Guided Wave Testing (GWT/LRUT) | Long-Range Pipe Screening | CUI Detection

NDT Method Guide · Updated February 2026

Screen hundreds of metres of pipe from a single test point. The primary NDT method for corrosion under insulation (CUI) screening, buried pipeline assessment, and rack piping — without full strip-out or excavation.

The GWT system consists of a transducer ring — a collar of piezoelectric or magnetostrictive transducer elements clamped around the outside of the pipe. These elements generate torsional (T-mode) or longitudinal (L-mode) guided waves at frequencies typically between 10 and 100 kHz. The waves propagate bidirectionally along the pipe wall, reflecting from any feature that causes a change in the pipe's cross-sectional area — welds, flanges, corrosion patches, pitting, or cracks.

Signal interpretation is fundamental to GWT. Symmetric reflections — typically from welds and flanges — appear as balanced signals. Asymmetric reflections — indicating localised metal loss such as corrosion or cracking — produce characteristic non-symmetric signal responses that allow a qualified technician to identify their location along the pipe. The data is displayed as an A-scan with a distance axis (similar to a radar trace), showing signal amplitude versus distance from the collar.

Corrosion under insulation (CUI) is one of the most costly and difficult-to-detect degradation mechanisms in oil and gas, petrochemical, and power generation facilities. Traditional CUI inspection requires removing insulation across the full length of suspect pipe runs — an enormously time-consuming and expensive process. Guided wave testing has transformed CUI inspection economics.

With GWT, a single insulation window approximately 200–300mm wide is cut at the collar position. The guided waves then propagate through the intact insulation and the pipe wall, detecting any corrosion or wall loss along the insulated run. GWT identifies which sections of the insulated pipe have potential issues — allowing insulation removal to be targeted only at those specific areas for confirmation by conventional UT.

CUI detection also used in conjunction with Pulsed Eddy Current (PEC) — see our complete ECT guide for comparison.

GWT procedures must reference applicable standards for equipment qualification, technique requirements, and reporting. The following standards govern guided wave testing in the oil & gas and industrial inspection sectors:

Understanding GWT limitations is critical for using it correctly within an inspection program. Misapplication of GWT — treating it as a sizing or confirmation tool — can lead to missed defects or incorrect fitness-for-service decisions.

GWT is a specialised technique requiring dedicated training and qualification beyond general UT certification. The following certification schemes are recognised in the industry:

Atlantis NDT provides ASNT-aligned GWT/LRUT training and certification support, online, live-virtual, or onsite at your facility. Our Level III UT experts develop GWT written practices and qualification procedures compliant with ASTM E2775 and API 570 Appendix H.

Atlantis NDT provides ASNT Level III GWT consulting services globally — procedure development compliant with ASTM E2775 and API 570, personnel qualification review, and inspection program design for CUI screening, buried pipe, and offshore riser programs. Contact our team to discuss your specific application.

Our ASNT Level III engineers develop GWT procedures, qualify personnel, and design CUI screening programs for refineries, pipelines, and offshore assets.

What this page covers

What Is Guided Wave Testing and How Does It Work?
GWT Inspection Process — Step by Step
Guided Wave Testing Applications
Governing Standards for Guided Wave Testing
GWT Limitations — What Guided Wave Testing Cannot Do
GWT vs Conventional UT vs Radiographic Testing
GWT Certification & Qualification Requirements
Related NDT Methods & Resources
Guided Wave Testing — Frequently Asked Questions
Plan a GWT / LRUT Inspection Program?
Key GWT Physical Principles
Deep Dive: CUI Screening with Guided Wave Testing
On This Page
Related NDT Methods

