How Do You Steer a Drill Below The Earth?

In December 2019, the City of Fort Lauderdale,  Florida experienced a series of catastrophic ruptures in a critical wastewater transmission  line, releasing raw sewage into local waterways and neighborhoods. Recognizing the need for  improvements to their aging infrastructure, the City embarked on a plan to install  a new pipeline to carry sewage from the Coral Ridge Country Club pumping station  across 7 miles (or 12 kilometers) to the Lohmeyer Wastewater Treatment Plant.  But just drawing a line on the map hides the enormous complexity of a project  like this.

Installing an underground pipeline through the heart of a major urban area while  crossing three rivers is not a simple task. Underground utilities are usually installed by  a technique known as trenching. In other words, we excavate a trench down from the surface, place  the line, backfill the soil, and repair whatever damage to the streets and sidewalks remains.

That  type of construction is profoundly disruptive, requiring road closures, detours, and pavement  repairs that never quite seem as nice as the original. Trenches are also dangerous for the  workers inside, so they have to be supported to prevent collapse. Beyond the human risk,  in sensitive environmental areas like rivers and streams, trenching is not only technically  challenging but practically unachievable because of the permits required.

In fact, trenching  in urban areas to install pipelines these days is for the birds. When the commotion of  construction must be minimized, there are many trenchless technologies for installing pipes  below the ground. One of those methods helped Fort Lauderdale get a 7-mile-long sewer built in less  than a year and half, and is used across the world to get utility lines across busy roadways and  sensitive watercourses.

I’m Grady and this is Practical Engineering. On today’s episode, we’re  talking about horizontal directional drilling. This video is sponsored by Curiosity  Stream and Nebula.

More on them later. If you’ve ever seen one of these machines on  the side of the road, you’ve seen a trenchless technology in action. Although there are quite  a few ways to install subsurface pipelines, telecommunication cables, power lines, and sewers  without excavating a trench, only one launches lines from the surface.

That means you’re much  more likely to catch a glimpse. Like laparoscopic surgery for the earth, horizontal directional  drilling (or HDD) doesn’t require digging open a large area like a shaft or a bore pit to get  started. Instead, the drill can plunge directly into the earth’s surface.

From there, horizontal  directional drilling is pretty straightforward, but it’s not necessarily straight. In fact, HDD  necessarily uses a curved alignment to enter the earth, travel below a roadway or river,  and exit at the surface on the other side. Let me show you how it works and at the end, we’ll  talk about a few of the things that can go wrong.

The first step in an HDD installation is  to drill a pilot hole, a small diameter borehole that will guide the rest of the  project. A drill rig at the surface has all the tools and controls that are needed.  These rigs can be tiny machines used to get a small fiber-optic line under a roadway or  colossal contraptions capable of drilling large-diameter boreholes for thousands  of feet at a time.

As such, many of the details of HDD vary across projects, but the  basic steps and equipment are all the same. As the drill bit advances through the earth,  the rig adds more and more segments of pipe to lengthen the drill string. Through this pipe,  drilling fluid is pumped to the end of the string.

Drlling fluid, also known as mud or slurry,  serves several purposes in an HDD project. First, it helps keep the drill bit lubricated and  cool, reducing wear and tear on equipment and minimizing the chances of a tool breaking  and getting stuck in the hole. Next, drilling fluid helps carry away the excavated soil or rock,  called the cuttings, and clear them from the hole.

Finally, drilling fluid stabilizes and seals  the borehole, reducing the chance of a collapse. I have here an acrylic box partly full  of sand, a setup you’re probably quite familiar with if you follow my channel. Turns  out a box of sand can show a lot of different phenomena in construction and civil engineering. 

Compared to soils that hold together like clay, sand is the worst case scenario when it comes  trying to keep a borehole from collapsing. If I pull away this support, the simulated  borehole face caves in no time. If I add groundwater to the mix, the problem is even  worse.

Pulling away the support, the wall of my borehole doesn’t stand a chance. Let me  show you how drilling fluid solves this problem. I’m mixing up a handcrafted artisanal batch of  drilling mud, a slurry of water and bentonite powder.

This is a type of clay created by  volcanic ash that swells and stays suspended when mixed with water. It’s pretty gloopy stuff,  so it gets used in cosmetics and even winemaking, but it’s also the most common constituent in  drilling fluids. If I add the slurry to one side of the demo, you can see how the denser  fluid displaces the groundwater.

It’s not the most appetizing thing I’ve ever put on camera, but  watch what happens when I remove the rigid wall. The drilling fluid is able to support the face  of the sand, preventing it from collapsing. In addition to supporting the sand, the drilling  fluid seals the surface of the borehole to reduce migration of water into or out of the interface. 

In most HDD operations, the drilling fluid flows in through the drill string and back out of the  borehole, carrying the cuttings along toward the entry location where it is stored in a tank or  containment pit for later disposal or reuse. So far HDD follows essentially the same steps as  any other drilling into the earth, but that first ‘D’ is important. Horizontal directional  drilling means we have to steer the bit.

The drill string has to enter the subsurface from  above, travel far enough below a river or road to avoid impacts, evade other subsurface  utilities or obstacles below the ground, and exit the subsurface on the other side  in the correct location. I don’t know if you’ve ever tried to drill something, but  so far when I do it, I’ve never been able to curve the bit around objects. So how is it  possible in horizontal directional drilling?

There are really two parts to steering a drill  string. Before you can correct the course, you need to know where you are in the first  place, and there are a few ways to do it. One option is a walkover locating device that  can read the position and depth of a drill bit from the surface.

