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.