Putting HDD Power to the Project

Push, pull, turn, pump - the basic tasks associated with horizontal directional drilling (HDD) sound pretty straightforward. Yet maneuvering around buried obstacles you can't see, and boring under existing structures/natural masses you can't move - often in less than ideal soil conditions - is anything but. Without the appropriate torque, pullback, horsepower and drilling fluid mix/flow, even a basic bore can turn into a frustrating and costly venture with inadvertent returns, locked up holes and damaged and/or lost tooling and drill pipe.

HDD rigs are officially categorized according to pullback force. And torque has historically garnered all the hype and attention. But matching power to the project goes beyond these two high-profile specs.

"It's about balance," says Richard Levings at the Ditch Witch organization. "[It's a] balance between torque, pullback, horsepower and drilling fluid. It's about matching the entire system to a job. A good directional drill will have a balance of torque, pullback and mud flow with sufficient horsepower to drive them all simultaneously."

Torque and pullback
When considering a new machine, take a look at the specifications (torque, pullback, horsepower, pump capacity). Is there any one number that is extremely high?

"If there is, ask questions," advises Levings. "Do some research with the dealer to determine if, for example, there is enough horsepower to drive any unusually high specifications. Also talk to contractors who own the same or similar models. Let them offer guidance."

Torque and pullback specs are especially critical on "maxi machines" rated above 100,000 ft.-lbs. These units, which are used for half-mile or mile-long bores and for installing very large product, utilize large cutters on the reaming process, Levings notes. "In these types of bores, you're also typically going through a multitude of soil conditions," he adds. "At times, you might be in zones that don't require a lot of torque. But somewhere along the line, you will get into a situation where you need more."

For any size job, torque is needed to drive the cutting tool. In the field, it is influenced by factors such as ground conditions, product diameter/length and steering deviations. To match an existing machine to a particular jobsite, you need to consider how these factors affect its performance and how they can be managed to best utilize torque.

Generally, torque requirements will increase in difficult ground conditions, such as rock and clay and, to some extent, even sand. They will also increase when installing large-diameter product over long distances. Jobs that require you to frequently maneuver up and down or side to side will also increase resistance in the hole, and thus torque.

But a lot of torque in the hole isn't necessarily a good thing. Adequate amounts are needed to push through certain conditions, but too much can be detrimental to the project and the machine.

"Excessive torque in the hole will slow you down and work the machine harder than necessary," says Mike Nameth, Astec Underground. "You also run the risk of losing rotation; then the game's over. High rotary pressures translate into trouble somewhere - either in the hole or with the machine. You want to take it slow and steady."

Ed Savage, Vermeer, recommends selecting a rig with multiple settings for rotational torque and speed. "That gives you different torque and rpm ratings, so you can choose what fits best for the soil conditions," he says. "You can spin it faster and not use as much torque, or vice versa where you need a lot of torque but don't need to spin as fast. Look for flexibility of the machine."

That flexibility can be beneficial for projects that may require more torque, such as when you're installing large-diameter product and need a large back reamer. "Installing larger diameter product typically requires more torque to spin the tool and cut the earth," says Savage.

Pullback is directly related to torque. It is necessary to pull the weight of the product being installed, and for applying the cutter force to the face of the soil. It is typically affected by the same jobsite conditions as torque.

Higher pullback may be needed for installing heavier product over long distances. "The longer the hole you're trying to dig, the more drag or friction you will encounter," says Savage. "You will also want additional pullback with heavier product, such as steel pipe."

A good operator will watch the rig's gauges to determine at what point he's beginning to push the limits of the machine. "Machines are pretty much run with hydraulics," says Savage. "At a certain psi, they will stall out. But a good operator will monitor the gauges so that doesn't happen. He should never run at more than a third or a half of the machine's capacity when pulling back. It's more finesse than simply forcing product through the ground."