A transmitter behind the bit  in the drill string sends a radio signal that can be picked up by a handheld receiver. Other  options include wire-line or gyro systems that use magnetic fields or gyroscopes to keep track  of the bit's location as it travels below the surface. Once you know where the bit is,  you can steer it to where you want it to go.

I’ve made up a batch of agar, which is a  translucent gel made from the cell walls of algae. I tried this first in the same acrylic box,  but the piping hot jelly busted a seam and came pouring out into my bathtub, creating a huge mess.  So, you’ll have to excuse the smaller glassware demo.

My simulated drill string is just a length  of wire. There are two things to keep in mind about directional drilling: (1) Although they seem  quite rigid, drill pipes are somewhat flexible at length. If I take a short length of this wire  and try to bend it, it’s pretty difficult, but a longer segment deflects with no problem.

And, (2)  you don’t have to continuously rotate the drill string in order to advance the borehole. You can  just push on it, forcing the bit through the soil. My wire pushes through the agar without much  force at all, and a drill string can be advanced through the soil in a similar way, especially  when lubricated with water or drilling fluid.

The real trick for steering a drill string is  the asymmetric bit. Watch what happens when I put a bend on the end of my wire and advance  it through the agar. It takes a curved path, following the direction of the bend.

If I rotate  the wire and continue advancing, I can change the direction of the curve. The model drill string is  biased in one direction because of the asymmetry, and I can take advantage of that bias to  steer the bit. I can steer the string left, then rotate and advance again to steer the bit  to the right.

I’m a little bit clumsy at this, but with enough control and practice, I could  steer this wire to any location within the agar, avoid obstacles, and even have it  exit at the surface wherever I wanted. This is exactly how many horizontal directional  drills work. The controls on the rig show the operator which way the bit is facing.

The  drill string can be rotated to any angle (called clocking), then advanced to change the  direction of the borehole. Sometimes a jet nozzle at the tip of the bit sprays drilling fluid  to help with drilling progress. If the nozzle is offset from the center, it can help create a  steering bias like the asymmetric bit.

Just like the Hulk’s secret is that he’s always angry, a  directional drill string’s secret is that it’s always curving. The rig operator’s only steering  control is the direction the drill string curves. And hey, if that sounds like something you’d like  to try for yourself, my friend Dan Shiffman over at the Coding Train YouTube channel built a  2D horizontal directional drilling simulator.

This is an open-source project, so  you can contribute features yourself, but it’s also really fun if you  just want to play a few rounds. If you’re into coding or you're wanting to get  started, there is no better way than working through all the incredible and artistic examples  Dan comes up with for his coding challenges. Go check him out his video on HDD after  this one, and tell him I sent you.

Once the drill string is headed in the right  direction, it can just be continuously rotated to keep the bit moving in a relatively  straight line. The pilot hole for an HDD project is just an exercise in checking the  location and adjusting the clock position of the drill string over and over until  the drill string exits on the other side, hopefully in exactly the location you  intended. But, not all soil conditions allow for a drill string to simply  be pushed through the subsurface.

Rocky conditions, in particular, make  steering a drill rig challenging. An alternative to simply ramming the bit through  the soil is to use a downhole hydraulic motor. Also known as mud motors, these devices convert  the hydraulic energy from the drilling fluid being pumped through the string to rotate a  drill bit that chews through soil and rock.

This allows for faster, more efficient drilling  without having to rotate the whole drill string. The housing of the mud motor is  bent to provide steering bias, and the drill string can be clocked to  change the direction of the borehole. Once the pilot hole exits on the other side, it  has to be enlarged to accommodate the pipe or duct.

That process is called reaming. A reamer is  attached to the drill string from the exit hole and pulled through the pilot toward the drill  rig to widen the hole. Depending on the size of the pipe to be installed, contractors may ream  a hole in multiple steps.

The final reaming is combined with the installation of the pipeline.  This step is called the pull back. The pipe to be installed in the borehole is lined up on  rollers behind the exit pit.

The end of the pipe is attached to the remaining assembly, and the  whole mess is pulled with tremendous force through the borehole toward the rig. Finally, it can be  connected at both ends and placed into service. That’s how things work when everything goes  right, but there are plenty of things that can go wrong with horizontal directional drilling too. 

Parts of the drill string can break and get stuck in the pilot hole. Drlling can inadvertently  impact other subsurface utilities or underground structures. The pipeline can get stuck or damaged  on pullback.

Or, the borehole can collapse. The controversial Mariner East II pipeline in  Pennsylvania experienced a litany of environmental problems during its construction between 2017  and 2022. Most of those problems happened on HDD segments of the line and involved  inadvertent returns of drilling fluid.

That’s the technical term for the situation when  drilling fluid exits a borehole at the surface instead of circulating back to the entrance  pit. The inadvertent returns in the Mariner East II line created water quality issues in  nearby wells, led to sinkholes in some areas, and spilled drilling fluid into sensitive  environmental areas. The pipeline owner was fined more than $20 million over the course of  construction due to violations of their permits, and they are still mired in legal battles and  extreme public opposition to the project to date.

In the case of Mariner East II, most of  the drilling fluid spills were partially related to the difficult geology in Pennsylvania.  Clearly HDD isn’t appropriate for every project. But in most cases, trenchless technologies  are the environmentally-superior way to install subsurface utilities because they  minimize disruptions on the surface to the people in urban areas and sensitive  habitat around rivers and wetlands.

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