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HORIZONTAL DIRECTIONAL DRILLING
Introduction
Horizontal directional drilling (HDD) was pioneered in the
United States in the early 1970s by an innovative road boring contractor who
successfully completed a 183 m (600 ft) river crossing using a modified rod
pushing tool with no steering capability (DCCA 1994). By integrating existing technology
from the oil well drilling industry and modern surveying and steering
techniques, today's directional drilling methods have become the preferred
approach for installing utility lines, ranging from large-size pipeline river
crossings to small-diameter cable conduits.
The HDD industry is divided into three major
sectors--large-diameter HDD (maxi-HDD), medium-diameter HDD (midi-HDD), and small-diameter
HDD (mini-HDD, also called guided boring)--according to their typical
application areas. Although there is no significant difference in the operation
mechanisms among these systems, the different application ranges often require
corresponding modification to the system configuration and capacities, mode of
spoil removal, and directional control methods to achieve optimal
cost-efficiency. Table 1 compares typical maxi-, midi-, and mini-HDD systems.
Table 1. Comparison of main features of
typical maxi-, midi-, and mini-HDD (Iseley and Gokhale 1997)
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Description
Directional
drilling methods utilize steerable soil
drilling systems to install both small- and large-diameter lines. In
most
cases, HDD is a two-stage process. Stage 1 involves drilling a pilot
hole
approximately 25 to 125 mm (1 to 5 in) in diameter along the proposed
design
centerline. In stage 2, the pilot hole is enlarged to the desired
diameter to
accommodate the pipeline. The pilot hole is drilled with a
surface-launched rig
with an inclined carriage, typically adjusted at an angle of 8 to 18
degrees
with the ground for entrance and 8 to 12 degrees for exit angle (Miller
the
Driller 2002). The
preferred minimum radius in feet for steel pipe is typically 100 times
the diameter of pipe in inch. For plastic pipe, the multiplication
factor is 40,
i.e., 40 times of diameter of pipe in inch.
Most systems adopt either fluid-assisted drilling or a
high pressure fluid jetting method to create or enlarge the bore hole. In a few
instances, some mini-HDD systems utilize dry bore systems (with compressed air)
in hard, dry soils and calcified or soft rock formations.
The progress of the pilot hole is monitored by a specially
designed surveying system, either a walkover system or an electromagnetic
down-hole navigational system. In a walkover system, the drill head is equipped
with a sonde (also called a beacon) transmitter behind the drill bit. The sonde
is powered by battery and emits signals continuously. These signals can be
picked up on the ground with a hand-held receiver. The receiver provides data
on the position, temperature, depth, and orientation of the drill bit. An
alternative detection system, the electromagnetic down-hole navigational system
can be used in conjunction with a series of four electrical cables positioned
directly above the desired path and secured in place. The cables, which can be
laid directly on top of the street or highway, do not interfere with traffic
flow. The cables transmit an electromagnetic signal that is picked up the
navigational instruments in the drill head. These instruments determine the
position of the drill head relative to the center of the cables and relay this
information continuously to a computer on the operator's console. In case of
deviations from the desired path, the operator can make necessary adjustments (Iseley
and Gokhale 1997).
After the drill head (or pilot string and washover
pipe) exits at the desired location, reaming devices are attached for the
pullback operation. This stage involves enlarging the pilot hole to the desired
diameter to accommodate the pipeline. The utility pipe is attached to the reamer,
with a swivel to ensure that the rotation (torque) applied to the reamer is not
transmitted to the utility. The reamer enlarges the bore hole to the required
size, and the utility is installed. For large diameter (greater than 500 mm (20
in.)), an intermediate prereaming may be required before pulling the utility
into place. Prior to the pullback operation, the pipeline is usually assembled
to its full length and tested.
The drilling process in HDD can be described as follows:
1. Site preparation
The construction site is prepared before
the main drilling operation. A drilling
rig is set up at the proper location. Slurry is prepared to stabilize the
borehole and to lubricate the surface of borehole. A transmitter is inserted
into the housing provided on the pilot drilling string near the drill
bit. Other equipment and facilities such as generators, pumps, storages, and
offices are prepared at this stage.
2. Pilot hole drilling
Drilling the pilot hole can be the most important phase of a HDD project,
because it determines the ultimate position of the installed pipe. A small
diameter (25 to 125 mm (1 to 5 in.)) drilling string penetrates the ground at
the prescribed entry point at a predetermined angle routinely between 8 – 18
degrees. The drilling continues
under and across the obstacle along a design profile.
3. Prereaming
In general, the final size of the
bore should be at least 50% larger than the outside diameter of the product
pipe. This overcut is necessary to allow for an annular void for the return of
drilling fluids and spoils and to allow for the bend radius of the pipeline. To
create a hole that accommodates the required size of pipe, prereaming is
necessary.
