Common Deviations in Horizontal Directional Drilling

I. Pilot Hole Trajectory Deviation
This is the most common and impactful type of deviation, manifesting as the actual drilling path deviating from the design curve. Main forms include:

1. Lateral Offset: The borehole deviates left or right from the design centerline, usually caused by external magnetic field interference or inaccurate rig positioning.

2. Longitudinal Offset: The drilling depth does not match the design, being too deep or too shallow, often due to uneven geological conditions or improper operating parameters.

3. Angular Deviation: The entry or exit angle does not meet design requirements, affecting pipeline slope and connection quality.

II. Borehole Undulation (Pipeline Undulation)
During drilling, obstacles or changes in strata at the drill tip can cause the drill bit to "raise its head" or "lower its head," creating a wavy borehole. This deviation increases pullback resistance and can even cause stress concentration in the pipeline, affecting its service life.

III. Drill Bit Deviation During Pullback

Even if the pilot hole is qualified, path deviation may still occur during the pullback stage. Reasons include:

Insufficient hole enlargement, resulting in steps or narrowing of the borehole;
Insufficient mud lubrication, increasing friction and causing deviation in the dragging direction;
Ground instability after disturbance, leading to local collapse and alteration of the original path.

IV. Causal Analysis

1. Equipment and Measurement Factors: Drill rig positioning deviation, inaccurate guidance system calibration, measurement errors, etc.

2. External Interference: Magnetic field interference from high-voltage lines and underground metal structures affects the accuracy of magnetic azimuth data.

3. Geological Conditions: Complex geological areas such as alternating hard and soft strata, pebble layers, and fault zones are prone to drill bit deviation.

4. Human Operation: Inexperienced operators failing to correct deviations in a timely manner or improperly adjusting parameters.

V. Control and Prevention

1. Accurately lay out the drilling data before construction to ensure that the drilling rig, entry point, and exit point are aligned in a straight line;

2. Measure the trajectory data every 3 meters of drilling, and immediately correct any deviation greater than 0.3 meters;

3. Use wired sensors or ground beacon systems to improve positioning accuracy in areas with strong interference;

4. Optimize the mud mix ratio and pump volume to maintain borehole stability and reduce the risk of drift.