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HDD Drilling Risk Identification in construction

Horizontal directional drilling construction is a high-risk engineering task. here we’ll analyze some common HDD drilling risks and solutions in construction

  1. Deviation of guiding trajectory

Deviation of the guide track often occurs during the guidance process, and the harm can be great or small.

Problem analysis and prevention

There are many factors that cause locator deviation from the trajectory, the common ones are:

(1) The guiding accuracy of the locator is not enough, or it is interfered by signals.

Preventive measures: Use reliable locator, use artificial magnetic fields to ensure guidance accuracy in large projects with complex working conditions, regularly monitor the position of the drill bit during the guidance process, and correct deviations in a timely manner if deviations are found. Blind tapping should be avoided even in horizontal drilling.

(2) Lagging or advancing the deflection will cause the horizontal section of the pilot hole trajectory to be lower or higher, or cause the drill bit to be unearthed in advance or lag behind.

Preventive measures: Check the deviation between the actual trajectory and the designed trajectory in time and correct it in time.

(3)The pilot drilling tool is not suitable for the formation, the deflection efficiency is low, and the trajectory deviates due to the inability to change direction in time.

Preventive measures: choose appropriate pilot drilling tools, such as using large plate surface guide plates instead of conventional guide plates in soft soil formations, adding bent joints when using cone pilot bits for long-distance soil crossing to enhance deflection performance, or using specially customized light mud motors to reduce the weight of drilling tools and improve deflection efficiency.

(4) After replacing the drilling tool, the pilot drill bit cannot enter the original hole. This usually happens in the process of passing through soft soil into rock, such as after the inclined plate pilot bit is replaced by the cone pilot bit, the cone pilot bit and the mud motor sink in the soft soil due to its own weight

To prevent refining, when encountering similar situations, you need to use casing to isolate the weak soil layer or use a light mud motor:

When the actual turn deviates from the design trajectory, to take corrective measures according to the situation, the correction process should be slowly, to prevent excessive correction, if the borehole trajectory is caused by a large deviation, then need to retreat – a fixed number of drill pipes, re-drill into the new hole, if it is the reason for the drilling tool and the formation, you can consider replacing the drilling tool or taking auxiliary measures after continuing to guide.

  1. Second, mud leakage and slurry

In the process of long-distance guidance and reaming, there will be no slurry return at the orifice, which is usually caused by slurry or mud leakage to the hole wall formation. The mud is the carrier for transporting drill cuttings, and once the mud flow decreases or the circulation stops, the drill cuttings will accumulate in the hole, affecting the construction safety. Pulp can also cause environmental pollution and even lead to the issue of economic compensation.

 Problem analysis and prevention

(1) When crossing long-distance and complex strata, due to complex geological conditions, it is easy to leak mud, and if the mud performance is not good, it is more likely to cause mud leakage. 

Preventive measures: To prevent slurry leakage, the first step is to improve the performance of slurry, for example, the water loss of slurry can be controlled and the viscosity can be increased by adding cellulose and other filter loss reducing agents.

(2) slurry accident mostly occurs in the weak part of the formation, because the buried depth is too shallow, the mud pressure is too large and other factors and causes the formation fracturing slurry, and the reason for the mud pressure is large is that the borehole is not smooth.

Precautionary measures: Before construction, we must first carry out geological data analysis and on-site survey, Choose appropriate drilling tools, and carry out corresponding solidification, filling and other treatments on the sections that need to be treated. In the construction process, the guidance, the speed of the drag back and the mud pressure, the amount of pumping, and the use of mud with good performance should be controlled to ensure that the drill cuttings are taken out of the hole in time to keep the borehole smooth.

Problem handling

(1) if there is no slurry, or the amount of slurry is reduced, first of all, check whether there is a slurry point along the construction line, if there is a small-scale leakage, it can be treated by adding a plugging agent to the mud. In the position of large slurry leakage, it can be used to solidify the soil or use a compacted pile in the slurry part to be treated. If the large-scale slurry occurs close to the soil entry and exit point, it can be directly excavated into a slurry return pit to dredge the slurry return channel and prevent the accumulation of drill cuttings in the hole. 

