Page 88 - Pipeline Rules of Thumb Handbook

Construction 75

Technology limits its way back to the drilling rig. Large quantities of slurry are
pumped into the hole to maintain the integrity of the hole and
The longest crossing to date has been about 6,000ft. Pipe to flush out cuttings.
diameters of up to 48in. have been installed. Although Pullback—Once the drilled hole is enlarged, the product
directional drilling was originally used primarily in the U.S. pipeline can be pulled through it. The pipeline is prefabri-
Gulf Coast through alluvial soils, more and more crossings cated on the bank opposite the drilling rig. A reamer is
are being undertaken through gravel, cobble, glacial till and attached to the drill string, and then connected to the pipeline
hard rock. 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
Advantages the pullback operation, rotating and pulling on the drill string
and once again circulating high volumes of drilling slurry.
Directional crossings have the least environmental impact The pullback continues until the reamer and pipeline break
of any alternate method. The technology also offers maxi- ground at the drilling rig.
mum depth of cover under the obstacle thereby affording
maximum protection and minimizing maintenance costs.
River traffic is not interrupted, because most of the work is Layout and design
confined to either bank. Directional crossings have a pre-
dictable and short construction schedule. Perhaps most sig- Access—Heavy equipment is required on both sides of
nificant, directional crossings are in many cases less expensive the crossing. To minimize cost, access to either side of the
than other methods. crossing should be provided with the least distance from an
improved road. Often the pipeline right-of-way is used for
access. All access agreements should be provided by the
owner. It is not practical to negotiate such agreements during
Technique
the bid process.
Pilot Hole—A pilot hole is drilled beginning at a pre-
scribed angle from horizontal and continues under and across Work space
the obstacle along a design profile made up of straight tan-
gents and long radius arcs. A schematic of the technique is Rig Side—The rig spread requires a minimum 100-ft.
shown in Figure 1. Concurrent to drilling a pilot hole, the wide by 150-ft. long area. This area should extend from the
contractor may elect to run a larger diameter “wash pipe” that entry point away from the crossing, although the entry point
will encase the pilot drill string. The wash pipe acts as a con- should be at least 10ft. inside the prescribed area. Since many
ductor casing providing rigidity to the smaller diameter pilot components of the rig spread have no predetermined posi-
drill string and will also save the drilled hole should it be nec- tion, the rig site can be made up of smaller irregular areas.
essary to retract the pilot string for bit changes. The direc- Operations are facilitated if the area is level, hardstanding
tional control is brought about by a small bend in the drill and clear of overhead obstructions. The drilling operation
string just behind the cutting head. The pilot drill string is not requires large volumes of water for the mixing of the drilling
rotated except to orient the bend. If the bend is oriented to slurry. A nearby source of water is necessary (Figure 2).
the right, the drill path then proceeds in a smooth radius bend Pipe Side—Strong consideration should be given to
to the right. The drill path is monitored by an electronic provide a sufficient length of work space to fabricate the
package housed in the pilot drill string near the cutting head. product pipeline into one string. The width will be as neces-
The electronic package detects the relation of the drill string sary for normal pipeline construction although a work space
to the earth’s magnetic field and its inclination. This data is of 100-ft. wide by 150-ft. long should be provided at the exit
transmitted back to the surface where calculations are made point itself. The length will assure that during the pullback
as to the location of the cutting head. Surface location of the the pipe can be installed in one uninterrupted operation.
drill head also can be used where there is reasonable access. Tie-ins of successive strings during the pullback operation
Pre-ream—Once the pilot hole is complete, the hole must increase the risk considerably because the pullback should be
be enlarged to a suitable diameter for the product pipeline. continuous (Figure 3).
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 “pre-reaming” the hole to successively larger Profile survey
diameters. Generally, the reamer is attached to the drill string
on the bank opposite the drilling rig and pulled back into the Once the work locations have been chosen, the area should
pilot hole. Joints of drill pipe are added as the reamer makes be surveyed and detailed drawings prepared. The eventual