Microduct Installation-A Method To Reduce Curve Deviation In Horizontal Directional Drilling(HDD).

Horizontal directional drilling (HDD) is a construction machine used to lay various underground public facilities without excavating the ground, such as microducts and cables. Based on the problems encountered during underground crossing construction with HDD, this paper analyzes the causes of deviation of the guide hole from the original designed curve during the crossing construction, and summarizes some methods to reduce curve deviation based on practical work experience.

Technical Characteristics and Application Prospects of Horizontal Directional Drilling (HDD).

Horizontal directional drilling (HDD) has been widely applied in the laying of pipelines for oil, gas, natural gas, water supply, power supply, and communications, and can generally be constructed in non-hard rock areas. Horizontal directional drilling technology offers a series of advantages such as low cost, fast construction, and high construction precision.

Over the past 20 years, this technology has achieved rapid development and is currently evolving in multiple directions, including miniaturization and large-scale application, adaptability to hard rock construction, automatic drill pipe extraction and stacking, and ultra-deep guidance. Its application prospects will be extremely broad.

Deviation of the guide hole from the original designed curve during the drilling process.

The first critical step in horizontal directional crossing technology is drilling the guide hole. The quality of the guide hole directly affects the success of pipeline backhaul and is a major factor determining the project's success. The primary purpose of the guide hole is to provide a pilot hole that meets the original design requirements. All pre-expanded holes at each level that meet the crossing requirements should be widened with the curve of the guide hole as the axis to achieve the diameter suitable for backhaul conditions. Therefore, whether the overall curve of the guide hole can meet the elastic radius of the entire pipeline directly determines whether pre-expansion at each level can achieve the elastic radius of the entire pipeline. The successful design of the guide hole serves as the foundation for the successful backhaul of pre-expanded holes at subsequent levels and is one of the critical components of the entire crossing project. However, during the drilling and crossing process of the guide hole, deviations from the original designed curve often occur.

Based on years of work experience with horizontal directional drilling technology, the causes of such deviations can be summarized into the following five points.

(1)Inevitable external factors.
such as terrain, underground soil moisture, and formation conditions cause deviations in the construction crossing curve.  

(2)Interference from external magnetic fields.
For example, high-voltage lines, underground optical cables, and formation conditions can all affect the magnetic azimuth angle during guide hole drilling, leading to deviations between the data displayed on the computer and the original design curve data.

(3)Deviations between the initial positioning angle of the drilling rig and the designed crossing centerline of the pipeline, which require correction of deviations as soon as the guide hole starts drilling.

(4)Caused by drilling rig operators.
Improper operation during guide hole drilling leads to deviations between the crossing trajectory and the design curve.

(5)Measurement errors.
including inaccuracies in measuring the magnetic azimuth angle of the crossing centerline, inaccuracies in measuring the elevation difference between the entry point and exit point, and inaccuracies in the drill pipe length input into the computer.

The above points are the main causes of deviations between the guide hole and the design curve during drilling. With a clear understanding of the causes, targeted solutions should be adopted to find methods to reduce curve deviations. Below are some methods to address these issues.

Methods to Reduce Deviations Between the Guide Hole Curve and the Design.

1. Use artificial magnetic field to reduce the deviation of the guide hole curve

The control system of horizontal directional drilling is the geomagnetic field, and the data of the guide hole is transmitted to the computer through the probe needle behind the drill bit. However, the geomagnetic field is very weak and easily affected by the magnetic field of other objects in the outside world, resulting in inaccurate magnetic azimuth data. The artificial magnetic field is created by adding a strong direct current and then setting a closed coil on both sides of the crossing center line. Therefore, the magnetic field generated by the artificial magnetic field is much larger than the magnetic field of the geomagnetic field and other objects, so the interference will be much smaller. It can even be ignored. This artificial magnetic field is simple to lay, but it is very effective. And it is economical. The artificial magnetic field can accurately reflect the specific position of the probe in underground work, the height of the crossing point, and the left and right offsets. The data obtained is a reflection of the true position of the probe.

