The Three API Product Specifications
Pipeline steel pipe is not covered by a single document. The American Petroleum Institute publishes separate specifications for the three families of tubulars used in the oil and gas industry, and each has its own scope, grade list and testing regime.
API 5L - specification for line pipe, covering seamless and welded pipe used to transport oil, gas and water. It is issued in two product specification levels, PSL1 and PSL2.
API 5CT - specification for casing and tubing, covering the wellbore tubulars that protect the borehole and convey production.
API 5DP - specification for drill pipe, covering the threaded string used to transmit rotation and drilling fluid to the bit.
Although the three documents share terminology and some test methods, they are not interchangeable. A line pipe grade and a casing grade with similar yield strength may have different chemistry limits, different dimensional tolerances and different inspection requirements, so the applicable specification must be stated on the purchase order.
Classification of Line Pipe Grades
| Group | Typical grades | Strength level | Usual duty |
|---|---|---|---|
| Low strength | Grade B | Minimum yield 35 ksi class | Low-pressure gathering and utility lines |
| Medium strength | X42, X52, X56 | 42 to 56 ksi | General transmission, distribution, station piping |
| High strength | X60, X65, X70 | 60 to 70 ksi | High-pressure trunk lines, large diameter transmission, sour service in PSL2 |
Grades are also classified by use, by manufacturing route and by delivery condition. The same grade can be produced as seamless, HFW/ERW, LSAW or SSAW pipe, and the delivery condition may be as-rolled, normalized, quenched and tempered or thermo-mechanically processed, all of which influence toughness and field weldability.
How the Pipe Is Produced
Material selection and steelmaking: carbon steel or microalloyed steel is selected to achieve the required strength, toughness and weldability, with desulphurization and inclusion control used for sour service qualities.
Steelmaking and casting: the heat is produced by converter or electric furnace route, then continuously cast into rounds or slabs. Each heat is identified so that it can be traced to the finished pipe.
Pipe forming: seamless pipe is pierced and rolled, welded pipe is formed from coil or plate and welded by HFW/ERW, longitudinal submerged arc or spiral submerged arc welding.
Heat treatment and sizing: normalizing, quenching and tempering or controlled cooling are applied as required by the grade and delivery condition, followed by sizing and straightening.
Testing and finishing: hydrostatic testing, non-destructive inspection, dimensional checks, end preparation and coating complete the process.
Key Parameters at the Ordering Stage
| Parameter | Why it matters |
|---|---|
| Grade and product specification level | Fixes the strength, chemistry and testing requirements |
| Outside diameter and wall thickness | Determines flow area, pressure containment and tonnage |
| Manufacturing route | Affects weld seam quality, dimensional tolerance and cost |
| Impact test temperature | Sets the minimum design metal temperature that the line can serve |
| End finish and coating | Determines field welding method and external corrosion protection |
Application Areas
API line pipe is used for oil and gas gathering and transmission, water injection and disposal lines, slurry pipelines and station piping. API 5CT casing and tubing is used to line the wellbore and to convey produced fluids to surface in conventional and unconventional wells. API 5DP drill pipe carries the drilling load in exploration and development drilling. Demand for higher strength and more corrosion-resistant qualities continues to rise as fields move into deeper water, colder regions and higher-sulphur reservoirs, and as wind, solar and other energy projects add requirements for structural and fluid-transfer tubulars.
Selection Points to Confirm Before Ordering
State the applicable specification, not only the grade, because the same strength can be supplied against different documents.
Decide whether PSL1 is sufficient or whether the toughness, chemistry and traceability of PSL2 are needed.
Confirm the delivery condition and the resulting welding procedure, since thermo-mechanically processed material has a lower allowable heat input than normalized material.
Verify the inspection level, including the extent of non-destructive examination of the weld seam and the acceptance criteria.
Reconcile the coating specification with the pipe specification, since coating and pipe are separate purchase items.
FAQ
Q: What is the difference between API 5L PSL1 and PSL2?
PSL1 defines a single set of requirements with routine testing. PSL2 adds tighter chemistry limits, a bounded yield strength range, a maximum yield-to-tensile ratio and mandatory impact testing.
Q: Can API 5L pipe be used as casing?
The two products are made to different specifications and are not interchangeable. Wellbore tubulars should be ordered to API 5CT so that the collapse, burst and connection requirements of the well design are met.
Q: Which grades are most common for transmission lines?
Grades X42 to X70 cover most transmission work, with X52 and X65 among the most widely specified for onshore and offshore pipelines respectively.
Q: What does the delivery condition change?
It changes the microstructure of the steel and therefore its toughness, its behaviour in welding and the heat input that the qualified welding procedure must respect.
Q: Why is impact testing specified on some pipelines and not others?
Impact testing is specified where the line must resist brittle fracture at the minimum design metal temperature, which depends on the route environment and the operating conditions.
Q: How is traceability maintained?
Each heat is identified during steelmaking and the number is carried through forming, heat treatment and finishing so that the finished pipe can be linked to its mill test certificate.
