Drill Rod of Rotary Drilling Rig: In-depth Analysis of Material, Number of Sections And Drilling Stability

        In modern infrastructure construction, rotary drilling rig has become an indispensable key equipment, widely used in the foundation construction of high-rise buildings, bridges, roads and other projects. Its efficient and precise pore forming ability greatly improves construction efficiency and engineering quality, providing strong support for the rapid development of the city. As one of the core components of the rotary drilling rig, the drill rod is like the skeleton of the human body, responsible for transmitting power, applying pressure, and driving the drill bit to rotate and drill, playing a decisive role in the overall performance of the rotary drilling rig. The material selection and section design of drill rods directly affect the stability of the drilling process, which in turn affects the efficiency, quality, and cost of construction. Therefore, it is of great practical significance to deeply explore the correlation between material, number of sections, and drilling stability in the drilling rod technology of rotary drilling rigs. It can not only provide scientific guidance for engineering practice, help construction units optimize construction plans, but also promote continuous innovation and progress in rotary drilling rig technology.

（1） Common material types and characteristics

        The commonly used materials for rotary drilling rig drill rods include Q235B steel, Q345B steel, 45 # steel, and 42CrMo steel, each of which has unique performance characteristics.

        Q235B steel belongs to carbon structural steel with low carbon content, which endows it with good welding and processing properties. It can be easily formed in various manufacturing processes and is widely used in fields such as construction, bridges, and vehicles. In terms of strength, its yield strength is ≥ 235MPa, tensile strength is ≥ 375MPa, elongation is ≥ 21%, and it can withstand a certain degree of external force. In the application of rotary drilling rig drill rods, it can meet the basic strength requirements for general construction.

        Q345B steel, as a low-alloy high-strength structural steel, has added microalloying elements such as vanadium (V ≤ 0.15%) and titanium (Ti ≤ 0.20%) on the basis of Q235B steel, significantly improving the material strength. Its yield strength reaches 345MPa, tensile strength is 450-630MPa, and elongation requirement is ≥ 17%. In actual testing, when subjected to a 30% overload condition, Q345B steel can still maintain a bearing capacity of over 85%, demonstrating superior strength performance and suitable for engineering scenarios with high strength requirements.

        45 # steel is a high-quality carbon structural steel with high strength, good toughness and plasticity performance. After tempering treatment, it can have good comprehensive mechanical properties and is generally used for components with certain requirements for material comprehensive properties such as the drilling bottom center axis of the rock socketed double bottom sand scoop. But its welding performance is poor, and preheating before welding and annealing after welding are required during processing to ensure welding quality.

        42CrMo steel belongs to alloy structural steel with high strength and permeability. After quenching and tempering, it has a high fatigue limit and resistance to multiple impacts. It has good low-temperature impact toughness and is suitable for components with large loads and high reliability requirements. It is commonly used in the manufacturing of key components such as shafts in rock formations. Compared with 45 # steel, 42CrMo steel has higher tensile strength and yield strength. At the same diameter value, 42CrMo steel is usually preferred to meet more stringent working conditions.

（2） How do material properties affect drilling stability

        Drilling stability is a key factor in the operation of rotary drilling rigs, and the material characteristics of drill rods are closely related to it. Drill rods made of high-strength materials can effectively resist deformation during drilling. For example, when facing significant axial pressure and torque, high-strength materials such as Q345B steel and 42CrMo steel, with their high yield strength and tensile strength, can reduce the deformation of drill pipes caused by bending, twisting, and other forces. Taking bridge pile foundation construction as an example, the depth of the pile foundation is relatively large, and the strength requirements for the drill rod are high. Using high-strength material drill rods can ensure good straightness during drilling, avoid drilling deviation caused by drill rod deformation, thereby ensuring drilling stability and improving drilling quality.

        Materials with good toughness can better cope with the impact and vibration of complex geological formations. When encountering complex formations such as rock layers and gravel layers, drill rods will be subjected to significant impact forces and vibrations. Materials with good toughness such as 42CrMo steel have high impact toughness and fatigue limit, which can absorb and buffer these impact forces, preventing brittle fracture of drill pipes. When passing through hard rock formations, even if the drill bit frequently impacts the rock, the resilient drill rod can effectively disperse the impact force, maintain the integrity and continuity of the drill rod, and maintain the stability of the drilling process based on its own characteristics.