When the probe of the horizontal directional drilling reaches the closed coil of the artificial magnetic field, the DC power supply is connected to generate a magnetic field. The offset of the crossing axis and the elevation difference are measured by this artificial magnetic field. And the azimuth of the probe at this time can be determined by comparing the left and right deviations of the artificial magnetic field and the geomagnetic field. Thus, the azimuth of the next drill rod is measured. In addition, since the artificial magnetic field can accurately measure the azimuth of the pipeline crossing under the interference of the geomagnetic field, and can correct the steering magnetic azimuth without being disturbed by the geomagnetic field, it can better control the deviation between the crossing curve and the designed curve. And the artificial magnetic field better ensures the smoothness of the crossing curve.

2. The interference of external magnetic field should be reduced.

The external magnetic field mainly includes underground pipelines, underground cables and some rigid buildings. These external magnetic fields will affect the strength and azimuth of the geomagnetic field, thereby affecting the azimuth of the control drill bit. The uncertainty and inaccuracy of the azimuth will directly affect the azimuth of the pilot hole drilling into the ground, making the direction of the pilot hole out of control. Therefore, in response to this problem, a field survey should be carried out when drilling to determine the range affected by the external magnetic field. In this way, the offset can be measured and the offset can be controlled during the drilling of the pilot hole.

When entering the external magnetic field, the azimuth has changed and is different from the steering azimuth. At this time, the interference is ignored and the drilling is carried out directly. However, when measuring the data, when the drill bit passes through the magnetic field interference area, the computer steering data can return to normal. The error at this time should be within the allowable range. However, if the difference between the two is large, the deviation is calculated first, and then the drill bit is pulled out and drilled again within the deviation range to combine the crossing curve with the design curve.

3. The position of the drilling rig must be combined with the center line of the crossing.

Before the drilling starts, the ground anchor must be installed first, and the position of the ground anchor and the drilling rig must be consistent. Therefore, when installing the ground anchor, the ground anchor system should be placed on the center axis of the crossing system. This ensures the precise positioning of the drilling rig. Before the drilling rig is in place, use a measuring instrument to release the pipeline to cross the center line, calculate the exact position of the drilling rig based on the drilling rig's entry angle, the size of the drilling rig itself, and a series of reference data, and mark it with white lime. After marking, not only should the white lime mark be used as a basis, but also the drilling rig's deviation should be calculated using a measuring instrument. The deviation is calculated to be zero when the drilling rig enters the soil, thereby ensuring the combination of the trajectory of the guide hole with the design curve.

4. Eliminate the phenomenon that the guide hole deviates from the original design curve due to human factors.

Before drilling, relevant personnel, such as surveying and laying-out personnel, direction control personnel and drillers, must be adequately trained to improve the quality of employees. Let employees cooperate with each other to prevent the deviation of the guide hole crossing curve from the design curve caused by human factors.

5. Accurate measurement must be ensured.

Accurate measurement includes three main aspects: accurate measurement of magnetic azimuth, re-measurement of the crossing centerline, and accurate measurement of drill rod length. Magnetic azimuth is the most important engineering data in directional drilling and is an important factor in ensuring the smooth curve of the guide hole. Therefore, the magnetic reverse azimuth must be carefully measured before the guide hole starts working. The original azimuth value of the guide hole control is the magnetic azimuth of the crossing centerline. Therefore, the magnetic azimuth must be accurate.

This is the most important step in horizontal directional drilling construction. The second accurate measurement is to review the crossing centerline. Before drilling, the intersection piles must be designed and the entry and exit points must be staked. The distance between the entry and exit and the elevation between the entry and exit must be re-measured, and the center pile must be accurately determined during the entry and exit measurements. The third accurate measurement is to carefully measure the length of the drill rod. Before drilling the guide hole, accurately measure the length of each drill rod in the order of the drill rod connections in mm and make accurate records.

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