Oilfield Pipe Overview: OCTG, Drill Pipe, Line Pipe, and Coated Pipe

Every oil and gas field requires multiple categories of steel pipe, each serving a distinct mechanical function under different loading conditions and regulatory frameworks. Selecting the wrong pipe category—or the wrong grade within a category—is one of the most costly engineering errors in oilfield procurement. A length of line pipe specified where casing is required will fail under the axial and burst loads of a live well; OCTG casing in a transmission pipeline will fail the weld inspection requirements of ASME B31.8.

This guide maps all four oilfield pipe categories—OCTG, drill pipe, line pipe, and coated pipe systems—covering governing standards, grade selection, typical size ranges, and the key distinctions that procurement engineers and EPC contractors must understand.

ZC Steel Pipe manufactures API 5CT casing and tubing, API 5DP drill pipe, and API 5L line pipe for oil and gas operators and EPC contractors across Africa, the Middle East, South America, and Southeast Asia. We hold PSL-2 production qualification, supply EN 10204 3.1 and 3.2 mill test certificates, and support third-party inspection by Lloyds, Bureau Veritas, and SGS.

What Is Oilfield Pipe?

Oilfield pipe is a broad category covering all tubular steel products used in the exploration, drilling, completion, production, and transportation of oil and gas. Despite sharing a common material (carbon or low-alloy steel) and a common form (round hollow sections), the four major categories are engineered to entirely different specifications and cannot be interchanged.

The governing standards divide broadly as follows:

Within each category, multiple grades define the mechanical performance level. Selecting the correct grade requires understanding both the loading conditions the pipe must survive and the corrosive environment it will encounter.

OCTG: Casing, Tubing, and Connections

Oil country tubular goods (OCTG) is the collective name for the casing, production tubing, couplings, and pup joints that make up the downhole tubular string in an oil or gas well. API Specification 5CT, 11th Edition governs all OCTG produced for the global market.

Casing

Casing is cemented in the wellbore at the end of each drilling interval and remains in place permanently. Its functions are to maintain wellbore integrity, isolate fresh water aquifers, contain formation pressure at the surface, and provide the structural anchor for wellhead equipment and blowout preventers. A typical well uses four to six separate casing strings, each run inside the previous one:

• Conductor casing — largest diameter (16" to 30" OD), shortest length (30 to 100 m), supports surface loads and prevents near-surface formation collapse
• Surface casing — provides wellbore integrity through shallow aquifer zones; sets the foundation for the BOP stack
• Intermediate casing — isolates abnormally pressured formations or lost-circulation zones encountered above the production target
• Production casing — the innermost string, set through the reservoir and perforated or slotted for production

API 5CT Grade Summary

Grade selection drives casing cost more than any other specification choice. The following table summarizes the API 5CT grades available under API Specification 5CT, 11th Edition:

Sour service grades (L80-1, C90, T95, C110) carry a maximum hardness limit under API 5CT because sulfide stress cracking susceptibility increases with hardness. Using a non-sour grade (P110, Q125) in an H₂S-containing well is the most dangerous grade selection error in OCTG procurement.

Production Tubing

Production tubing is run inside the production casing string and carries produced fluids from the reservoir perforations to the surface. Unlike casing, tubing is designed to be pulled and replaced during workover operations. The governing specifications for tubing dimensions and grades are the same as for casing (API 5CT), but tubing uses smaller OD sizes—typically 1.05" to 4½" (26.7 to 114.3 mm)—and is connected with EU (external upset) or NU (non-upset) threaded connections in standard applications, or premium metal-to-metal seal connections in HPHT and sour service.

L80-13Cr and Super 13Cr tubing is the preferred grade for CO₂-corrosive gas condensate wells where a carbon steel string would suffer unacceptable internal corrosion. These martensitic stainless alloys are not sour service grades—they must not be specified in H₂S-containing environments unless combined with appropriate corrosion management protocols per NACE MR0175 / ISO 15156.

Connections

The connection is the threaded joint between adjacent casing or tubing joints. API threaded connections—short thread (STC), long thread (LTC), and buttress thread (BTC)—are the baseline for shallow and medium-depth sweet wells. For HPHT, deviated, or sour wells, premium connections with metal-to-metal radial and shoulder seals (VAM Top, Tenaris Blue, and similar proprietary designs) are required to achieve gas-tight sealing under combined axial, bending, and pressure loading.

For the complete API 5CT grade specifications, dimensional tables, and connection selection guidance, see the API 5CT specification tables →

To match a well environment to the appropriate casing or tubing grade, use the AI Pipe Grade Selector →

Drill Pipe and Drill Stem Components

Drill pipe is the rotating tubular string that connects the surface rig to the drill bit. It transmits torque for bit rotation, carries drilling fluid (mud) downward through its bore to the bit for cooling and cuttings removal, and provides the tensile member that allows the drill string to be pulled out of the hole when required.

API Specification 5DP (ISO 11961) governs the material, dimensions, mechanical properties, and inspection of drill pipe. Unlike casing and tubing—which use threaded API or premium connections—drill pipe uses heavy-wall, shouldered forged steel tool joints at each end. Tool joints are mechanically stronger and more wear-resistant than pipe body connections and allow rapid make-up and break-out during tripping operations.

API 5DP Grades

Drill Collar, Heavy-Weight Drill Pipe, and Subs

The drill string also includes drill collars (thick-wall heavy pipe that provides weight on bit above the drill pipe), heavy-weight drill pipe (HWDP) used as a transition between drill collars and standard drill pipe, and various subs and stabilizers. These components are not governed by API 5DP but by API Specification 7-1 for rotary drilling equipment.

