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How Does Directional Drilling Work and What’s the Process

Directional drilling rig ready to start boring, Image by LKyle89 / CC BY-SA

Thinking about how a boring bit can be seen deep underground and steered in any direction at will is fascinating.  In this article, we take a look at how horizontal directional boring can accomplish these feats, and the process steps that get the job done.

Here’s the short answer to how horizontal directional drilling works…

Horizontal directional drilling bores underneath obstacles with a guided bit. The operator steers the bore head utilizing an angled pipe, pressurized fluids, and a steering tool that provides depth and positioning feedback. After reaming and swabbing the borehole, the pipeline string is pulled back through the hole.

Horizontal boring allows for pipelines to be installed without disturbing highways and railroads.  It avoids damaging sensitive ecosystems such as lakes and wetlands.  And it allows pipelines to be installed under city blocks without disturbing the buildings and urban activities.

Now let’s take a closer look at the steps involved in completing the HDD process…

Horizontal directional drilling process steps

To run a pipeline under a body of water, highway, or a city block, it takes experts that know exactly how to get the job done safely and accurately.  HDD operators need to make sure the borehole steers down to the correct depth and back up to the surface at the right location.  They need to avoid hidden underground obstacles such as utilities.  They need to do it in a safe manner that will not collapse the borehole.  And they need to make sure the pipe string will pull through easily without becoming damaged in the process.

There are four main steps in the horizontal directional drilling process.  We are going to cover these four steps one by one and make this intriguing process not so mysterious.  Let’s get started…

Step 1: Pilot Bore—A pilot hole is bored through the designated path of the borehole

After locating the directional boring rig at the right location, a boring assembly is drilled into the ground.  A series of boring pipes are screwed together called the drill string.  Additional pipe lengths are added to the drill string until the borehole is completed.

The boring assembly depends on the type of geological formation.  It consists of some form of a boring bit, a steering tool, and a bent section of pipe called the “bent sub.”  To adjust directions, the bent sub is angled to orient the bit in the right direction.  To continue in the same direction, the bent sub and entire drill string are continually rotated, providing equalized drilling force at all rotation angles.

In soft geological formations, the bent sub orients a jetting assembly to hydraulically blast the dirt away on the desired side of the bit.  This moves the bit in the direction of least resistance and steers the bit through the designated boring path.  In hard formations, a hydraulic drill motor is used, with the drilling fluid rotating a turbine that turns the boring bit.  This way, the bent sub does not rotate, maintaining the proper bend angle while still rotating the drill bit.

The drilling fluid used for the jetting assembly or hydraulic drill motor performs many additional functions.  This mixture of water and bentonite clay keeps the boring bit cool by both pulling the heat out of the bit and lubricating the bit.  It also stabilizes the borehole by keeping it filled and pressurized.  Additionally, it keeps the boring bit free of the soil and bedrock cuttings, carrying them out of the borehole back to the drill rig location.  The drilling fluid is then separated from the cuttings and recycled back into the boring process.

After navigating through the designated drill path, the drill bit emerges at the opposite side of the borehole project.  The exit pit keeps the residual drilling fluid and tailings contained.  It also creates an easier entry point for the next step of the process, reaming.

Step 2: Reaming—The borehole is reamed to 150% the size of the pipeline

Once the borehole reaches the far side of the directional drilling operation, a reaming bit is attached to the drill string.  The borehole is reamed, enlarging it to about one and a half times the size of the pipeline to be pulled into the hole.

A single reaming pass may be sufficient if drilling into soft dirt.  However, consecutive passes may be needed when drilling into a hard formation or if the pipeline to be installed is of a wide diameter.

Step 3: Swab Pass—Run swab passes to clean out and lubricate the borehole

The last reaming operation is called a swab pass, or mud pass.  Swabbing the hole verifies the borehole is clear and ready to pull the pipeline through.  It will clean out any remaining tailings, as well as further compact and lubricate the walls of the hole.

This swab pass reaming tool is sized bigger than the pipe to be installed, but smaller than the largest reaming already accomplished.  As the hole is swabbed, the ease of the pull and the amount of debris is assessed.  Only one swab pass might be sufficient.  On other occasions, multiple passes may be required to make sure the hole is clear, secure, and well lubricated.

Step 4: Pullback—The pipe string is pulled through the borehole

Once the hole is sufficiently reamed and swabbed, the HDD process is ready for the next and last step, pulling the pipeline through the borehole.

Pipes are brought to the exit side of the borehole and positioned to be welded together.  After welding, they receive a protective coating and are quality tested.  High-pressure testing, also known as hydrostatic testing, ensures that no leaks occur under more severe pressure than the pipes will be experiencing once in operation.  X-ray tests find hidden imperfections in welds and correct them before any problems surface.

The completed pipe string is a little longer than the length of the borehole.  This pipe string is placed on rollers in preparation to be pulled through the borehole. It is then positioned to minimize the resistance as it is pulled through the borehole.

The pipe string is attached to the drill string and pulled through the borehole.  Once the pipe string emerges at the drill rig, it is again checked and tested to make sure no damage occurred during the pullback process. Once a successful pullback is completed, the worksites are cleared of equipment and restored to preexisting conditions.

Wrapping it up

Horizontal directional drilling is an intriguing process.  Being able to see and steer a horizontal bore bit that’s deep underground attests to how far technology has advanced.  When you break the process down to its four basic steps of boring the pilot hole, reaming it to size, swabbing the hole, and pulling back the pipe string, it all makes sense.

Hanging H is a leading directional boring company with decades of experience.  In addition to providing the HDD expertise, we are full-service midstream pipeline installers.  We can handle your pipeline project from feasibility reviews to ROW services, pipeline installation, horizontal drilling, and compressor station construction.  We’ve got you covered, aboveground and underground.

In our next article, we’ll check out nine benefits of using horizontal directional drilling. We’ll also see the world record in HDD reach and a video showing how they accomplished this great feat.

See the nine benefits of horizontal directional drilling

May 14, 2020

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