Horizontal Directional Drilling (HDD): How the Drill Bit is Steered

Planning and Navigation

Successful HDD projects begin with thorough planning to ensure accuracy and efficiency. Before drilling starts, HDD professionals assess soil composition, map the bore path, and configure the necessary steering tools.

A pre-planned route is essential for successful drilling. Professionals review urban plans and use specialized software to determine the most efficient path while considering ground conditions and obstacles such as existing underground utilities.

During drilling, the operator adjusts the drill’s direction in real-time using steering tools, guided by a teammate known as the ‘tracker.’ The tracker operates the location system, ensuring the drill maintains precise depth and direction.

Steering in Soft Formations

When drilling through loose soil or soft formations, a specialised drill bit with an asymmetrical leading edge is used. This allows for directional steering by applying thrust without rotation, creating a natural bias toward a specific direction.

How Soft Formation Steering Works:

• The drill bit’s leading edge is asymmetrical, creating a natural directional bias.
• A transmitter sonde, housed in protective casing, sends real-time positional data to the operator.
• The operator aligns the drill bit using a clock-face system: 12 o’clock steers up, 9 o’clock steers left, 6 o’clock steers down, etc.
• By applying thrust without rotation, the drill bit moves in the desired direction.
• Once the adjustment is complete, the operator rotates the drill bit while applying thrust to maintain a straight bore path.

This method ensures a controlled and uniform drilling path through soft ground conditions.

Steering in Hard Formations

In rock or compacted soil, a rock motor or air hammer is required to break through the material while maintaining directional control.

Steering with a Rock Motor:

• A bent sub at the drill head houses an internal drill motor, which allows the drill bit to rotate independently of the outer drill string.
• The bent sub is oriented in the desired direction before drilling.
• The drill string is pushed forward, following the bent sub’s deflection.
• Continuous monitoring allows for precise adjustments to maintain trajectory.

This method provides greater control and accuracy in challenging ground conditions.

Steering with an Air Hammer:

• Works similarly to the rock motor but uses compressed air to activate a pneumatic hammer.
• The air pressure drives the hammer action, breaking through tough rock formations.
• The bent sub is rotated to align with the desired direction before drilling forward.

Both systems allow HDD operators to maintain precision in hard ground conditions where traditional drill bits would struggle.

Real-Time Monitoring and Adjustments

Throughout the drilling process, guidance software and downhole sensors provide continuous data on depth, angle, and trajectory. Even small deviations are corrected immediately to maintain accuracy, ensuring that the bore path stays on course.

Conclusion

HDD is not just about drilling—it’s about precision, planning, and efficiency. The ability to steer a drill bit accurately through varying soil conditions makes HDD an indispensable technique in modern underground construction.

Whether working in soft formations or tackling hard rock with rock motors or air hammers, mastering steering techniques is essential for success.

For the best results in HDD projects, having the right equipment is just as important as skill and strategy.

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