Underground steering is an essential component of horizontal directional drilling (HDD). It is very rare to have a project where there are no other underground installations already in place, some of which may be undocumented. Steering enables you to navigate past these obstacles.
The tighter the steering control, the closer you can be to other installations without hitting anything.
Another benefit of modern steering systems is the record keeping of the actual underground route. This record makes future trenchless projects in the area much simpler because of the known underground obstacles at any point.
The Basic Principles of Underground Steering
Steering an HDD rig assumes access to the following:
- Knowledge of the direction the drill is pointing in.
- The ability to adjust this direction.
- Calculated or externally verified drill position underground.
When an HDD rig is drilling the pilot hole, it uses a non-rotating drill string. The drill bit itself has an asymmetrical leading edge. In order to change the direction, the drill string is turned so that the leading-edge faces in the new direction. The method is simple and allows operators to change the direction of the drill at any time during the process.
The more difficult task is to know the actual position of the drill and how much the direction needs to change. Generally speaking, the direction of a drill is set using two parameters. Azimuth gives an indication of the direction of the drill relative to North, while inclination indicates whether the drill is pointing up or down and to what extent.
There are two main types of HDD steering tools. The gyro tool uses fiber optic gyroscopes and accelerometers to calculate the position of the drill, while the magnetic tool uses magnetic fields. Wires connect the steering tools to equipment on the surface where calculations are made, and adjustments implemented.
Gyro steering tools
Gyro steering tools have the advantage of not relying on any form of magnetic field. This is helpful in projects where external magnetic fields exist, which would interfere with a magnetic steering tool. High voltage cables, railways and cathodic protection systems can all interfere with a magnetic steering tool.
However, gyro tools have disadvantages too. There is no way to externally verify the gyro tool’s position under the ground. Their insensitivity to magnetic fields also means that they cannot pick up other underground structures and may collide with them if their location is unknown to the operator.
Gyro steering systems tend to be expensive and require trained operators to use them. However, they are quicker to deploy because they do not need magnetic coils installed on the surface.
Magnetic Wireline Steering Tools
Magnetic wireline steering tools rely on magnetic fields to determine their position and direction. A major advantage is the ability to track the drill from the surface using a coil and its magnetic field. It’s important for the actual steering tool to be located in a magnetic collar so that no other magnetic fields interfere with its readings.
Of course, infrastructure or environmental elements like lakes may prevent tracking from the surface.
Magnetic steering systems are cheaper and simpler to use than gyro systems, but the setup time is longer due to the surface tracking system.
Tracking an HDD drill from the surface is a major contributor to steering accuracy. The walkover system actually provides the most accurate steering because it continuously monitors the actual position of the drill from the surface. However, this method is impractical in most applications due to the presence of infrastructure or lakes etc., which make standing over the drill impossible.
The next best tracking method uses a coil on the surface to generate a magnetic field. Magnetic sensors on the drill provide readings that allow operators to locate the drill precisely. The actual position is then used to update the steering calculations. These adjustments ensure that the steering system is always using the actual position as the base for determining what adjustments in steering should be made.
It is important to note that there is no method available to externally verify the position of a gyro steering tool. Because magnetic fields do not have any effect on the gyro, this steering method relies on calculations only. It is not possible to offset any errors during the drill and any error that exists in the system will be compounded along the drill route. For this reason, some trenchless practitioners consider the magnetic steering solution to be the most accurate.
DC loops for magnetic tracking systems provide the highest levels of accuracy. This is because the tracking system uses both sides of the wire loop to calculate position. An AC system uses only one side of the wire loop to calculate position, but the signal from the return wire could interfere with this signal reducing the accuracy.
The future of steering
Underground steering is an essential component of HDD success. As such, the technology for gyro and magnetic steering systems is constantly improving. There is no one solution that is right for every application. Engineers and drilling companies must assess the specific conditions for each job and recommend the system that is best suited.
Both gyro and magnetic systems offer benefits for HDD operators, but the tracking ability of magnetic sensors makes them the most accurate steering system on offer at this stage.