What Is S135 Heavy Drill Pipe?
S135 refers to the steel grade defined in API Specification 5DP for drill pipe. The number 135 represents the minimum yield strength in thousand psi (135,000 psi). It sits at the high-strength end of standard drill pipe grades, above E-75, X-95, and G-105, offering superior load-carrying capacity. Its mechanical properties typically include:
Minimum yield strength: 135 ksi (931 MPa)
Minimum tensile strength: 145 ksi (1000 MPa)
Hardness: Typically controlled to prevent sulfide stress cracking (often 30 – 36 HRC when specified for sour service)
Heavy drill pipe, often called heavy weight drill pipe (HWDP), is not just a thicker version of standard drill pipe. It features several distinct design elements:
Thick wall tube: The wall thickness is significantly greater than regular drill pipe of the same OD, increasing weight per foot roughly 2 to 3 times. This added mass provides concentrated weight for drilling while maintaining a manageable hydraulic diameter.
Center upset or wear pad: An extra-thick section in the middle of the tube protects the pipe from buckling and wear against the wellbore. It acts as a stabilizer, reducing bending stresses.
Long tool joints with hardfacing: HWDP tool joints are longer than standard to accommodate repetitive recuts and to accept hardbanding (e.g., Arnco or similar alloys), which combats abrasive wear on the connection OD.
Stress relief features: Many S135 HWDP pipes incorporate a spiral groove or machined stress-relief grooves at the box and pin base to mitigate fatigue in the highly stressed thread root area.
Combining the S135 grade with the heavyweight design creates a pipe that can withstand high tensile loads, severe bending, and compression simultaneously—ideal for use in the transition zone between drill collars and normal drill pipe.
Understanding NC38 Thread Connection
NC38 stands for Numbered Connection 38, a rotary shouldered connection originally designed for 3-1/2 inch inner-flush (IF) profiles. The NC designations were introduced to unify and standardize the many proprietary and semi-proprietary connections in the oilfield. Today, NC38 is interchangeable with the longstanding 3-1/2 IF connection but offers the clarity of a numeric system.
Key dimensional and thread form characteristics of NC38 (as per API 7-2 and related standards) are:
Taper: 1 in 6 on diameter (standard for most rotary connections)
Threads per inch: 4
Thread form: V-0.038R (V-profile with a rounded crest and root radius)
Pitch diameter at gauge point: Typically 3.0938 inches for the pin
Major OD of pin (new): Approx. 4.250 inches
Nominal ID of connection: Based on drill pipe size, often around 2-3/4 inch for HWDP
Make-up torque: Ranges from roughly 12,000 to 18,000 ft-lbs depending on OD, ID, and friction factor; exact values must come from the manufacturer’s torque chart.
Why is NC38 frequently selected for heavy drill pipe? It hits a sweet spot for pipe sizes commonly used as HWDP: 4-1/2 inch and 5 inch outer diameter. It provides a robust cross-sectional area to resist make-up and break-out stresses while keeping the connection OD manageable for downhole clearances. The connection’s 4 TPI and steep taper give it good tolerance to multiple break-outs and field handling, which is critical for a component that is often racked back and run multiple times.
Where S135 HWDP with NC38 Fits in the Drill String
Heavy drill pipe’s primary job is to provide a smooth stiffness transition between the very rigid bottom hole assembly (BHA) and the relatively flexible drill pipe above. This reduces stress concentration at the drill collar/drill pipe interface—historically the site of many fatigue failures. S135 HWDP with NC38 is typically run in:
Vertical wells: Placed immediately above the drill collars, it adds weight directly on bit while isolating the upper string from harmful bending cycles.
Directional and horizontal wells: Used in the bend and tangent sections where the pipe must rotate through high doglegs. The thick wall and center wear pad prevent wall collapse and reduce buckling risk when running compression in the horizontal segment.
Extended-reach drilling (ERD): High strength S135 supports the massive tensile loads needed to push the string far out. The NC38 connection offers a proven balance of tensile and torsional capacity for such complex profiles.
Slim-hole and deep gas wells: Because the connection OD is relatively trim, NC38 HWDP can be run inside production casing or smaller open-hole sections without sacrificing weight or stiffness.
Purchasing and Inspection Considerations
When ordering S135 heavy drill pipe with NC38 threads, several specifications must be confirmed beyond just “S135” and “NC38”:
Tube OD, wall thickness, and weight per foot: Common sizes include 4-1/2 in. OD x 2-1/4 in. ID (approx. 40 lb/ft) and 5 in. OD x 3 in. ID (approx. 50 lb/ft). These are standard API HWDP weights.
Connection condition: New, used, recut, or re-faced. Clearly state whether you need phosphating (copper or zinc) on the threads to prevent galling.
Materials traceability: Request full mill test reports (MTR) showing chemistry, Charpy impact values (especially for low-temperature or sour service), and hardness.
Hardbanding type: Specify “casing-friendly” hardbanding (e.g., TCS 9000, Arnco Duraband) if the pipe will rotate inside casing.
NDT and drift requirements: Mandate EMI or full-body ultrasonic inspection for used pipe. For new pipe, confirm that the mill performs a standard drift test and hydrostatic proof test.
Sour service specifications: For H2S environments, S135 must be ordered with restricted hardness and often larger radius stress grooves, per NACE MR0175. Not all S135 HWDP is automatically NACE-compliant.
Handling and Care on the Rig
S135 heavy drill pipe with NC38 connections demands careful rig floor practices:
Proper doping: Use the manufacturer-recommended thread compound with the correct friction factor (typically 1.0 or 0.8 with API Modified compounds). Inconsistent doping leads to over-torquing or under-torquing.
Controlled make-up: Always apply final torque with a calibrated torque turn system. Relying on “merchant right” (tightening till it stops) is a recipe for galled threads and connection failure.
Stabbing alignment: The long tool joints on HWDP are heavy and can easily misalign during stabbing. Use a stabbing guide and stab slowly to prevent crest-to-crest impact.
Lift subs and pup joints: Avoid lifting directly on the NC38 pin unless using a properly fitted lift sub. Many HWDP strings include lift subs installed on the top pin to protect the thread.
Storage: Long-term storage should have thread protectors with fresh dope, and pipes should be racked with adequate support to prevent sag.
Conclusion
The pairing of S135 heavy drill pipe with NC38 thread ends is not arbitrary; it reflects decades of drilling engineering evolution. The high yield strength addresses extreme tensile and compressive loads, the heavyweight design smooths the BHA transition and adds concentrated weight, and the NC38 connection delivers a reliable, field-proven threaded interface. Whether you are planning a simple vertical well or a complex ERD campaign, understanding these components in detail—from metallurgy to make-up torque—helps build safer, more efficient drill strings. When sourcing, always look beyond the grade and connection name: confirm the exact dimensional specs, supplementary tests, and handling protocols to get the full value from these workhorse joints in the field.
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