(2)If the mud is mainly leaked into the formation, it is necessary to adjust the performance parameters of the mud, reduce the water loss of the mud, improve the viscosity of the mud, and use the plugging agent appropriately. If it occurs during long-distance soil guidance, it is advisable to use a spray short to replenish the borehole with mud and lubricate the drill pipe.

  1. Guidance signal is lost or abnormal

During the guidance process, especially when using a wired geomagnetic guidance system, the guidance signal is often lost. In severe cases, it may even be necessary to withdraw all the drill pipes from the hole for inspection, resulting in a large amount of repetitive work.

Problem analysis and prevention

(1) When using wireless guidance instruments, the loss or abnormality of the guidance signal is mostly caused by factors such as battery shortage, depth exceeding the limit, probe damage, signal interference and other factors.

Preventive measures: choose the appropriate guidance method and guidance instrument according to the guidance distance and surrounding environment, if using a wireless guidance instrument. It is necessary to confirm the battery power, guide burial depth, and select the appropriate signal frequency.

(2) If the signal is lost when using wired guidance, it is mostly caused by circuit breakage or leakage. The guide signal wire is subject to strong impact and force in the drill pipe and collides with the drill pipe nozzle. If the insulation layer is thin, it is easy to wear and leak. If the wiring is not in place, it is easy to cause leakage.

Preventive measures: Standardize wiring operations, strengthen protection of guide wires, and use special guide cables (with thickened insulation layers). It is also recommended to use thread fixers during long distance guidance.

 Problem handling

(1) When using wireless guidance, if the signal is lost or abnormal, the drill pipe should be withdrawn and the signal condition should be checked (to ensure that the receiving instrument is working normally). If there is still no signal, the probe needs to be taken out and checked for damage or battery. Whether the battery is exhausted. If there is a signal, there may be interference or shielding. You can drill 1 to 2 drill pipes in a straight line and then detect the signal again. If there is still no signal, you need to make a new steering plan.

(2)When using wired guides, once the guide signal is found to be lost, the lines should be checked in time. If the external lines of the drill pipe are intact, the drill pipe should be withdrawn and the wires inside the drill pipe should be checked for wear and fracture. Usually, the guide wire should be cut off and reconnected for inspection after withdrawing one drill pipe. until the guidance signal is restored.

  1. Abnormal torque and back-towing speed

When pulling back to expand the hole, we occasionally encounter the problem of high rotational torque of the HDD machine and low hole expansion efficiency.

Problem analysis and prevention

(1) drilling tools is unreasonable and the drilling tools have been severely worn. For example, when using a squeeze-expanding reaming drill bit in dense clay strata, the cone wear will be serious during rock reaming, which will greatly affect the reaming speed.

Preventive measures: Choose an appropriate reaming drill bit, and ensure the quality and usage status of the reaming bit. During the reaming process of complex formations, especially rocky formations, appropriately increase the number of hole cleanings to remove drill cuttings in the holes.

(2) If the hole expansion step span is too large, the hole expansion rotation torque will increase and the speed will inevitably decrease.

Preventive measures: Develop a reasonable hole expansion grading plan based on the drilling rig performance and formation properties. If the hole expansion span is large, attention should be paid to appropriately increasing the pump volume and discharging the drill cuttings in the hole in a timely manner.

Problem handling

(1) Return the reaming drill bit to a certain distance (about 2m), increase the mud displacement, rotate it again and drag it back slowly. If the problem still cannot be solved, you should consider whether the reaming drill bit is damaged and replace the reaming drill bit at the same time. Exit in time to prevent the drill from getting stuck.

(2) If necessary (not suitable for the formation or seriously worn), the reaming grading plan can be adjusted and the reaming drill bit can be replaced.