（3） Material selection strategies under different geological conditions

        Different geological conditions have different requirements for drill pipe materials, and selecting materials reasonably can improve drilling efficiency and stability. In soft soil formations such as clay, silt, and silty soil, the strength of the formation is relatively low, and the strength requirements for drill rods are relatively low. However, drill rods need to have certain flexibility and corrosion resistance. Q235B steel, due to its good processing performance and moderate strength, can meet the drilling needs of soft soil layers, while having relatively low cost and good economy. Moreover, in some soft soil formations with high groundwater levels, Q235B steel can also be treated with anti-corrosion measures to enhance its durability in humid environments.

        In rock formations, including strongly weathered, moderately weathered, and slightly weathered rock layers, the hardness of the formation is high, and the drilling rod needs to withstand enormous pressure and torque during drilling. At this point, high-strength and high toughness materials such as 42CrMo steel are ideal choices. It can maintain structural stability under high pressure, effectively transmit power, resist rock reaction forces, reduce drill pipe wear and damage, and ensure smooth drilling in rock formations. In tunnel construction in hard rock formations such as granite, using 42CrMo steel drill rods can significantly improve drilling efficiency and ensure construction progress.

（1） Basis for determining the number of drill pipe sections

        The determination of the number of drill pipe sections in a rotary drilling rig is not arbitrary, but is based on scientific calculation and analysis of multiple key factors. Firstly, the drilling capacity parameters of the drilling rig are one of the important criteria, which determine key indicators such as the torque and axial pressure that the drill rod can withstand. Different models of rotary drilling rigs have different drilling capabilities. For example, the maximum output torque of the power head of the Sany SR280C rotary drilling rig is 280kN · m, and the maximum output torque of the XCMG XR360E rotary drilling rig can reach 360kN · m. This requires the matching number of drill pipe sections to meet the corresponding torque transmission requirements. The drilling diameter and drillable depth are also closely related to the number of drill pipe sections. The larger the diameter of the borehole, the greater the lateral force on the drill rod during the drilling process, requiring stronger overall rigidity to ensure drilling stability, which may require more drill rods to disperse the force. The drillable depth directly determines the total length of the drill pipe, and each section of the drill pipe has its effective length. By dividing the drillable depth by the effective length of each section of the drill pipe and adjusting it according to the actual situation, the number of drill pipe sections can be preliminarily determined. In general, the effective length of each drill rod is around 8-12 meters. If the designed hole depth is 60 meters, preliminary calculations may require 5-7 drill rods. The stability of the mast under effective height and self weight conditions cannot be ignored. As a key component supporting the drill rod, the effective height of the mast limits the total length of the drill rod. If the drill rod is too long and exceeds the effective height of the mast, it will not only fail to work properly, but also pose a safety hazard. At the same time, the weight of the drill rod itself will increase with the number of sections, and too many sections may cause the drill rod to bend, deform, and affect drilling stability under its own weight. Therefore, when determining the number of drill pipe sections, it is necessary to comprehensively consider these factors and find the most suitable section configuration through accurate calculation and simulation analysis.

（2） The specific role of section number in drilling stability

        The number of drill pipe sections has a significant impact on drilling stability, which is reflected in various aspects such as stress, center of gravity distribution, and overall rigidity. When there are too many drill pipe sections, stress concentration is prone to occur due to the connection points between each section of the drill pipe when transmitting torque and axial pressure. During the drilling process, with the continuous changes in torque and pressure, stress concentration at the connecting parts may cause local damage to the drill rod, such as keyway wear, weld cracking, etc., which in turn affects the continuity and stability of drilling. From the perspective of center of gravity distribution, having too many sections will make the center of gravity distribution of the drill rod more complex and difficult to maintain balance. During the process of lifting and lowering the drill rod, the deviation of the center of gravity may cause the drill rod to shake, increase the risk of collision with the hole wall, and easily cause the collapse of the hole wall, affecting the quality and stability of drilling. In terms of overall rigidity, too many sections will relatively reduce the overall rigidity of the drill rod. When encountering large resistance in complex formations, the drill rod is more likely to bend and deform, leading to borehole deviation and seriously affecting drilling accuracy and stability.

