New underground infrastructure construction is an important aspect for a developing municipal environment. Installing this new infrastructure using traditional trenching techniques, particularly open-cut construction, can equate to high social costs. These social costs include noise pollution, traffic disruption, aesthetic factors and negative public perception. The use of trenchless construction methods can enable installation of pipelines and other conduits under these sensitive areas, while providing minimal disruption in comparison to traditional trenching methods.
Horizontal directional drilling (HDD) has the capacity to install a wide variety of pipe materials into the ground. This process provides an alternative to the traditional open-cut methodology, while providing a number of benefits. For example, the HDD process can decrease the cost of installing underground conduit, as the operation can be performed more quickly, requires less working space, and can be conducted without disruption to surface activities (traffic and pedestrian areas). When utilized under a watercourse, HDD can provide reduced environmental impacts and increased productivity.
HDD is one of the fastest growing trenchless construction methods. Smaller drilling rigs typically are used for the installation of telecommunications services. Larger rigs are capable of installing pipelines up to 48 inches in diameter. With this growth comes an ever-increasing need for gaining a better understanding of the physical nature of this construction process and its influence on the surrounding medium. While HDD has been employed in North America since the 1970s, there still are some municipalities and regulatory agencies that are wary of allowing the process, due to negative perceptions regarding the annular space region of the installed pipe. In particular, some agencies are concerned that voids will be found in the annular space region, due to the use of drilling fluids to replace native soils. Voids could result in instability of the installed product pipe. It is hoped that the results of recent research will make these municipalities and regulatory agencies more aware of the capabilities of the HDD process, and curb any concerns regarding the long-term effects on the annular space created during these operations.
Drilling Procedures
The installation of pipe and conduit typically is performed in three distinct phases – pilot bore, reaming and pipe pullback. The pilot bore phase consists of using a small-diameter drill head launched from the surface at an entry angle between 8 degrees and 16 degrees to the horizontal. This pilot bore proceeds downward until the desired depth is achieved and the orientation of the drill is brought to horizontal. Drilling continues along a horizontal path – or a given grade in the case of gravity sewers – until it is brought to the surface through a predetermined exit location. During the pilot bore operation, the drilling rig operator has the ability to steer the drilling head in any direction. Steering is accomplished by pushing the sloped drill head without rotation to the required alignment.During the drilling process, the location, orientation, rotation and depth of the drill head are tracked (or surveyed) by either a manual walkover or a wireline location system. Typically, a magnetic device – known as a transmitter, or sonde – is placed inside the drilling apparatus. The transmitter emits an electromagnetic signal field that can be tracked using a hand-held locating device. These hand-held devices interpret the magnetic signals from the transmitter and display the depth, pitch and roll of the drill head. Hand-held devices have their limitations, as they can receive interference from sources including buried utilities, steel structures and power lines, and they are risky to use for watercourse crossings and under surface obstacles.
When hand-held tracking systems are unable to be utilized due to surface site access restrictions or drilling depth, directional-drilling contractors can implement the use of a wireline system. These systems consist of a measuring instrument that is mounted in the drill head, where the transmitter for a walkover system normally would be located. This measuring device tracks the bore path and transmits information through the wires that run inside the drill pipe. The azimuth and inclination of the drilling tool is collected and calibrated through the use of a computer system. The wireline system is accurate within 2 percent in both plan and profile, regardless of the borehole depth. However, it also is very time-consuming, as the wireline needs to be reconnected with each additional drill rod. Wireline systems are useful when the bore path navigates under a deep-water body, where conventional tracking units would not be effective.
Once the pilot hole has reached the exit pit, the drill head is removed and a reamer is attached. The purpose of the reaming operation is to enlarge the borehole prior to the installation of the permanent product pipe pullback process. Throughout the reaming operation, the borehole typically is enlarged to 1.5 times the diameter of the product line that is to be installed. This may be adjusted, however, according to the soil conditions encountered and the overall length of the installation. This oversizing allows for a reduction in the frictional effects that are imposed upon the product pipe during installation, and can reduce the associated bending stresses near the entry and exit regions. For larger diameter pipes, several reaming passes typically occur, with increasing reamer diameter sizes used to reach the desired upsizing to the final diameter. The last reaming pass is conducted in conjunction with the pipe pullback process.
During the pipe pullback process, the product line is attached to the reamer with a swivel link assembly. This swivel link allows the reamer to rotate without rotating the product pipe that is being installed. This helps to decrease the torsional stresses on the pipe during the pullback phase. Additionally, the swivel link may be designed to break if the force on the product pipe exceeds a pre-calculated limit. This prevents the product pipeline from being overstressed, as the breakaway link will fail prior to any structural damage of the permanent pipe. Additionally, during a pullback operation, it is preferred that the pipe is completely fabricated prior to the pullback operation. The risk of the pipe becoming stuck in the borehole can increase substantially if the pulling operation is stopped for the incremental connection of the individual pipe segments.
Drilling fluid plays an important role in the directional drilling operation, and is utilized for all stages of the HDD operation. Drilling fluid primarily is composed of bentonite clay mixed with water, and may have polymers and other agents added. During the pilot bore phase, the drilling fluid serves several purposes, such as stabilizing the borehole, removing the drilled cuttings, reducing the torque on the drill string, lubricating the drill pipe, and cooling both the drill bit and the housing containing the electromagnetic transmitter.
Drilling fluid is dispersed through the reamer orifices during the reaming phase and aids in the cutting of the native soil, provides lubrication of the reamer drill string in the borehole, transports the cuttings, provides hole stability, and prevents the enlarged reamed borehole from collapsing. During the pullback phase and the final reaming phase, the drilling fluid not only aids in the reaming action, as mentioned previously, it also provides lubrication to the product pipe. This lubrication decreases the frictional effects from the contact of the pipe with the borehole wall, and may reduce the chances of the product line becoming stuck during the pullback phase.
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