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What
is Directional Drilling?
Directional
boring, commonly called horizontal directional drilling
or HDD, is a method of installing underground pipes
and conduits along a prescribed bore path from the surface,
with minimal impact to the surrounding area, Installation
lengths up to 6,500 feet have been completed and diameters
up to 48" have been installed in shorter runs.
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Applications:
(Top)
The
process is used for installing telecommunications & power
cable conduits, water lines, sewer lines, gas lines, oil lines,
product pipelines and environmental remediation casing. It
is used for crossing waterways, roadways, shore approaches,
congested areas, environmentally sensitive areas and any area
where other methods are more expensive.
The
Process: (Top)
The
process begins when a directional bore machine pushes a bore
head connected to hollow pipe into the ground at an angle.
As each joint of drill pipe is pushed into the ground, a new
one is added behind.
Most directional boring machines use drilling
fluid (mud) with a few machines designed to use air or air
and foam. Air & Foam machines are used for rock. Drilling
is generally a mixture of Bentonite clay and water, with additives
used to improve performance. In softer soils the high-presure
jet of fluid is pumped down the hole and the cuttings are
carried back out to the surface. Drilling fluid is classified
as non-toxic and can disposed of accordingly.
In softer soils, an angled bit is used
and the pipe is rotated, if necessary, to bore straight. To
steer, rotation is stopped, the angle of the bit is aligned
to the desired directions and forward thrust is applied. The
directed jet of the drilling fluid and forward thrust cuts
a new path.
In rock, a mud motor, which converts the
hydraulic pressure of the drilling fluid into mechanical rotation,
is used to rotate the bit and the drill pipe. Steerage is
accomplished by aligning the angle of the mud motor to the
desired direction.
In cases where the ground is unstable, a
wash over pipe, or casing, can be pushed down the borehole
to prevent the collapse of the hole walls. Some systems use
a dual pipe exclusively. |
Upon
reaching the exit point, the bit is detached and the end of
the drill pipe is attached to a reamer or hole opener (for rock),
if the borehole must be enlarged. The reamer is pulled back
while rotating the drill pipe to enlarge the borehole with as
many consecutive passes as required. Drill pipe is added behind
the reamer so that there is always drill pipe in the borehole.
When the bore hole is at least 25% larger
than the pipe to be installed, the end of the drill pipe is
connected to a reamer attached to a swivel connected to the
product pipe. |
Drilling
a Pilot Bore
Back
Ream & Product Pulling
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Drilling fluid is pumped down the hole to provide lubrication
and the product pipe is pulled in while rotating the drill
pipe and reamer. The swivel prevents rotation of the product
pipe.
For some telecommunications or power cable
projects, the drill pipe itself becomes the conduit and is
left in the ground upon reaching the exit point. This type
of installation is known as "drill and leave".
Once the pipe is installed the exit and
entry points are excavated if necessary and connections made
as needed.
Directional
Bore Machines: (Top)
Directional
bore machines are rated by thrust and pullback force and rotary
pressure. Sizes range from small machines with a few thousand
pounds of thrust and pullback to the largest with a few smaller
machines designed to be used in pits.
Locating
& Guidance: (Top) |
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The
most commonly used equipment for determining the location of
the bore head is called a "walk-over" locating system.
A sonde, or transmitter, behind the bore head registers angle,
rotation, direction and temperature data. The information is
then encoded into an electro-magnetic signal, which is transmitted
through the ground to the surface. At the surface a receiver
is manually positioned over the sonde and the signal decoded
and steering directions relayed to the operator of the bore
machine.
When conditions do not allow a receiver to
be positioned over the sonde or interference causes degradation
of the signal, a "downhole system" is used. The most
commonly used type of downhole system is called a "wire
line" and uses a wire to transmit the data up the inside
of the drill pipe.
At the surface a computer decodes the data
from the wire and provides depth, angle, rotation, direction
and other information. Gaining in popularity are newer downhole
wireless systems such as the Polaris EM System, which transmits
the data through the ground by an electro-magnetic signal to
a stationary receiver.
To compensate for potential magnetic interference
which might distort magnetic readings in downhole system, an
artificial electro-magnetic grid is created a the surface using
what is called a Tru-Traker ™ System. |
Costs:
(Top)
Directional
boring has evolved steadily over the last 20 years and is
now the preferred method on many installations due to its
low cost and low impact on surroundings. It is generally less
expensive that other methods such as micro tunneling, jack
& bore and open trenching in urban areas. In urban areas
it cannot only save a considerable amount on installation
cost it can provide a tremendous amount of public goodwill.
Danger is usually greatly reduced along
with excavation. Workers are not in dangerous trenches or
around heavy earth-moving machines. Disruption of property
owners and traffic is much less when open trenches are eliminated,
and the risk of damage and disruption to nearby utilities
is much less when digging is reduced.
Installation costs also go down when excavation
is reduced because so much heavy equipment is unnecessary.
As exposure to liability goes down, so do insurance rates.
Pipe
Reaming: (Top)
A
variation of directional boring called pipe reaming can be
used to replace existing clay, asbestos cement, non-reinforced
concrete and PVC pipe. A reamer is pulled through the existing
pipe which cuts the pipe into small pieces. The pipe pieces
are flushed out the borehole with the drilling fluid. A new
HDPE or PVC pipe is pulled in behind the reamer. Pipe reaming
can often be used instead of pipe bursting. The advantages
of pipe reaming are lower cost, faster installation, no compaction
of the surrounding formation and much greater up sizing capabilities.
Limitations:
(Top)
Directional
boring can be used in a wide variety of conditions, but is
not the optimal method in all conditions. The most difficult
ground formation for any method is un-consolidated soils (cobble).
In some cases the un-consolidated soils can be grouted and
then bored. Directional boring can be used for sanitary sewers,
but only when ground conditions permit a straight path.
Design
Considerations: (Top)
When
designing a project for directional boring it is important
to have accurate geotechnical data, sufficient space for the
bore rig and support equipment and enough space for laying
out the pipe on the other side. It is best to allow extended
work hours for pull back. Additional considerations may be
required for specific projects. Pipe, which can be used for
directional boring installations, includes HDPE mechanical
joint PVC and steel. |
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