Underground Construction - December 2020 - 24

Intersect HDD Gets Better with Experience

Currently Michels' intersect project count is 206, with the
number continuing to go up. The company's longest HDD
pilot-hole intersect HDD to date is the 13,247-foot installation
of a 20-inch pipe under Lake Sakakawea and the Missouri River
in North Dakota.
Mears Group, Neil Smith, senior vice president,
engineering; and Steve Sommerfeld,
steering technician manager
Primary factors to consider in determining whether a project
could be best accomplished with an intersect include:
* Is it feasible?
* Does the geometric design of the crossing allow
for a reasonable place to propose an intersection?
* Would an intersect sufficiently benefit the project?
* Do its benefits outweigh the additional cost incurred
to accomplish it?
Generally, the situations appropriate for intersect pilot-hole
drilling are crossings that are very long; where using intersect
technology will alleviate potential problems concerning excessive
annular pressures; where there are adverse geotechnical
conditions at both the entry and exit points (often necessitating
pre-installation of surface casings); and those with complicated
geometries that would make it difficult to maintain directional
control, or maintain adequate weight on the bit, with a single
pilot drilling operation.
The intersect point is determined after evaluating the geometrical design of the crossing, as well as the subsurface ground
formations to be encountered, for the most strategic location that
provides the best chance of success. A straight section in the drill
and a competent formation are both important considerations.
Guidance technology used depends on the particulars of each
situation. We have utilized conventional magnetic guidance
tools, gyroscopic tools and a combination of both to accomplish
numerous intersects.
A serious risk is that a " dogleg " can occur at the intersect point,
which will be detrimental to the reaming and ultimately to the
product installation phase of the HDD process. Care needs to be
taken to ensure holes are merged together gradually, so they don't
exceed radial design limitations of the product pipe to be installed.
Depending on the type of guidance system selected for the project, once the relational difference between the two opposing pilot
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DECEMBER 2020 | UCONonline.com

holes has been determined, one string is extracted while the other
is advanced towards the intersect position. It's important to steer
the advancing string at a merge rate that does not allow it to enter
the existing hole at an unacceptable deflection deviation. Once the
intersection is accomplished, radius calculations of the merge into
the existing hole are analyzed for acceptable tolerances, and the
strings are advanced/retracted to pilot hole completion.
Location for the product preparation side is normally
determined by available space and right-of-way restrictions.
Once the intersection is completed, the reaming rig and pull
rig location can be situated at either end, depending on the
particulars of each individual site.
To date, Mears has completed 29 HDD intersects. The longest
is a 7,400-foot-long crossing for 130V underground pipe/cable
crossing the Elizabeth River in Norfolk, Va.
Hard Rock Directional Drilling,
Cory Baker, general manager
When applied correctly, the intersect method is one of the greatest
tools in the HDD industry. It's primarily considered because of
length and concerns about inadvertent fluid return (IR).
An intersect may be needed to build the section of the exit
curve on very long bores when it's necessary to steer to build
the angle in the exit curve, and push at the bit is lost, preventing
it gaining angle. Therefore, it helps to set up a rig at the exit end
to drill the exit curve so the main rig on the entry can follow
the exit rig up the exit curve.
The intersect method also helps prevent inadvertent returns.
A fracture analysis is used to determine the maximum pressure
the soil can handle. Then the drilling contractor can calculate
the anticipated annular pressure the soil can handle for the pilot
hole. If the pressure goes higher than the maximum allowable -
which is a safety factor of two - then an intersect rig is needed
to reduce the annular pressure downhole. This is one of the
best methods in the industry to reduce IRs.
Before planning an intersect, geo cores should be completed
to help decide where to make the intersect. Planning also
requires a review of the entire crossing to identify what
downhole tools will be used for the soil conditions.
Guidance technology is determined by each crossing's
requirement. Hard Rock has completed intersects with all types
of systems and combinations. If the intersect is under a body of
water, a system with radar is great - it all depends on the job.
What does not change in all intersect projects is the entry and
exit rig. The larger rig will always be at the entry side, so it can
complete the reaming and pulling operations. The side with the
space to lay out product determines which side is which.
Hard Rock has completed many intersects over the years.
The longest to date is 11,265 feet of 10-inch pipe under
Clear Lake at Kemah, Texas. Drill rigs of 1 million and
330,000 pounds of pullback were used. *
FOR MORE INFORMATION:
Michels Corporation, (920) 924-4300, www.michels.us
Mears Group, (281) 448-2488, mears.net
Hard Rock, 210-403-2086, http://hardrockhdd.com

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Underground Construction - December 2020

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