        On the contrary, if the number of drill pipe sections is too small, although it can reduce stress concentration and center of gravity distribution at the connection point to a certain extent, it will also bring other drawbacks. Too few sections may not meet the drilling depth requirements, resulting in the inability to complete construction tasks. Even if it can meet the depth requirements, due to the limited length of the drill rod, it cannot effectively disperse the force when transmitting torque and pressure, resulting in excessive local force on the drill rod, which can easily cause damage to the drill rod. Moreover, drill rods with fewer sections lack sufficient flexibility and adaptability when dealing with complex formations, making it difficult to adjust drilling parameters according to changes in the formation, thereby affecting drilling stability and construction efficiency.

（3） Case study: Performance of drill rods with different numbers of sections in practical engineering

        In a certain bridge pile foundation project, the construction site is mainly composed of silty clay and medium sand layers, with a designed pile diameter of 1.5 meters and a pile depth of 50 meters. Initially, a rotary drilling rig with 4 drill rods was selected for construction. During the drilling process, when drilling to about 30 meters, a thick layer of medium sand was encountered. Due to the high resistance of the medium sand layer to the drill rod, local stress concentration occurred in the four sections of the drill rod when transmitting torque and pressure, resulting in wear on the keyway of one of the drill rods and slight shaking of the drill rod. The drilling stability was affected, and the borehole deviated to a certain extent. To solve this problem, the construction party replaced the rotary drilling rig with 5 drill rods. The length and number of sections of the 5-section drill pipe have been optimized to better distribute torque and pressure, reducing stress concentration. In the subsequent construction, the shaking of the drill rod was significantly reduced, the problem of borehole deviation was improved, the drilling stability was significantly improved, and the construction efficiency was also improved, successfully completing the pile foundation construction task.

        For example, in the foundation construction of a high-rise building, the site strata are relatively complex, with the upper part being miscellaneous fill soil and the lower part being moderately weathered mudstone sandstone. Design pile diameter of 1.2 meters and pile depth of 45 meters. In the early stage of construction, a drilling rig with three drill rods was used. Due to the loose nature of the mixed soil, the distribution of the center of gravity of the three drill rods was relatively unstable. The drill rods shook greatly during lifting and lowering, frequently colliding with the hole wall, resulting in the collapse of the hole wall and affecting the drilling progress and stability. Later, a drilling rig with four drill rods was used, and the addition of one drill rod made the overall center of gravity distribution of the drill rods more reasonable. When crossing mixed fill soil and moderately weathered siltstone, the shaking of the drill rods was effectively controlled, and the collision with the hole wall was reduced. The drilling stability was significantly enhanced, and the foundation construction was successfully completed, ensuring the quality of the project. Through these practical cases, it can be clearly seen that there are differences in the performance of drill pipes with different numbers of sections in actual engineering. Reasonable selection of drill pipe sections is crucial for ensuring drilling stability and construction quality.

（1） Other influencing factors besides material and number of sections

        In addition to the material and number of drill rods, drilling stability is also significantly affected by many other factors. Torque is a key factor that directly affects whether the drill bit can effectively break rock and soil. When the torque is insufficient, the drill bit cannot fully cut the rock and soil, which can easily lead to slippage, resulting in reduced drilling efficiency and decreased stability. Excessive torque, exceeding the bearing capacity of the drill rod, will subject the drill rod to excessive stress, increase the risk of damage to the drill rod, and also affect drilling stability. When drilling in hard rock formations, if the torque is insufficient, the drill bit will have difficulty cutting into the rock, resulting in frequent jumping and irregular drilling, which will affect subsequent construction.

        Pressure is also an important factor affecting drilling stability. Appropriate pressure can ensure full contact between the drill bit and the soil, ensuring cutting effectiveness. The pressure is too low, the drill bit cannot effectively break the rock and soil, and the drilling speed is slow; Excessive pressure may cause the drill rod to bend, deform, or even break, and may also cause the hole wall to be subjected to excessive compression, leading to the collapse of the hole wall. In soft soil layers, excessive pressure may cause excessive deformation of the soil, leading to instability of the pore wall.

