Development of Rolling Process for AISI 4145H Steel Tubular Drill Collars Used in Oil Drilling

Abstract

To solve the problem of smooth rolling during the production of ultra-long, extra-thick-walled drill collar tubing (diameter-to-wall thickness ratio ≤3.7) using the Assel mill and to improve the uniformity of wall thickness both longitudinally and transversely, a series of technical measures were implemented. These included selecting optimal billet shapes, reasonably distributing deformation, adjusting parameters of piercing and rolling mills, and optimizing parameters and speed regimes of the finishing mill. The finished drill collar tubing met length specifications, achieved high dimensional accuracy including uniform wall thickness both longitudinally and transversely, and satisfied performance criteria. User feedback indicates that compared to solid bar stock, using tubular stock for manufacturing drill collars offers advantages such as material savings, higher yield rates, increased productivity, and reduced tool consumption.

Drill collars are an essential component of oil drilling equipment, connecting the drill bit below and heavy drill pipes above during drilling operations. They experience significant alternating stress. The API Spec 7-1 standard imposes strict technical requirements on them. Drill collars can be classified into solid and helical types; their inner holes are always circular, with the main characteristic being a thick wall (4 to 6 times thicker than regular drill pipes), providing substantial weight and rigidity. They are typically manufactured by drilling holes through rolled or forged AISI 4145H chrome-molybdenum steel bars.

Using solid bars for drilling to manufacture drill collars has four disadvantages:

  1. Low material utilization;
  2. Low yield for small-diameter holes;
  3. Low processing efficiency;
  4. High tool consumption. These factors increase production costs and limit capacity. Manufacturers aim to reduce costs and enhance efficiency while maintaining product quality.

Objective

Hubei Xinyegang Co., Ltd., leveraging the world’s largest and most advanced Assel three-roll hot rolling mill complex, developed a series of thick-walled drill collar tubes. User trials have shown positive results, effectively addressing the issues associated with drilling from solid bars. This transition to using tubes instead of bars represents a cost-effective solution, generating significant economic and social benefits.

Technical Requirements for Drill Collar Tubing

The following outlines our company’s technical requirements for drill collar tubing:

  1. Chemical composition as shown in Table 1;
  2. Dimensions and tolerances as specified in Table 2;
  3. Ovality of outer diameter and wall thickness variation along the entire length must be less than 70% of the tolerances for outer diameter and wall thickness;
  4. Length and tolerance: 9,700 ± 100 mm;
  5. Bow not exceeding 2 mm/m, with total bow over the entire length not greater than 4 mm;
  6. Delivery condition: annealed;
  7. Mechanical properties determined by heat treating test samples according to ASTM A370 standards, meeting the requirements outlined in Table 3.

From these technical requirements, it is evident that manufacturers of drill collars demand high-quality raw tubing characterized by purity, high dimensional accuracy, straightness, and excellent comprehensive mechanical properties. Notably, the stringent requirements for dimensional accuracy (outer diameter tolerance, wall thickness tolerance, and uniform wall thickness along the entire length) and straightness highlight the advantage of using tubing over solid bars for expanding the inner hole.

Development of Rolling Process

Table 4 lists the diameter-to-wall thickness ratios (D/S) of various sizes of drill collar tubing developed by our company, ranging from 2.94 to 3.69. This indicates that these are extra-thick-walled tubes, even challenging for the Assel mill, known for rolling medium-thick-walled tubes. Given the super-length of approximately 9.7 meters for drill collar tubing, conventional billet shapes cannot be used. Therefore, selecting special billet shapes and developing processes to roll drill collar tubing to meet size and length requirements is the primary challenge. Secondly, mastering the deformation characteristics at each stage of the rolling process for AISI 4145H drill collar tubing, understanding the factors affecting dimensional accuracy, optimizing deformation distribution, adjusting rolling mill parameters, improving tool design, and selecting optimal tools are necessary to achieve the required dimensional accuracy, particularly uniform wall thickness both longitudinally and transversely.

Selection of Suitable Billets

As shown in Table 5, to ensure the final length meets requirements, the billet shapes selected for each size of drill collar tubing are one size larger than those typically used for the same outer diameter in the Assel mill, meaning the total reduction is 1.5 to 2.0 times that of conventional products.

Formulation and Optimization of Processes

To address issues related to smooth rolling and wall thickness precision, adjustments were made to piercing and rolling processes, including reducing the expansion during piercing to minimize the impact of large reductions on the final product’s shape and uniformity.

Conclusion
  1. By reasonably selecting rolling mill adjustment parameters and optimizing mandrel size, smooth rolling of extra-thick-walled drill collar tubing can be achieved.
  2. Using reduced expansion piercing with the largest possible mandrel and plug diameters within process limits maximizes piercing stability and improves the uniformity of the pierced billet’s wall thickness. When rolling drill collar tubing with extreme reductions, mandrel stability becomes crucial for maintaining uniform wall thickness. Optimizing rolling mill parameters, especially angles and speeds, and adjusting guide devices can mitigate the impact of mandrel instability on wall thickness uniformity. Reducing the reduction amount and optimizing the reduction mill pass design and speed settings can effectively minimize the formation of square and hexagonal cross-sections, achieving better longitudinal and transverse wall thickness uniformity.
  3. Users have confirmed that using tubing for manufacturing drill collars compared to solid bars provides four major advantages: material savings, higher yield rates, increased productivity, and reduced tool consumption.