I. Causes of Fracture
1. To save investment, the directional drilling equipment used by the construction contractor does not match the actual needs of the project. The tonnage of the directional drilling equipment is too small, maintenance is not timely, and the failure rate is high, resulting in uneven drilling force. At the same time, intermittent construction leads to poor hole quality and irregular holes, and the drilling tools are prone to sticking, causing drill pipe fracture;
2. The reamer model is not reasonably selected and fails to be matched according to actual geological conditions, which increases drilling torque and drill pipe wear, leading to fracture;
3. The drill pipe selection is unreasonable. Due to the lack of tracking management of drill pipes and the lack of statistical data on drill pipe inspection, the service life of a set of drill pipes is uneven. Among them, the more worn drill pipes fracture due to stress concentration;
4. The deviation control of the pilot hole is unreasonable. If the deviation is too large, during the later correction process, due to the unreasonable actual curve, the bending moment borne by the drill pipe is too large, which easily leads to increased stress in the drill pipe or directly causes surface friction heat cracking and fracture of the drill pipe. In particular, the drill pipe joint bears high lateral friction. When it periodically contacts and rubs against the hole wall, a large amount of heat is generated, causing the joint surface material to undergo periodic heating and cooling, producing brittle martensite. Under the simultaneous influence of excessive bending moment, longitudinal cracking of the internal thread of the drill pipe joint occurs;
5. The reaming stages are unreasonable. To rush the schedule, the construction unit reduces the reaming stages, which greatly increases drilling torque and easily leads to increased drill pipe wear and fracture;
6. Due to unfavorable geological conditions, the hole is broken and collapsed, and there is excessive drill cuttings, which increases the risk of drilling tool sticking and leads to drill pipe fracture.
II. Solutions
1. In response to the unreasonable selection of directional drilling rigs and reamers, strengthen the start-up management of directional drilling. Before the start of directional drilling, the relevant design documents (especially the relevant geological survey data) should be carefully studied, and the construction plan submitted by the construction contractor and the directional drilling equipment to be used should be reviewed to ensure that the configured directional drilling equipment and drilling tools match the actual needs of the project. Those that do not meet the requirements are not allowed to start, and tracking and adjustment should be made in a timely manner according to actual drilling conditions after start-up.
2. For the problem of unreasonable drill pipe selection, strictly control the selection and management of drill pipes.
① The drill pipe selection should match the drilling rig parameters. Based on previous construction experience, the maximum torque borne by the drill pipe should be selected as 2 times the maximum estimated torque (the calculation formula for the maximum estimated torque can refer to relevant technical documents);
② The drill pipes entering the site should be inspected and graded for use according to the "Drill Pipe Classification Inspection Method" (SY/T5824). For projects with high schedule risks, Grade I drill pipes should be used;
③ A drill pipe management file should be established to record the use of each drill pipe throughout the process. During use, they should be rotated to ensure uniform service life of each drill pipe;
④ For directional drilling projects with high schedule risks, finite element simulation analysis should be used to conduct stress analysis and life evaluation of drill pipes;
⑤ 2 to 3 thickened drill pipes should be set before and after the reamer to transition between the drill pipe and the drill collar.
3. For the problem of excessive pilot hole deviation, strengthen the control of pilot hole deviation. The deviation between the actual curve of the pilot hole and the design curve should not exceed 1% of the crossing length, and the deviation should comply with relevant regulations. After the pilot hole construction is completed, it must be accepted by the supervisor or owner, and only after passing the acceptance can reaming be carried out.
4. For the problem of unreasonable reaming stages, scientifically carry out reaming stages. Construction supervisors and owners should urge the construction unit to reasonably stage the reaming. For pipelines with a diameter exceeding 400 mm, reaming should be staged and carried out multiple times. The reaming stage difference should be determined according to the drilling rig, mud system, drill pipe specifications, geological conditions, etc. Generally, the first stage reaming diameter is 20″–24″, and then staged reaming is carried out at 4″–10″.
5. Regarding problems such as unfavorable geological conditions, strict monitoring of the reaming process is required. During the reaming process, the changes in various parameters should be closely tracked and analyzed. If excessive torque or pulling force is found, hole cleaning operations should be carried out in a timely manner, and reaming should continue after hole cleaning is completed. For directional drilling in broken rock geology, hole cleaning should be carried out at least once after each reaming.