| I. OVERVIEW
A. DEVELOPMENT
AND USES - Originally used
in the 1970s, directional crossings are a marriage of conventional road
boring and directional drilling of oil wells. The method is now the
preferred method of construction. Crossings have been installed for
pipelines carrying oil, natural gas, petrochemicals, water, sewerage
and other products. Ducts have been installed to carry electric and
fiber optic cables. Besides crossing under rivers and waterways, installations
have been made crossing under highways, railroads, airport runways,
shore approaches, islands, areas congested with buildings, pipeline
corridors and future water channels.
B. TECHNOLOGY LIMITS -The longest crossing
to date has been about 6,000 ft. Pipe diameters of up to 48 in. have
been installed. Although directional drilling was originally used primarily
in the U.S. Gulf Coast through alluvial soils, more and more crossings
are being undertaken through gravel, cobble, glacial till and hard rock.
C. ADVANTAGES
- Directional crossings
have the least environmental impact of any alternate method. The technology
also offers maximum depth of cover under the obstacle thereby, affording
maximum protection and minimizing maintenance costs. River traffic is
not interrupted, as most of the work is confined to either bank. Directional
crossings have a predictable and short construction schedule. Perhaps
most significant, directional crossings are in many cases less expensive
than other methods.
D. TECHNIQUE
1. Pilot Hole - A pilot
hole is drilled beginning at a prescribed angle from horizontal and
continues under and across the obstacle along a design profile made
up of straight tangents and long radius arcs. A schematic of the technique
is shown in Figure 1. Concurrent to drilling pilot hole, the contractor
may elect to run a larger diameter "wash pipe" that will encase the
pilot drill string. The wash pipe acts as a conductor casing providing
rigidity to the smaller diameter pilot drill string and will also save
the drilled hole should it be necessary to retract the pilot string
for bit changes. The directional control is brought about by a small
bend in the drill string just behind the cutting head. The pilot drill
string is not rotated except to orient the bend. If the bend is oriented
to the right, the drill path then proceeds in a smooth radius bend to
the right. The drill path is monitored by an electronic package housed
in the pilot drill string near the cutting head. The electronic package
detects the relation of the drill string to the earth's magnetic field
and its inclination. This data is transmitted back to the surface where
calculations are made as to the location of the cutting head. Surface
location of the drill head also can be used where there is reasonable
access.
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| 2. Preream
- Once the pilot hole is complete, the hole must be enlarged to
a suitable diameter for the product pipeline. For instance, if the pipeline
to be installed is 36 in. diameter, the hole may be enlarged to 48 in.
diameter or larger. This is accomplished by "prereaming" the hole to
successively larger diameters. Generally, the reamer is attached to
the drill string on the bank opposite the drilling rig and pulled back
into the pilot hole. Joints of drill pipe are added as the reamer makes
its way back to the drilling rig. Large quantities of slurry are pumped
into the hole to maintain the integrity of the hole and to flush out
cuttings. |
| 3. Pullback
- Once the drilled hole is enlarged, the product pipeline can be
pulled through it. The pipeline is prefabricated on the bank opposite
the drilling rig. A reamer is attached to the drill string, and then
connected to the pipeline pullhead via a swivel. The swivel prevents
any translation of the reamer's rotation into the pipeline string allowing
for a smooth pull into the drilled hole. The drilling rig then begins
the pullback operation, rotating and pulling on the drill string and
once again circulating high volumes of drilling slurry. The pullback
continues until the reamer and pipeline break ground at the drilling
rig. |
