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.
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 (movie (This wmv file is 2.8 MB. Windows Media Player is required to run this file.)). 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).
Source : https://nodignews.files.wordpress.com/2013/04/hdd-e1369148108705.jpg
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 (picture, movie (This wmv file is 4.0 MB. Windows Media Player is required
to run this file.)) 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 (picture). 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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