        The complexity of geological conditions has a direct impact on drilling stability. The hardness, viscosity, particle composition, and other characteristics of different strata vary. In hard rock formations, the rock has high hardness, making drilling difficult and causing severe wear on the drill rod and drill bit. This can easily lead to vibration and shaking of the drill rod, affecting drilling stability. In soft soil layers, especially those with high moisture content such as silty soil or flow plastic cohesive soil, the bearing capacity of the soil is low, and the borehole wall is prone to collapse, which can also pose a threat to drilling stability. When drilling in sand layers, due to the low friction between sand particles, it is easy to cause hole collapse, which affects the continuity and stability of drilling.

        The operational skills cannot be ignored either. Proficient and standardized operations can adjust drilling parameters in a timely manner according to actual situations, maintaining a smooth drilling process. If the operator cannot accurately control the drilling speed, torque, and pressure, such as sudden acceleration or deceleration during the drilling process, it will cause the drill rod to be impacted, resulting in unstable drilling. Improper operation during the lifting or lowering of the drill rod, such as too fast or too strong, may cause the drill rod to collide with the hole wall, resulting in damage to the hole wall and affecting drilling stability.

（2） The synergistic relationship between material, number of sections, and other factors

        There is a close synergistic relationship between material, number of sections, and other factors that affect drilling stability. They interact and influence each other, jointly determining the stability of the drilling process. High strength and high toughness materials can better withstand torque and pressure. When the rotary drilling rig operates in hard rock formations, a large torque is required to break the rock. At this time, high-strength materials such as 42CrMo steel drill rods, with their excellent mechanical properties, can effectively transmit torque, resist stress caused by torque, reduce deformation and damage of drill rods, and provide guarantee for stable drilling. If the strength of the drill rod material is insufficient, even if the torque and pressure are within the theoretical range, it may still cause the drill rod to fail due to inability to withstand them, affecting drilling stability.

        The number of drill pipe sections is also closely related to torque and pressure. The number of sections will affect the overall rigidity and force distribution of the drill rod. When a drill rod with a larger number of sections transmits torque and pressure, stress concentration may occur due to the increased number of connecting parts, which may reduce the overall performance of the drill rod. Therefore, it is necessary to select the appropriate number of drill pipe sections based on the torque and pressure output capacity of the drilling rig, to ensure that the drill pipe can uniformly receive force when transmitting power and maintain drilling stability. In large-diameter drilling construction, a large amount of torque and pressure are required. If the number of drill pipe sections is too small to effectively disperse the force, it will cause excessive local stress on the drill pipe, affecting drilling stability; And having too many sections will increase the risk of stress concentration at the connection points.

        The geological conditions, materials, and number of sections are also interrelated. Different formations require different materials and numbers of drill rods to adapt. In complex geological formations, such as those with soft soil in the upper part and rock in the lower part, the upper soft soil layer has relatively low requirements for the strength of the drill rod, but requires the drill rod to have a certain degree of flexibility to adapt to the deformation of the soil; The lower rock formation requires drill rods to have high strength and toughness to resist the reaction force of the rock. Therefore, it is necessary to select suitable materials and drill rods with appropriate number of sections according to the changes in the formation, and adjust drilling parameters to ensure drilling stability. If unsuitable low strength material drill rods are used in rock formations, or the number of sections is not reasonably configured, it will cause the drill rods to be unable to withstand the resistance of the rock during drilling, resulting in problems such as fracture and deformation, seriously affecting the stability of drilling.

        The operation technology, materials, and number of sections are also coordinated with each other. Skilled operators can reasonably control parameters such as torque, pressure, and drilling speed based on the material and section characteristics of the drill rod. For high-strength materials and drill rods with a large number of sections, operators can increase torque and pressure appropriately to accelerate drilling speed, but also pay attention to avoiding uneven force on the drill rod due to improper operation. For drill rods with softer materials and fewer sections, operators need to control parameters more carefully to avoid excessive stress causing damage to the drill rods. In practical operation, operators also need to adjust the operation mode in a timely manner according to changes in geological conditions to ensure that the drill pipe can maintain a stable working state under various working conditions.

