NDT of Steel Pipe: Methods, API Requirements, and What to Specify

Non-destructive testing separates the pipe that passed dimensional inspection from the pipe that is fit for the well. Every API 5CT casing specification lists minimum inspection requirements — but for most grades, those minimums stop at hydrostatic testing and visual examination. For J55 surface casing or N80 intermediate string, the mill is fully compliant delivering pipe with no ultrasonic or electromagnetic testing unless the purchase order explicitly requires it. Many procurement teams assume NDT was run. It was not.

ZC Steel Pipe supplies API 5CT casing and tubing and API 5L line pipe to operators in West Africa, the Middle East, and South America. We have processed purchase orders where the NDT requirement was missing, underspecified, or conflated with the hydrostatic test. This guide exists to close that gap.

What NDT Covers and What It Does Not

NDT is a family of inspection methods, not a single test. Each method detects a different category of flaw, and no single method detects everything. The table below is the starting point for every inspection test plan.

The hydrostatic test is not an NDT method in the technical sense — it does not locate or characterize flaws. It confirms that the pipe holds pressure to a defined level. A pipe can pass hydrostatic testing and still contain laminations, seams, or weld flaws that will propagate under cyclic or corrosive loading. This distinction matters when writing ITPs.

API 5CT NDT Requirements by Grade

The table below summarizes what API Specification 5CT, 11th Edition requires at the mill for each grade without supplementary requirements. "Mandatory" means the mill must perform this inspection for the MTC to be valid; "purchaser option" means the mill will not run the test unless the PO specifies it.

P110 is the inflection point. Below P110, the purchaser carries the responsibility for specifying NDT. At P110 and above, the mill cannot omit UT or ET and remain API-compliant.

Ultrasonic Testing (UT)

Ultrasonic testing introduces high-frequency sound waves into the pipe wall through a transducer. When the wave encounters a discontinuity — an inclusion, lamination, crack, or weld defect — part of the energy reflects back to the transducer and is recorded as an indication. The equipment is calibrated against a reference standard before each inspection run.

Straight-Beam UT

Straight-beam UT directs sound perpendicular to the pipe wall surface. It is the primary method for detecting laminar defects — planar flaws parallel to the pipe surface — and for measuring wall thickness. A lamination does not affect hydrostatic test results but can cause catastrophic failure under axial loading or when pipe is cut for coupling makeup.

Angle-Beam UT

Angle-beam UT directs sound at an oblique angle, typically 45° to 70°, making it sensitive to transverse cracks, longitudinal cracks, and weld seam defects. For ERW pipe, angle-beam UT of the weld seam is the primary method for detecting hook cracks and lack-of-fusion defects in the bond zone. API 5L PSL2 ERW pipe requires angle-beam UT of the seam per the standard's inspection requirements.

Acceptance Criteria

API 5CT defines acceptance thresholds using reference notches machined into a pipe sample of the same OD and wall thickness. The standard longitudinal reference notch is 12.5% of the nominal wall thickness in depth. Any signal equal to or greater than the reference notch signal triggers rejection. The mill may accept the pipe after re-inspection if the indication cannot be reproduced on the second pass, but the decision must be documented.

Purchasers who specify "UT per API 5CT" without further qualification get 12.5% of wall as the threshold. For sour service wells, subsea applications, or critical HPHT strings, many project specifications tighten this to 5% of wall — but this must be explicitly written on the purchase order.

End-Area Coverage

Automated UT equipment has a physical dead zone at each pipe end where the transducers cannot generate valid signals — typically 50 to 100 mm depending on the equipment configuration. Pipe body UT does not cover this zone. If end-area integrity is required — and for threaded connections it always is — a separate end-area inspection by UT or MT must be specified. Failing to specify this is among the most common ITP gaps we see on inbound orders for P110 and L80.

Magnetic Particle Testing (MT)

Magnetic particle testing magnetizes the pipe — either by passing DC current through it or by applying a magnetic yoke — and then applies iron particles (dry or suspended in liquid) to the surface. Magnetic flux leaks at discontinuities, attracting particles that form visible indications.

MT is the most reliable method for surface cracks, seams, laps, and near-surface laminar defects in ferromagnetic carbon steel. For pipe ends, coupling faces, and connection areas, MT is standard. API 5CT Section 10.13 covers MT application for OCTG ends and thread areas.

Acceptance criteria under API 5CT for MT indications: linear indications greater than 25 mm in length are cause for rejection or repair. Rounded indications greater than 5 mm in diameter are cause for rejection. These thresholds apply at the pipe surface — sub-surface detection capability is limited to approximately 2–3 mm depth.

Critical limitation: MT cannot be used on non-magnetic materials. L80-13Cr, Super 13Cr, duplex 2205, super duplex 2507, and austenitic stainless steel grades are not ferromagnetic and do not respond to MT. Applying MT to a 13Cr coupling produces no useful result. Liquid penetrant testing (PT) is the substitute for these materials.

Liquid Penetrant Testing (PT)

Liquid penetrant testing applies a low-viscosity penetrant to the cleaned pipe surface. The penetrant is drawn into surface-open discontinuities by capillary action. After a dwell time, the excess is removed and a developer is applied. The developer draws the penetrant back to the surface, revealing the outline of any open flaw.

PT detects only surface-open defects — any flaw with a closed surface cannot be found. Its advantage over MT is material independence: PT works on 13Cr, Super 13Cr, duplex, stainless steel, and any other CRA grade where MT is not applicable.

Under API 5CT, PT is the required surface examination method for L80-13Cr and other CRA grades where MT is not appropriate. Acceptance criteria mirror the MT thresholds: linear indications greater than 25 mm are cause for rejection; rounded indications greater than 5 mm are rejected.