Typically, the reamer is attached to
the drill string at the pipe side and pulled back into the pilot hole. Large
quantities of slurry are pumped into the hole to maintain the borehole and to
flush out the soil cuttings (DCCA 1994). The type of reamer varies based on the
soil type. A blade reamer is used for soft soils, a barrel reamer for mixed
soils, and a rock reamer with tungsten carbide inserts is used for rock
formations.
4. Pullback
Once the prereaming is completed, the pipe or conduit can be pulled back into
the reamed hole filled with drilling fluid. The pipe is prefabricated and tested
at the pipe side. If the pipe is made of steel, it is recommended that the pipe
be placed on rollers to reduce the friction and to protect pipe coating.
However, this operation is usually not required for High Density Polyethylene (HDPE) pipe
installation.
The drill pipe is connected to the
product pipe using a pull head or pulling eye and a swivel.
The swivel is a device used to prevent the rotation of the pipeline during
pullback. A reamer is also located between the pull head and the drill string to
ensure that the hole remains open and to allow lubricating fluid to be pumped
into the hole during the pullback. The pullback operation will continue until
the pipe or conduit surface at the drill rig. The pull head is disconnected, the
drill rig removed, and clean-up and tie-ins are started. For
small diameter pipes, the prereaming process and pullback process can be
performed at the same time.
Main Feature and Application Range
Diameter range
In maxi- and midi-HDD, the size of pipe installed can
range from 75 mm (3 in.) to 1,200 mm (48 in.) in diameter. Multiple lines can
be installed in a single pull, but only in the case of small-diameter pipes.
The installation procedure for multiple lines is the same as for single lines,
with the bundle being pulled back as a single unit along the prereamed profile.
A significant multiple line crossing is more than 600 m (2,000 ft) in bore
length and consists of five separate lines, pulled as one, ranging in size from
150 mm (6 in.) to 400 mm (16 in.). The largest pipe that can be installed
by the mini-HDD system is 300 mm (12 in.) in diameter.
Depth of installation
Mini-HDD can install pipes up to 4.5 m (15 ft) in depth.
This depth limitation comes from the restriction in the capacity of walkover
tracking system. However, for the larger machines, such as midi- and maxi-HDD,
the maximum installation depth for HDD is 61 m (200 ft).
Drive length
The length of bore in HDD is determined by the type of
soil and site conditions. Bore spans can range from 120 m (400 ft) to 1,800 m
(6,000 ft) for maxi- and midi-HDD. However, small lengths are not economically
feasible because of the high operational costs of these systems. Mini-HDD is
capable of installing pipelines and utilities 180 m (600 ft) in one continuous
pass to a specified tolerance.
Type of pipe
In general, the pipe to be installed is limited to one
that can be joined together continuously, while maintaining sufficient strength
to resist the high tensile stresses imposed during the pullback operation. In
maxi- and midi-HDD, steel pipe is the most common type of casing used. However,
butt-fused, high-density polyethylene pipe (HDPE) also can be used. HDPE pipe,
small-diameter steel pipe, copper service lines, and flexible cables are some
of the common types of pipe materials being used today in mini-HDD.
Required working space
The directional drilling process is a surface-launched
method; therefore, it usually does not require access pits or exit pits. If
utility installation is being undertaken, pits may be required to make
connections with the existing utility. The rig working area should be
reasonably level, firm, and suitable for movement of the rig. For maxi- and
midi-HDD, an area of 120 m (400 ft) b y 60 m (200 ft) is considered adequate.
The equipment used in mini-HDD is portable, self-contained, and designed to
work in congested areas.
Soil condition
Clay is considered ideal for HDD methods. Cohesionless
fine sand and silt generally behave in a fluid manner and stay suspended in the
drill fluid for a sufficient amount of time; therefore, they are also suitable
for HDD.
High-pressure fluid drilling systems (mini-HDD and
midi-HDD) normally do not damage on-line existing utilities and thus are safe
for subsurface-congested urban areas. Fluid cutting systems, which are most
suitable in soft soil conditions, have been used widely in sand and clay
formations. Although small gravel and soft rock formations can be accommodated
by higher fluid pressure and more powerful jets, steering accuracy might
suffer.
Generally, mechanical drilling systems (mini-HDD) can be
applied in a wider range of soil conditions than fluid jetting methods. A pilot
hole can be drilled through soil particles ranging from sand or clay to gravel,
and even in continuous rock formations, by using suitable drill heads; however,
problems might occur in spoil removal, pilot hole stabilization, and
backreaming operations. Today's technology enables large drilling operations to
be conducted in soil formations consisting of up to 50 percent gravel.
Productivity
HDD systems have the highest pilot hole boring rate of
advancement among all trenchless methods for new installations. For mini-HDD rigs, a
three-person crew is sufficient. In suitable ground conditions, a pipeline as
long as 180 m (600 ft) can be installed in 1 day by a regular work crew.
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