（3） Overall strategy to enhance drilling stability

        Improving drilling stability requires comprehensive consideration from multiple aspects and adopting a holistic strategy. Optimizing drill pipe design is crucial. In terms of material selection, suitable materials should be accurately selected according to different engineering requirements and geological conditions. For engineering under complex geological conditions, the development and application of new composite materials, such as the composite of high-strength alloys and fiber-reinforced materials, can be considered to further enhance the comprehensive performance of drill pipes. In terms of section design, advanced mechanical analysis software is used, combined with actual engineering data, to accurately calculate the most suitable number of drill pipe sections. At the same time, the connection structure of the drill pipe is optimized, and new connection methods are adopted, such as high-strength bolt connection or a combination of welding and mechanical connection, to reduce stress concentration at the connection part and enhance the overall rigidity and stability of the drill pipe.

        Matching construction parameters is also a key step. Before construction, a detailed survey of the geological conditions at the construction site should be conducted to obtain accurate geological data. Reasonably adjust parameters such as torque, pressure, and drilling speed based on geological conditions and drill pipe performance parameters. When drilling in hard rock formations, increase the torque and pressure appropriately, and reduce the drilling speed to ensure that the drill bit can effectively break the rock, while reducing the vibration and wear of the drill rod; When drilling in soft soil layers, reduce the pressure appropriately, increase the drilling speed, and prevent the collapse of the hole wall. Real time monitoring of parameter changes during the construction process should also be carried out, and adjustments should be made in a timely manner according to the actual situation to ensure that the construction parameters are always in the best matching state.

        Improving operational skills is equally indispensable. Strengthen the training of operators, regularly organize professional skills training courses, invite industry experts to give lectures, so that operators can have a deep understanding of the working principle, performance characteristics, and operating skills of rotary drilling rigs under various working conditions. At the same time, establish strict operating standards and assessment systems, requiring operators to strictly follow the standards for operation, regularly assess the skill level of operators, reward those who perform well, and provide retraining or job adjustment for those who do not meet the requirements, ensuring that operators have good operating skills and a sense of responsibility, can proficiently and accurately control the rotary drilling rig during the construction process, and ensure drilling stability.

        There is a close and complex relationship between the material, number of sections, and drilling stability of the rotary drilling rig drill rod. From the perspective of materials, different material characteristics have their own advantages and disadvantages in terms of strength, toughness, etc., which directly determine the ability of drill rods to resist deformation, respond to impact and vibration during drilling, thus profoundly affecting drilling stability, and requiring precise selection based on different geological conditions. In terms of the number of sections, it is necessary to comprehensively consider multiple factors such as the drilling capacity of the drilling rig, drilling diameter, drillable depth, effective height of the mast, and self weight stability. The number of sections has a significant impact on the stress, center of gravity distribution, and overall rigidity of the drill rod, which in turn affects the drilling stability. The appropriate number of sections is the key to ensuring drilling stability. In addition, other factors such as torque, pressure, formation conditions, and operating techniques work together with material and number of sections to ensure drilling stability.

        As one of the core technologies of rotary drilling rigs, drill rod technology plays a crucial role in the overall performance and construction effectiveness of rotary drilling rigs. It is not only related to the efficiency and quality of construction, but also closely related to project costs, safety, and other factors. With the continuous promotion of infrastructure construction and the continuous development of engineering technology, higher requirements have been put forward for the drilling rod technology of rotary drilling rigs. In the future, drill pipe technology is expected to develop towards material performance optimization, intelligent section design, and deep integration with other technologies. In terms of materials, we will continuously develop new high-performance materials to enhance the comprehensive performance of drill pipes; In terms of section design, advanced computer simulations and intelligent algorithms are utilized to achieve more precise and efficient designs; In terms of technological integration, it will be combined with automation control, intelligent monitoring and other technologies to further enhance drilling stability and construction intelligence level, providing more reliable and efficient technical support for engineering construction.