The ability to locate the drill head is paramount to the success of any bore. In the early days of horizontal directional drilling, the bore often ended with the drill head coming out in the middle of the street, missing the intended target. As the industry developed, locating systems became more sophisticated. Now, there are several manufacturers that specialize in locating equipment, and it is a very competitive industry. Because of the competition, locator technology is constantly advancing. Today, the locator technology is to the point where it is possible to install sewer lines on grade using HDD.
The type of locator most commonly used in HDD is a walkover system. Walkover systems, as the name implies, require that the locator operator "walk over" the top of the drill head with the receiver to determine the depth and position of the drill head.
Walkover systems consist of two major components, a receiver and a sonde. The sonde, also known as a transmitter or probe, fits inside the drill head and is powered by "C" cell batteries. The sonde sends out a signal that is picked up by the receiver. The receiver processes this signal into three important pieces of information: PITCH, ROLL, and DEPTH.
Pitch is the inclination of the drill head and can be expressed in degrees or as a percent of slope, depending on the locator being used. Pitch as a percent of slope is used most often in the field. If the pitch is zero, the drill head is level from end to end. If there is a minus pitch reading, the drill head is pointing down. A positive reading means the drill head is coming up. By knowing what the pitch reading is, it is easy to calculate how much depth change there will be in the next 10 feet. For example, if the pitch reading is +10 percent, the head will rise one foot in the next 10 feet that the bore advances.
Roll is the rotational position of the duckbill and is very important when making a steering correction. Steering changes are made by thrusting forward without any rotation. The rotational position of the duckbill determines which direction the drill head will move. Roll is commonly referred to as the "clock face" in the field. When the operator of the drill rack faces the direction that the drill is advancing, 12 o'clock is up, 6 o'clock is down, 3 o'clock is right, and 9 o'clock is left. The duckbill can be positioned to move two directions at the same time. An example is the 2 o'clock position. This would cause the drill head to move mainly to the right and a little up. A 7 o'clock steer would cause the drill head to move mainly down and a little to the left. Because of patents, some locator manufacturers have broken the clock face into more segments. However, the vast majority of the operators in the field use a 12-segment clock when giving and receiving steering commands.
To determine the depth of the drill head, the receiver must be directly over the top of it. The receiver converts the signal coming from the sonde and displays the depth. Depending on the manufacturer, depth can be displayed in inches or feet and inches. Most locators can also be programmed to display depth in metric.
A third and important part of a walkover locator package is the remote display. Remote displays attach to the drill rack and display the pitch, roll, and depth information that is on the receiver. This makes it possible for the drill rack operator to see what the locator operator is seeing. They are very useful for the drill rack operator when trying to position the drill head at a specific clock position or for determining how much of a pitch change is being made while steering.
Wire-line locating systems are similar to walkover systems. The major difference is the sonde itself. Instead of being powered with "C" cell batteries, power is provided through an insulated wire that exits the rear of the sonde. The wire runs through the drill stem and exits at the rack. The wire is then attached to a 12V or 24V automotive-type battery. Consequently, the battery life of the sonde is not a consideration since the battery is above ground and can be recharged without pulling the drill stem out of the hole. Another advantage of the wire-line system is that the drill head can be located at depths of up to 100 feet. Obtaining depth readings and determining the location of the drill head are done the same as when using a walkover system. However, the pitch and roll information is transmitted up the same wire that powers the sonde and this information goes directly to the remote display on the rack. Wire-line systems are particularly useful for extremely long or extremely deep bores. Boring with a wire-line system is more time-consuming than using a standard walkover system because of the time required to attach a new section of wire every time the rod is changed.
Steering ToolsSteering tools are a locating technology that has been adapted from the oil field. Because of their physical size, they are somewhat cumbersome to work with. However, they are very accurate and the head can be located from the drill rack without walking over the drill head. Since all of the data is transferred either through the drill stem or through a wire, it is possible to drill in areas where interference is too great for a walkover-type system. They are also good for deep bores where accuracy is crucial. The major downside of a steering tool is the price. They can be five to 10 times higher priced than a walkover system, and they require substantially more training to operate. For these reasons, they are not used for normal drilling applications.