Underground Construction - December 2020 - 23

Directional Drilling

Intersect HDD Gets Better with Experience
Jeff Griffin | Senior Editor

An " intersect " project may be the most complex application in
horizontal directional drilling (HDD).
Two pilot holes - one at each end of the projected bore path
- meet somewhere in between to make a single pilot hole for
pulling in the product. Each segment usually is long, and the
diameter of the pipe being installed is larger. The intersects'
point of connection may be under a river or larger body of water.
For the two bore paths to intersect takes meticulous planning
and precise execution. While, today, an intersect project is not
uncommon, few contractors have the equipment and capabilities
to undertake an intersect project and the challenges it presents.

Early days
The first successful intersect was completed in 2003 by Michels
Directional Crossings for a segment of Maritimes and Northeast
Duke Energy Hub-Line Project in Boston. The Georges Island
HDD crossing a was 5,174-foot-long, hard-rock, water-to-water
crossing to install 30-inch diameter pipe.
Lengths of intersects and diameters of product installed have
steadily increased in the years since the first intersect installation.
Underground Construction queried representatives from
three veteran HDD contractors with several successful intersect
projects on their resumes about this challenging method of
underground construction.
Michels Directional Crossings,
Jeffrey S. Mueller, senior vice president
Intersects have become common in the HDD industry and are used
for a variety of reasons to minimize risk and increase productivity.
Various circumstances lead us to consider pilot hole intersects
instead of single bore. The most obvious one is length. Depending
on diameter of the pipe and the geological conditions, at a certain
point even the most powerful rig would not have the horsepower
needed to provide the thrust and rotary torque required to cross
the entire span.
Other factors are the anticipated annular pressure of
drilling fluid required to remove the cuttings, right-of-way
considerations and time frames. If timing is tight, it theoretically
takes less time for two rigs to drill from opposite sides of a
project and meet in the middle than it would for one rig to
complete the entire drill. Right-of-way restrictions determine
where the product pipe is staged and assembled for pullback.
As with all HDD projects, accuracy and attention to details
are critical to pilot-hole intersect projects. With two rigs
drilling from opposite sides, tracking tools are important to
ensure each rig stays on the predetermined alignment. Severe
pilot-hole misalignment can cause tool failure, or the product
pipe to become lodged due to inappropriate radii through the
intersect transition.

Typically, a tangent section of the bore, rather than a midpoint, is selected as the location of an intersection. As technology
has advanced in this industry over the past two decades, the point
of intersect is not as critical or difficult a decision as it had been
in the method's infancy. The sophistication and skill of those
involved in this industry have grown exponentially.
We use a variety of equipment, including surface guidance,
magnetic survey instruments and gyroscopic tooling. For most
drills, primary and secondary tracking systems are important.
Many factors affect our decisions on the best methods and
equipment for picking up signals, for and determining the precise
position of the drill heads.
Downhole guidance sensors are utilized to align the drill paths,
resulting in a successful intersect. Many intersects are completed
underwater because HDD is typically done in conditions where
traditional or open-cut installations would not be possible - and
that is often under water or in other environmentally sensitive
areas. The execution of the intersect does not vary based on what
is above the location.
UCONonline.com | DECEMBER 2020

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Underground Construction - December 2020

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