions. The soil condition will determine the specific type of reamer that should be selected.

A small rotating and steerable drill bit is launched from the surface at an angle of 10 - 15° and is used to drill a 90mm diameter mud filled pilot hole. During the drilling operation a 125mm diameter washover pipe is drilled over the pilot string and following some 100m behind the head. Alternate drilling then continues on the pilot string and the washover pipe until the exit point on the far side of the obstacle.

The pilot string is removed and the bore is enlarged by a rotating barrel reamer attached to and pulled back by the washover pipe, drilling mud being used to flush away the cuttings and to support the reamed hole. Subsequent reaming continues until the required diameter is achieved. The product pipe is then attached to the reaming head and pulled through the bore. Drives of more than 1.5km and of up to 1200mm diameter have been carried out.

HDD drilling employs an excavation or soil displacement head with compact light weight rig for rapid mobilisation. Small diameter jets, mechanised cutting tools or displacement heads attached to a flexible drill string are positioned to form a bore as the head is thrust forward. The drilling head is launched from the surface at an inclined angle. Steering, in both vertical and horizontal planes, is effected by controlling the orientation of a slant faced drilling bit at the head.

For a straight bore the drill string is continually rotated. When a deviation is required the rotation is stopped and the slant face orientated to produce the required deviation. The drillstring is then pushed forward. With the drill string rotating once more, excavation or displacement is continued. Monitoring of the alignment takes place using a transmitter in its the head and a locating device at ground level.

Having established the pilot bore, backreaming equipment is drawn through the hole to enable it to accept the product pipe, duct or cable, using an impact mole.

The full length of product pipe is joined and often pressure tested prior to installation in the area of the drill exit point. The gas pipe is then attached to the reaming head via a swivel joint and pulled through the newly formed bore using the pullback capacity of the drilling rig. This is normally carried out at the same time as the final back-reaming operation.

The percussive impacts on the highly flexible drill stems can be adjusted at a rate up to 1,000 impacts per minute. This percussive action can create an extra ramming energy of up to 62,900 pounds of force allowing the HDD to successfully steer the drill head where other, standard, comparably sized drill rigs can’t.

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