Without enough horsepower, you can't maintain all the necessary functions of a drill rig. "You can have high torque and pullback specs, as well as mud flow, but none of them will do you any good without the horsepower to turn them," says Levings. Conversely, you could have all the horsepower in the world; but without sufficient torque, pullback or fluid, all that horsepower would be wasted, and you may simply be burning excessive fuel.

Higher horsepower drills are needed for long bores in rocky soils. But it's not always necessary to have such high torque, pullback or fluid power.

"It's very dependent on the experience level and patience of the operator," says Levings. "There have been contractors who have installed product with equipment that shouldn't have been able to do it. But they were patient, methodical and willing to take the time to do the job right. Sometimes contractors can be tempted to throw large equipment at a job for some added security."

Mud management
In the end, balancing all the previous specs is for naught if you don't pay attention to fluid management. The right flow levels and mud mix recipes can make or break almost any bore, because the drilling fluid lubricates the cutter and the product you're installing. It also provides a medium for carrying spoils out of the hole.

"Mud is typically the key to success," says Nameth. "For the most part, it is the reason for failure and problems associated with directional drilling."

Mud suppliers and many drill manufacturers provide calculations to determine the appropriate amounts of mud, as well as the approximate length of time necessary to complete a bore in a given soil condition. "These are good pre-planning tools that will help you gauge how much mud product you need so you can be prepared," says Nameth.

For most small- and mid-size drilling rigs, fluid is delivered via pumps driven by the same engine as the drill rig. For larger machines, fluid pumps are driven separately. "It's critical to balance horsepower loads so you supply enough horsepower to do all the tasks you need it to do," says Levings. "You also have to have a pump with enough pressure and volume to sustain pressure drop at extended distances."

Flow ratings are important, since they affect pullback speeds. So you will want to make sure you have enough flow for the size of the hole. "If you don't have enough capacity, you will have to wait for the mud pump to get enough slurry in the hole," says Savage. "If you don't slow down, you will lock up the material in the hole. You basically outrun your drilling fluid. You end up with a dry hole and you can't pull the product any further."

Soil and water properties
It's important to do some research to identify what soils you will be working in, since this can influence the fluid mix required.

To ensure the greatest success on large bores, you will likely rely on formal geotechnical surveys and core samples taken at various lengths along the intended distance. But for bores of any size, it's also beneficial to talk with other contractors who have worked in the same area to determine the specific types of soils they have encountered.

"If you're trying to understand a particular soil condition such as sand, identify if it's wet and running sand or dry and compacted," says Savage. "Time of the year can also influence soils. In the Midwest, the summer months can be dry, so our clay can get very hard. But it's a different animal in the Spring and Fall when there's more moisture available. You will want to ensure you're talking about the same depths, since soil changes at various depths."

It can be beneficial to dig down a few feet and do your own visual inspection, as well. "Also pay attention to the initial cuttings coming from the pilot bore," suggests Tom Tibor, Baroid Industrial Drilling Products. "It can reconfirm your thoughts or serve as an initial check."

Mud mixes broadly fall into two categories: bentonite mixes designed for coarse, non-reactive soils, such as sand, rock and gravel; and polymer-based mixes for fine, reactive soils such as clay and shale.

For unstable soils such as sand, bentonite mixes build a wall around the tunnel to prevent water loss and intrusion, so the hole will stay open long enough for the tooling and product to pass through uneventfully. "This will also help reduce torque and pullback requirements," says Savage. "If that soil caves in or comes around the product you're pulling in, it acts like a piece of sandpaper when you wrap it around a steel rod. It grips down on it and locks up so you can't pull it or rotate it."

Plus, you can't get spoils out, adds Nameth. "You should be able to recognize rotary torques going up by watching the gauges," he notes. "And you will notice that it's getting harder to turn. At that point, stop. Don't keep drilling yourself into problems. You might get lucky and get through the pilot hole, but you will likely have problems on the ream. The hole may collapse entirely and you may lose tooling and drill pipe."

