Maximise HDD accuracy with combined guidance technologies

Vector Magnetics director of marketing Jed Sheckler discusses in Trenchless Australasia how the trenchless industry can palliate this pressure through horizontal directional drilling (HDD). HDD has become an essential part of the expansion and replacement of critical data, electrical, water, energy, and waste networks, with proven effectiveness across a diverse range of installations.

Exploring the practical limits of HDD accuracy has become a popular topic of discussion within the trenchless community. In a year when both the North American Society for Trenchless Technology (NASTT) and European Drilling Contractors Association (DCA) are updating their best practices and guidelines, it is important to have a clear understanding of the role of guidance systems in maximising the accuracy of the HDD process.

Guidance overview

HDD guidance systems have generally been classified as either walkover (small rig) or wireline (large rig). However, the term wireline simply refers to one type of power and data transmission method which may be utilised by rigs of any size. The term downhole more appropriately describes a guidance system capable of operating without surface access, and with no inherent depth limitation. Downhole systems may be classified as magnetic downhole, gyroscopic downhole or combo systems. As shown in Table 1, there are several methods of guiding a HDD pilot bore. While details may differ depending on the manufacturer’s approach, a complete HDD guidance system must provide the following capabilities:

• Steering puts the directional in HDD: The drilling assembly is fitted with an offset bit, sub, or motor. An accelerometer in the steering tool informs the driller of the current inclination and roll-angle. Orienting the offset and pushing (sliding) the drill string without rotation provides directional control of the borehole.
• Surveying calculates the location of the pilot bore as it progresses: Surveys are taken at discrete positions (typically when making a drill rod connection) when there is no fluid flow, and the sensors are most stable. HDD survey tools (downhole magnetic or gyro) measure the orientation of the tool relative to the earth’s poles (azimuth) and gravitational field (pitch). These values are combined with total drill pipe length (measured depth) to calculate the current position relative to a known starting point, typically the bore entry point (Figure 1).
• Tracking verifies the calculated survey position by making direct measurements relative to external reference points. A tracking source (ParaTrack2 electrical cable, Beacon Tracker, Microcoil, etc.) is placed at a known location on surface or run through an adjacent, completed conduit. The source is periodically electrified and emits an electro-magnetic signal sensed by the magnetometers in the survey tool. The signal is emitted from a known location, at a known intensity, at a known frequency, providing a series of additional tie points that greatly increases the accuracy of the surveyed position (Figure 2).

Sources of error

While technological advancements have greatly improved the quality of HDD surveys, it is important to understand the types of errors that may act independently or in combination to influence the accuracy of an HDD guidance system. Random errors are unpredictable and may appear as though no one sample repeats (low precision).

However, the impact of random errors is minimised by increasing the sample size and averaging. Adequate sampling makes it possible to obtain accurate answers from highly random data sets. Systematic errors are consistent and reproducible, imparting a bias to the data. When drilling on survey data alone, without the benefit of additional tie points from an external tracking source, systematic error will typically become the main source of positional uncertainty over long-distances (>300m). Gross errors are, in short, typically caused by human error.

Gross errors can be especially difficult to detect as they cannot be modeled or effectively planned for. Gross errors include input errors (moving a decimal point), transcription errors (flipping a sign, transposing a digit), or operational errors due to a lack of training, etc.

Ellipse of uncertainty

While all three types of errors may act independently or in combination, it is widely accepted that the majority of borehole surveying uncertainty is due to systematic errors in the surveying system. Small errors in inclination, azimuth, and depth systematically accumulate. This accumulation forms an ellipse of uncertainty regarding the true position of an inclined borehole that grows as a function of drilled length (Figure 3).

Maximum accuracy, minimum risk

The positional uncertainty that results when a pilot bore is guided from a single tie point presents a risk that all parties must be aware of when considering the available technology. A guidance system combining a precision survey (gyro or magnetic) with secondary tracking (ParaTrack2 electrical cable, Beacon Tracker System, Microcoil) that identifies and corrects for any induced error results in the highest achievable accuracy for HDD (Figure 4). Modern HDD job sites fuse the latest technological advancements with the realities of getting product in the ground.

Above all is the need to identify potential sources of risk that could slow progress, result in an accidental strike, or worse yet, cause the project to fail entirely. A clear understanding of the capabilities and limitations of available guidance technology ensures constructable designs with tolerances that can be realistically followed, maximising the chances of a successful outcome.

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