(3) Clean the hole in a timely manner, strengthen the mud performance and dosage, and clean the drilling cuttings at the bottom of the hole in a timely manner.

  1. Buried drill, stuck drill

Buried drill and stuck drill are also common construction problems in horizontal directional drilling construction.

Problem analysis and prevention

(1) Buried drill and stuck drill accidents often occur in unstable strata, such as pebbles, coarse sand, etc., which have poor cementability and are prone to collapse, or flow-plastic strata. If the soil structure is unstable or the mud used during construction has poor performance, If it is not done well, it is very easy to cause buried drilling,

Preventive measures include appropriately increasing mud viscosity and density, balancing formation pressure, stabilizing hole walls, and preventing hole collapse. If the unstable strata have shallow bedding depths, the clay can be excavated and replaced. If the burial depth is large, casing isolation can be used.

(2) When performing rock hole expansion, some strongly weathered rock formations may easily cause drill sticking due to the development of stratigraphic fissures and broken rock structures, and the rocks on the hole wall may fall off.

As a precaution, use a rock reaming drill bit that is in good condition. The centralizer enhances the stability of the reaming drill bit and trims the hole wall, promptly clearing the drilling chips at the bottom of the hole to keep the drilling smooth.

(3) Water-sensitive mudstone will absorb water in the mud and expand. Or because the drill cuttings stick to the reaming drill bit, the drill pipe forms a mud bag, and drill stuck accidents are also prone to occur at this time.

Preventive measures include selecting appropriate reaming drill bits and reducing mud water loss.

Problem handling

(1) First try to drag or jack the drill pipe in the opposite direction to the construction direction to relieve the stuck drill. If the back-pulling force is insufficient, you can try using a larger drill rig or pulley block.

  • When the drilling rig cannot be unstuck by pulling back, you can try to use a pipe tamping hammer to impact and vibrate the drill pipe to unstuck it or perform casing cleaning to unstuck it.
  1. Pipe dragging

Pipe dragging is similar to buried drill and drill stuck. It usually refers to the phenomenon that the pipeline is blocked during the dragging process. It is recommended to conduct a trial dragging during large-scale directional drilling construction to check the drilling quality.

Problem analysis and prevention

(1) Unstable formations cause tube collapse due to hole collapse.

Preventive measures: Increase the density and viscosity of the mud in a targeted manner to maintain the stability of the hole wall. Pay attention to reduce the disturbance and erosion of the hole wall during hole cleaning and dragging back. The lubrication performance of the mud should be increased when the pipeline is dragged back.

(2) If there are many drilling cuttings in the hole and the pipeline is towed back before being discharged, then during the dragging process, the mud in the hole will move forward with the drilling cuttings and gradually accumulate in front of the pipeline, hindering the pipeline backdragging.

Preventive measures: During the hole expansion construction, attention should be paid to promptly discharging the drill cuttings in the hole, and necessary hole cleaning should be performed before the pipeline is pulled back. In the rock formation stage, hole cleaning must be carried out in time, and trial back dragging can be carried out.

(3) The trajectory of the guide hole is too curved, or there is a stepped hole due to stratum reasons, causing the pipeline to drag back and get stuck.

Preventive measures: Strictly control the steering parameters and drill according to the designed trajectory. When constructing in complex strata, measures such as reasonable drilling tools (centralizers) and auxiliary techniques (casing tamping) are used, and trial back-towing can be carried out.

Problem handling

(1)If the pipeline is about to be pushed back into place, boosting equipment can be used to increase the push-back force or thrust to continue the towing construction.

(2) To rescue the pipeline, connect the pipeline from the side of the pipeline into the soil to the outside of the hole, and use a larger drilling rig Or use a pulley block to increase the back thrust.

(3) After the pipeline rescue is completed, analyze and find the cause of the problem, and clean the hole. If a trial back-towing is not performed, a trial back-towing can be performed first to check the situation in the hole. After the cause of the problem is eradicated, the back-towing can be performed again.

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