Line Pipe

Line pipe is the oilfield designation for steel pipe used in surface and subsea gathering, transmission, and distribution of oil, gas, condensate, water, and CO₂. API Specification 5L, 46th Edition (ISO 3183) governs line pipe grade designations, mechanical properties, chemistry, testing, and traceability requirements.

Line pipe is classified by grade designation (Grade B, X42, X52, X60, X65, X70, X80) and by product specification level (PSL1 or PSL2). PSL2 imposes additional chemistry limits, impact testing, and traceability requirements and is required for sour service (per Annex H) and for offshore and subsea applications.

API 5L Grade Reference

ZC Steel Pipe manufactures seamless line pipe in sizes from NPS 2 to NPS 16 and LSAW (JCOE-formed) large-diameter line pipe in sizes from NPS 18 to NPS 56. ERW line pipe is available in NPS ½ to NPS 24 for moderate-pressure applications.

Coated Pipe Systems

Any oilfield pipe category may be ordered with an external or internal protective coating when the service environment creates corrosion or contamination risk that bare steel cannot tolerate.

External Coatings

External coatings protect buried or submerged line pipe from soil and seawater corrosion. The main systems in order of increasing thermal resistance:

• FBE (fusion-bonded epoxy): Single-layer powder-applied epoxy, excellent adhesion, suitable for temperatures up to 80 °C. Per CSA Z245.20 or AWWA C213.
• 3LPE (three-layer polyethylene): FBE primer + adhesive copolymer + HDPE topcoat; the global standard for onshore buried pipeline; rated to 80 °C continuous.
• 3LPP (three-layer polypropylene): Same structure as 3LPE but with polypropylene topcoat; rated to 110 °C for high-temperature gathering lines and subsea applications.
• CWC (concrete weight coating): Applied over 3LPE or 3LPP to provide negative buoyancy for offshore pipelines laid on the seabed.

All external coating systems are governed by ISO 21809 series standards. Specify which part applies: Part 1 for 3LPE/3LPP, Part 2 for FBE alone.

Internal Coatings

Internal coatings for gas transmission line pipe reduce frictional resistance and protect against internal corrosion:

• Flow-efficiency FBE: A thin (75 to 125 μm) FBE coating reduces surface roughness from approximately 50 μm (bare steel) to 5 to 10 μm, increasing the effective pipeline flow capacity by 5 to 8 percent.
• Internal liquid epoxy: For gathering systems handling produced water or wet gas, a thicker (250 to 400 μm) amine-cured epoxy coat provides corrosion protection against H₂S, CO₂, and brine.

OCTG casing can also receive an internal coating—typically phenolic epoxy applied to the full string before running—when production of highly corrosive fluids (high CO₂, produced water) would otherwise cause rapid internal wall loss and jeopardize well integrity.

Comparison: Oilfield Pipe Categories

How to Select the Right Oilfield Pipe Type

Selection begins with answering four questions:

1. What function does the pipe serve? Wellbore structural liner → casing. Produced-fluid conduit inside casing → tubing. Rotational drilling → drill pipe. Surface gathering or transmission → line pipe.

2. What is the corrosive environment? H₂S in the well fluid → API 5CT sour service grade (L80-1, C90, T95, C110) for OCTG; API 5L PSL2 Annex H for line pipe. CO₂ without H₂S → chromium-bearing OCTG grades (L80-13Cr, Super 13Cr) for tubing; standard PSL2 for line pipe with internal coating. Sweet gas → standard grades with PSL1 acceptable for lower-risk pipeline segments.

3. What is the design pressure and temperature? For OCTG, higher yield grade reduces required wall thickness for the same collapse and burst resistance. For line pipe, wall thickness is calculated using the design pressure and specified minimum yield strength per ASME B31.8 or B31.4.

4. What connection performance is required? API connections (BTC) are adequate for straight vertical wells with moderate loads. HPHT, highly deviated, or sour wells require premium connections with independently evaluated CAL ratings per ISO 13679.

Purchase Order Guidance

When purchasing oilfield pipe from any mill or distributor, require the following on the purchase order:

1. Standard designation, edition, and grade — e.g., "API Specification 5CT, 11th Edition, Grade P110, Group 3." Specifying edition matters: grade tables change between editions, and an 11th-edition P110 test requirement differs from a 10th-edition requirement.
2. PSL level (for line pipe) — state PSL1 or PSL2 explicitly; PSL2 requires supplemental chemistry, CVN impact testing, and traceability that PSL1 does not.
3. Thread type and API or premium designation — for OCTG, specify "BTC per API 5B" or the premium connection trade name and applicable licensed drawing number.
4. MTC format — EN 10204 3.1 for standard supply; 3.2 when third-party witness is required by the project specification.
5. Inspection and test plan — for critical HPHT or sour service casing, specify 100% ultrasonic inspection (UT) of pipe body and hydrostatic test witnessed by a third-party inspector.

Procurement trap: Accepting a "dual-marked" or "dual-stenciled" pipe that carries both an API 5L and an API 5CT stencil without verifying which tests were actually performed. A pipe dual-marked 5CT / 5L must meet all testing requirements of both standards—but some mills apply dual stenciling based on dimensional compliance alone while only performing the less stringent standard's tests. Review the MTC test record page by page and confirm that every required test (hydrostatic, UT, CVN impact if PSL2, hardness if sour service) is documented with an actual result, not just a "meets requirements" notation.