We supplied 3,800 joints of 4½" L80-13Cr PSL2 flowline pipe to an African gas project where the operator's ITP initially called out MT for all coupling face and pin-end inspection — a carry-over from the standard carbon steel OCTG template. L80-13Cr is martensitic stainless steel; it does not respond to MT. We flagged the mismatch before production began and the ITP was revised to PT for all 13Cr joints. Catching it at the ITP review stage avoided a hold at third-party inspection, which would have required the inspector to return for a second visit after the procedure was corrected.

Electromagnetic Testing (ET)

Electromagnetic testing encompasses two related techniques used widely in pipe mill production:

Eddy current testing induces alternating electrical currents in the pipe wall. Discontinuities alter the eddy current flow and are detected as changes in the signal. Eddy current is sensitive to surface and near-surface defects and is fast — high-speed automated systems can test full production runs.

Magnetic flux leakage (MFL) saturates the pipe wall with a strong magnetic field and detects the flux that leaks at discontinuities. MFL is better than eddy current for detecting defects beneath surface scale or light oxidation.

API 5CT Section 10.12 permits ET as an alternative to UT for grades that require NDE — including P110. In practice, Chinese mills running large J55 and N80 orders frequently use ET rather than UT because the throughput is higher. For these grades, ET is commercially sensible and technically defensible.

The limitation appears at wall thickness. For walls above approximately 16 mm, ET sensitivity to internal flaws degrades. UT penetrates the full wall thickness regardless of how thick it is. For heavy-wall P110 or thick-wall line pipe, project specifications that permit "UT or ET" should be read as accepting ET on standard-wall sizes and requiring UT on heavy-wall.

API 5L NDT Requirements for Line Pipe

For line pipe, the NDT requirements are defined in API Specification 5L, 46th Edition and depend on PSL level and pipe type.

PSL1 line pipe: No mandatory NDT beyond hydrostatic testing and visual inspection. This applies to seamless, ERW, LSAW, and SSAW pipe at PSL1.

PSL2 welded pipe (ERW, LSAW, SSAW): Weld seam NDT is mandatory. For ERW pipe, angle-beam UT of the full weld seam and heat-affected zone is required. The reference notch for ERW seam UT is typically a notch in a calibration pipe cut from the same heat — the threshold system is the same as API 5CT's notch method.

PSL2 seamless pipe: Full-body UT or ET is required for the pipe body. Acceptance criteria use the same reference notch approach.

For sour-service PSL2 pipe (HIC-tested grades), full-body UT for laminar defects is often added to the project specification in addition to the API 5L standard requirement, because laminations create stress concentration sites that promote hydrogen-induced cracking. This is not in the base API 5L standard — it must be specified.

Purchase Order Guidance

Specifying NDT — the Right Language

Insufficient PO language: "API 5CT P110 — NDE per API 5CT standard."

This language satisfies the minimum for P110 (UT or ET per Section 10.12), but specifies nothing about:

• Reference notch depth (defaults to 12.5% of wall)
• Method (UT or ET — the mill chooses)
• End-area coverage (dead zone may not be covered)
• Weld seam coverage for ERW (straight-beam body UT alone misses seam defects)
• Calibration record delivery requirements

Complete PO language for P110 (example):

"Full-body ultrasonic testing per API 5CT Section 10.12. Reference notch depth: 12.5% of nominal wall thickness. Angle-beam UT of full weld seam and HAZ (ERW pipe only). End-area UT to within 25 mm of each pipe end. UT calibration records, reference standard certificate, and equipment calibration log to be included with MTC package. Calibration frequency: per API 5CT, minimum once per production shift."

Common Procurement Traps

Trap 1 — Assuming NDT is included for lower grades. A PO for J55 surface casing that does not invoke SR15 or explicitly specify UT will receive pipe with only hydrostatic test and visual inspection. The mill is fully API-compliant. If the project specification requires UT on all casing regardless of grade, that requirement must appear on every PO.

Trap 2 — "NDE per API 5CT" on a CRA grade. API 5CT does not specify a particular surface examination method — it permits MT or PT depending on material. A PO for L80-13Cr that says "MT per API 5CT" is technically incorrect because MT cannot be applied to martensitic stainless steel. Write "PT per API 5CT for L80-13Cr" explicitly.

Trap 3 — Not distinguishing body UT from seam UT. For ERW casing or line pipe, straight-beam body UT does not reliably cover the weld seam. The hook crack failure mode in ERW pipe — the most common ERW weld defect — is a transverse or angular crack that is found only by angle-beam scanning of the seam. Specify both body UT and seam UT as separate line items.

Trap 4 — Waiving hydrostatic test when UT is specified. Some purchasers reason that if UT is run, the hydrostatic test adds nothing. This reasoning is wrong: UT finds planar flaws, but it does not confirm that the pipe system holds pressure without leakage through the weld bond zone. The two tests are complementary. Retain the hydrostatic test unless the project specification explicitly prohibits it for a stated reason.

Receiving Inspection Checklist

Before releasing an NDT-inspected shipment at the receiving yard:

1. Confirm the MTC identifies the NDE method applied (UT, ET, MT, PT) and the standard section invoked
2. Verify calibration records are included — equipment serial number, reference standard traceable to a calibration body, shift start/end times
3. Check that end-area coverage is documented — if it is absent, it was not done
4. For P110 and Q125, verify the UT operator's qualification level (minimum Level II per ISO 9712 or SNT-TC-1A)
5. Cross-reference the pipe joint serial numbers on the NDE records against the joints in the shipment — partial records are a common finding on large orders

For the full API 5CT grade requirements and dimensional data, see the API 5CT specification tables →

To match a casing grade to your well conditions and inspection requirements, use the AI Pipe Grade Selector →