What Is OCTG?
OCTG (Oil Country Tubular Goods) covers the casing and tubing that make up an oil or gas well, manufactured to API Spec 5CT. Casing lines the borehole; tubing carries the produced fluid to surface. Unlike line pipe (API 5L), OCTG is selected by yield strength and connection integrity as much as by chemistry, because it must withstand collapse, burst, tension and corrosion downhole.
API 5CT Grade Families
| Grade | Min Yield (MPa) | Max Yield (MPa) | Typical Use |
|---|---|---|---|
| H40, J55, K55 | 379 | 552 | Shallow wells, structural casing |
| N80-1 / N80Q | 552 | 758 | Standard mid-depth wells |
| L80-1 | 552 | 655 | Sour service (restricted yield) |
| C90, T95 | 621 / 655 | 724 / 758 | Sour service, higher strength |
| P110 | 758 | 965 | Deep wells, high pressure |
| Q125 | 862 | 1034 | Deep, high-pressure, harsh service |
Restricted-yield grades (L80, C90, T95, C110) have a maximum yield cap plus hardness control so the material meets NACE MR0175 / ISO 15156 requirements for H2S-containing wells. Never substitute a higher-strength grade for a restricted-yield grade in sour wells simply because it is stronger — higher hardness increases sulfide stress cracking risk.
Casing, Tubing and Connections
Common casing sizes run from 4-1/2 in up to 20 in OD; tubing runs from 1.090 in to 4-1/2 in. API 5CT defines three standard connections — STC (short round thread), LTC (long round thread) and BTC (buttress thread) — plus integral premium connections supplied by mill or licensee for gas-tight sealing, high torque and well-integrity demands.
Selection order: well depth and pressure → collapse/burst/tension design → H2S / CO2 environment → grade and connection. Document the design basis in the inquiry so the mill can propose the right grade.
What Buyers Should Specify
For each inquiry: size and wall, grade (including type, e.g. N80Q), connection, coupling type, service environment (sour/none), and required supplementary testing such as slip/collapse testing or full-body ULT. Every OCTG shipment must carry an EN 10204 3.1 mill certificate referencing API 5CT; third-party inspection is strongly recommended for casing strings.
Casing Design Basics: Collapse, Burst and Tension
A casing string is sized by three load cases: collapse (external pressure from mud and cement — critical near the bottom), burst (internal pressure during kicks and tests — critical near the surface) and tension (string weight, critical at the top joint). Design factors are typically 1.0–1.125 for collapse, 1.05–1.1 for burst and 1.6–1.8 for tension, set by company policy and local regulation.
| Load Case | Worst Location | Typical Design Factor |
|---|---|---|
| Collapse | Bottom of string | 1.0–1.125 |
| Burst | Top of string | 1.05–1.1 |
| Tension | Top joint | 1.6–1.8 |
Production Casing vs Tubing: Different Jobs
Casing is cemented to the formation and stays for the life of the well; tubing is the replaceable conduit inside it and is packed off at surface with a tubing hanger. That is why tubing tolerances, threads and metallurgy are held tighter — it sees the produced fluid directly and is pulled and re-run during workovers.
Frequently Asked Questions
What does the P in P110 mean?
Nothing by itself — P110 designates a yield strength of 110,000 psi (758 MPa) minimum. The letter is simply the API grade series; L, C and T grades are the restricted-yield (sour-capable) families.
Can P110 be used in sour wells?
Not as a standard grade. NACE MR0175 / ISO 15156 restricts carbon steel to 22 HRC, which P110 exceeds. In sour wells use L80, C90, T95 or C110, or clad/compound solutions for extreme conditions.
Which certificate comes with API 5CT pipe?
Each shipment carries an EN 10204 3.1 mill certificate referencing API 5CT, with heat chemistry, mechanical results, heat-treatment cycle and test pressures. Third-party inspection can verify traceability back to heat numbers.
What is a premium connection?
A proprietary threaded or integral joint (mill-designed) that seals metal-to-metal for gas-tightness and handles higher torque and compression than API STC/LTC/BTC threads. Specified when API threads cannot meet sealing or load requirements.
Ordering OCTG from China: Due-Diligence List
Confirm the mill holds a valid API 5CT monogram license (license number is printed on the coupling and the MTC). Request the license copy with your quotation, check its scope covers your size/grade, and verify markings on every length at pre-shipment inspection. For critical strings, witness slip-collapse or full-body ULT testing at an accredited lab.
Ask for the API monogram license number at quotation stage — not after the PO. It is the fastest way to filter out traders repacking non-monogrammed stock.
Need a quotation for steel pipe? CREATEEL supplies carbon steel, alloy and stainless pipes to API, ASTM and EN standards with EN 10204 3.1/3.2 certification. Contact us for a fast offer.
Standards & references: American Petroleum Institute (API Spec 5CT) · AMPP (NACE MR0175)
