During horizontal directional drilling (WDD) construction, the drill pipe (https://www.eastern-steels.com/products/drill-pipe.html) inside the borehole must withstand the combined effects of various complex stresses, including compression, tension, bending, and torsion. It also frequently experiences frictional wear from the rock and gravel surrounding the borehole wall, leading to a gradual weakening of its strength. This is especially true during long-distance, large-diameter pipeline laying, where the forces faced by the drill pipe are even more severe and complex.

Drill pipes endure various stresses such as compression, tension, bending, and torsion, as well as frictional wear from the ground. This is particularly severe during long-distance, large-diameter construction, affecting project quality. Therefore, the quality of the drill pipe directly impacts project quality. If a drill pipe breaks or falls inside the borehole, it can not only lead to the failure of the entire project but also delay the construction period, severely impacting economic benefits.

Signs of Potential Drill Pipe Fracture
During WDD construction, the following phenomena may indicate that the drill pipe has fractured:

1. During construction, a sudden and significant decrease in torque, feed force, or pullback force usually means that the drill pipe cannot withstand the current stress, leading to its fracture.

2. A significant drop in mud pump pressure may be due to mud flow obstruction caused by drill pipe breakage, resulting in pressure reduction.

3. No significant change or alteration in the signal received by the detection instrument may be due to signal transmission obstruction caused by drill pipe breakage.

4. Although the drill pipe is still rotating, the drill bit signal does not rotate accordingly, which may also indicate that the drill pipe has broken and cannot transmit rotational power normally.

Causes of Drill Pipe Breakage
The causes of drill pipe breakage can be various, including but not limited to the following:

1. Material Defects
Defects in manufacturing or the drill pipe material (https://www.eastern-steels.com/newsdetail/what-material-is-drill-pipe-made-of.html) itself may lead to insufficient strength or toughness, resulting in breakage during use.

2. Excessive Stress
Drill pipe may be subjected to excessive torque, feed force, or pullback force during construction, exceeding its tolerance and causing breakage.

3. Corrosion and Wear
Drill pipes operating in underground environments for extended periods may be subject to corrosion from corrosive substances and wear from friction with the formation. These factors gradually weaken its structural integrity, increasing the risk of breakage.

4. Improper Operation
Improper operation during construction, such as unreasonable torque control or excessive feed rate, can also lead to drill pipe fracture due to stress concentration.

Prevention of Drill Pipe Fracture
1. Regular Inspection and Operating Procedures
Drill pipes used for extended periods may suffer significant surface damage, even approaching or exceeding their fatigue life. Failure to perform timely maintenance or replacement can lead to fracture.

Regular Comprehensive Drill Pipe Inspection: After a period of use, drill pipes must be carefully inspected for wear, bending, and surface scratches. Drill pipes with excessive wear, significant bending, or deep surface scratches should be discarded immediately to ensure the quality of drill pipes used is up to standard.

Strict Adherence to Operating Procedures: During construction, the drill pipe must be used strictly according to its permissible tensile force, torque, and radius of curvature. Strict adherence to operating procedures is crucial to avoid excessive stress on the drill pipe due to improper operation.

2. Strengthen Prevention and Optimize Mud
Selecting an inappropriate reamer model, or failing to match it to actual geological conditions, can increase drilling torque, leading to drill pipe breakage due to increased wear.

In sections with severely over-diameter boreholes, drill pipe breakage may occur. This is usually due to substandard mud wall protection. In strongly weathered and fractured zones, gravel layers, or sandy soil strata, the mud loses a large amount of water, and free water seeps into the borehole wall under pressure differential, causing borehole wall collapse and subsequent drill pipe breakage.

Adding a centralizer during construction and using high-quality mud to maintain borehole wall stability are crucial. Adding a centralizer at the front end of the reamer can effectively reduce torque and the negative impact of alternating stress on the drill pipe, thereby reducing the risk of drill pipe breakage.

3. Reaming and Thread Oil
When the radius of curvature of the borehole trajectory is too small, and the feed or pullback is too violent, the drill pipe may break due to the enormous torque and tension. Especially during reaming operations, if the reamer suddenly encounters significant resistance and becomes stuck, while the operator continues to apply torque or pull-back force, the drill pipe will bear enormous stress, leading to breakage.

During the reaming stage, the drill pipe is subjected to combined tensile and torsional stresses and frequently experiences alternating stress. Scientific design of the reaming gradation and reasonable construction are crucial. Optimizing the reaming gradation is essential; for issues with unreasonable reaming gradation, a scientific reaming gradation design should be implemented.

Pay attention to the use of thread lubricant. High-quality thread lubricant can effectively prevent drill pipe sticking, reduce torque during uncoupling, and reduce wear on the threaded surfaces.

4. Drill Pipe Instability and Pilot Hole Deviation Control
Because the drill pipe is mechanically a slender rod, it is at risk of instability under pressure. Therefore, during the pilot hole construction stage, the free-state drill pipe needs to be restrained to prevent unstable bending during directional drilling. Generally, the length of the free section of the drill pipe should not exceed 20 times the drill pipe diameter to prevent instability.

Strictly control pilot hole deviation. The deviation between the actual curve of the pilot hole and the designed crossing curve should be controlled within 1% and meet the relevant specifications. After construction is completed, the hole can only be enlarged after it has been inspected and approved by the supervisor or owner.

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