Fluid mixes are designed to combat the tendency clay soils have to swell and get sticky when hydrated with water alone. "Clay can swell up to 10 times its original size," says Savage. "It can settle around the drill stem and product pipe and lock it up so you can't move it. Polymers keep the clay from swelling and they also add a lot of lubrication to make the tooling spin and pull easier."

It's also important to identify properties of any water you use for mixing additives, notes Tibor. "Some water - even city water - may have a low pH with a lot of calcium," he says. "That can affect the ability of the additives (bentonite and polymers) to mix properly and be utilized to their full extent."

In that effort, Tibor encourages contractors to test the water for pH and hardness. In general, soda ash should be mixed into the make-up water to raise the pH and reduce excess calcium. The soda ash assists in creating an environment that will enhance the yield and performance of selected fluid additives and optimize drilling fluid properties. It also is key in minimizing any waste of the fluid additives due to not mixing completely.

Additional fluid
Drills rigs equipped with more than one fluid tank can prove beneficial. "That gives you the ability to mix in one and use the second one to supply the drill," Tibor says. The alternative is to stop drilling once you've used the fluid in the single tank, fill the tank with water and additives, then mix everything. "That takes time. And for optimum benefits, you should wait 15 to 20 minutes for the fluid to mix properly."

Oftentimes what happens is that contractors don't want to wait. "They mix on the fly. They keep drilling - fill the tank with water and mix," says Tibor. However, until the additives and water are thoroughly mixed, they are basically pumping water.

"Plain water doesn't have the properties you need for boring. It doesn't have the ability to suspend the cuttings that you create when you drill," Tibor points out. "And if you don't remove enough of the cuttings, you can get stuck, stretch the product, fracture the surface or hump the road because there simply isn't enough room for the product to fit. You don't save money by trying to get by without any mud. Fluid should be considered a tool, just like a bit or reamer."

Nameth agrees, noting, "It may be tempting to try to get away with minimal amounts of fluids or just water alone. That may work in small-diameter installations. But when you step up into installations above about 6 in., fluid management becomes more important.

"The process boils down to push, pull, turn and pump. No one part is difficult," he continues. "But if you miss one or two details, you will start to have problems. Take the time to do it right the first time, or you will have to make time to do it right the second time."

Sizing It Right
For Erv and Darrin Hoge, owners of Earth Energy, Boise, ID, the right balance of drilling specs is based heavily on what they've learned through personal experience. The brothers entered the directional drilling industry in the mid-1990s. Since then, they have owned numerous rigs used mostly for commercial boring. Their current fleet includes six directional drills that range from 10,000-ft.-lb. machines to their newest model, a 100,000-ft.-lb. unit purchased four months ago.

Determining which rig to send out on a particular job often hinges on ground conditions, says Erv Hoge. Soil conditions for many of the jobs consist of gravel, sand, cobble, hardpan and cemented cobble (similar to caliche), with very little topsoil. "Cemented cobble conditions are very abrasive," he says. "It's almost like drilling through a solid formation. It's a bit easier than drilling through rock, but it's still hard on equipment."

These conditions can sometimes force them to move up to a larger machine. Such was the case for some recent jobs where they opted to use the 100,000-ft.-lb. unit. "At the Capital in Boise, we used the larger machine because of the number of ducts involved," he says. "We also weren't always certain what soil conditions we would encounter at depths of 17 to 18 ft. We knew that machine would be able to get the job done."

On a job for Idaho Power in nearby Napa, the brothers also opted for the bigger unit, largely due to the diameter (18 in.) and length (500+ ft.) of pipe that needed to be installed. "The water table and sandy conditions were also issues," Erv explains. "We needed to have a machine that could handle the extra pullback force when we needed it."

While the Hoges are seeing a trend toward larger product sizes, they will place anything from 3/4-in. water lines to 20-in. water mains. Pullback power is necessary when placing larger product at long distances. "But if we develop the hole with the right torque and mud flow for the ground conditions, we can sometimes get away with a smaller machine to pull back product," says Erv. "It can be a bit forgiving if we do everything else right and have patience."