Key points covered

Guided Wave Testing (GWT), also called Long-Range Ultrasonic Testing (LRUT), is a non-destructive testing method that propagates low-frequency ultrasonic waves along the length of a pipe or structure. Unlike conventional UT which tests one small spot at a time, GWT sends waves from a single test point that travel hundreds of metres in both directions, screening the entire pipe for areas of metal loss, corrosion, or cracking. It is primarily used as a screening tool to identify areas for further investigation with conventional UT.
In ideal conditions — clean, bare steel pipe — guided waves can travel 100 metres or more in each direction from the transducer collar. Practical inspection range is typically 20–50 metres each direction and depends heavily on pipe condition. Factors that reduce range include: heavy bitumen or viscoelastic coatings, liquid-filled pipes (especially viscous fluids), soil loading on buried pipe, corrosion severity, and pipe fittings like tees and reducers that reflect or scatter the wave. GWT reports specify the inspection range achieved based on signal attenuation measured during the test.
GWT is suitable for most metallic pipes: carbon steel, stainless steel, alloy steels, and duplex grades. Common applications include: process piping in refineries and chemical plants (especially insulated pipe), buried pipelines at road/rail crossings, rack piping in elevated pipe racks, offshore risers and subsea risers, water injection lines, gas distribution pipelines, and firewater mains. GWT is less effective on pipes with heavy viscoelastic coatings (internal or external), pipes with multiple elbows in close succession, and pipes smaller than approximately 2 inches nominal diameter.
Detect corrosion under insulation without full strip-out. A small window cut in the insulation gives access for the collar. GWT screens the full insulated run and identifies suspect areas for targeted insulation removal and conventional UT sizing — reducing inspection cost by 60–80%.
Screen road crossings, river crossings, and buried pipeline sections without excavation. GWT accesses the pipe at an exposed section (valve box, above-grade riser) and propagates waves through the buried section — detecting corrosion or wall loss that would otherwise require costly dig-up.
Inspect elevated pipe racks from ground level without scaffolding. The collar is installed at an accessible location on the rack; guided waves screen the full rack run from a single position. Particularly useful for detecting rack-support interface corrosion where pipe contacts the rack structure.
Screen splash zone and submerged riser sections where direct access is costly. GWT from the accessible topside section propagates waves downward through the riser, detecting corrosion in the difficult-to-access splash zone and below waterline — reducing or eliminating costly diving or ROV inspections.
Detect underside corrosion of tank annular plates from the tank rim without emptying the tank. GWT propagates along the annular plate, identifying areas of corrosion or pitting on the underside that would otherwise only be visible during a full tank entry inspection to API 653.
Standard Practice for Guided Wave Testing of Above Ground Piping — primary GWT standard
Standard Practice for Guided Wave Testing of Above Ground Steel Piping with Magnetostrictive Transduction (screening; follow-up required)
Recognises guided wave examination as a screening technique to locate areas for follow-up inspection
Employer-based qualification and certification of GW examination personnel (Guided Wave is a listed method)
Screening tool only — cannot accurately size defects or measure wall thickness precisely
Cannot quantify corrosion depth without follow-up conventional UT or PAUT
Signal attenuation reduced by heavy viscoelastic coatings (bitumen, rubber linings) — may limit range to 5–10m
Liquid-filled pipe (especially viscous fluids) attenuates signal more than gas-filled pipe
Dead zone at collar location — approximately 2–3 pipe diameters either side of collar cannot be inspected
Cannot inspect through certain fittings: tees, reducers, and valves limit wave propagation and range
Requires interpretation by qualified Level II/III technician — signal interpretation is more complex than conventional UT
Not suitable for very small bore pipe (below ~2 inch NPS) or heavily corroded pipe where wall loss exceeds ~30%
Site preparation: identify collar location — ideally at an insulation window or accessible section. Mark test point. Clean pipe surface at collar location for good acoustic coupling.
Collar installation: clamp transducer ring around pipe circumference. Ensure all elements are in uniform contact. Set transducer frequency appropriate for pipe diameter and expected range.
System calibration: verify signal quality using known weld reflections. Establish noise floor and signal-to-noise ratio. Set alerting thresholds based on ASTM E2775 requirements.
Data acquisition: transmit guided waves bidirectionally. Record full waveform response from collar to the limit of inspection range in each direction. Identify all reflectors (welds, supports, flanges).
Signal interpretation: qualified Level II technician analyses A-scan trace. Classifies each reflector as symmetric (structural feature) or asymmetric (potential defect). Flags anomalous asymmetric reflections.
Range assessment: record maximum inspection range achieved based on signal-to-noise at pipe ends or at known reflectors at maximum distance. Document any zones of reduced sensitivity.
Follow-up recommendation: all flagged anomalies are assigned a priority for follow-up conventional UT inspection. Provide location (distance from collar ± direction) for each flag.
Reporting: written report per ASTM E2775 or ASTM E2929 — including collar location, inspection range, signal quality, all reflector classifications, and follow-up recommendations.
UT Level II with GWT/LRUT endorsement. USA-based qualification scheme. Written practice developed by employer must specify GWT as a sub-method with specific experience and training hours.
PCN (Personnel Certification in Non-Destructive Testing) Long Range Ultrasonic Testing category. UK-based, internationally recognised. Level 1 and Level 2 qualification.

Guided wave testing for plant piping: screening, range and follow-up

Direct answer: Guided wave testing (GWT, also called GW, GWUT or long-range UT) sends low-frequency ultrasonic waves along the pipe wall from a ring of transducers clamped at one location, and records reflections from changes in cross-section such as welds, supports and corrosion. It screens long or hard-to-reach lengths of pipe quickly, but it is a screening tool: it does not measure remaining wall thickness. Any significant call-out needs follow-up with UT, corrosion mapping, profile radiography or another quantitative method before an inspector can use it in an API 570 corrosion-rate or remaining-life decision.

This section adds the practitioner detail that matters when GWT is used inside an in-service piping program: what the ASTM standards actually cover, what shortens range, how call-outs are followed up, and how the results fit the owner's API 570 obligations.

What the ASTM guided wave standards cover

Two ASTM practices describe GWT on above-ground steel piping. ASTM E2775 covers guided wave testing using piezoelectric transduction; ASTM E2929 covers magnetostrictive transduction. Their stated scopes are similar and are worth knowing, because they define the conditions under which the technique is on firm ground:

Material and size. Carbon steel or low-alloy steel pipe, NPS 2 to 48, with wall thickness between 0.15 and 1 inch (3.81 to 25.4 mm).
Basic configuration. Straight pipe of a single size and schedule, fully accessible at the test location, joined by girth welds, held on simple contact supports, and free of internal or external coatings. E2929 notes that insulation and paint are acceptable within its basic configuration.
Screening only. Both standards state that GWT is a screening tool and does not provide a direct measurement of wall thickness.
Follow-up expected. Where a possible defect is found, follow-up of the suspect area with UT or other NDT is normally required to obtain detailed thickness information.

The current active versions listed by ASTM are E2775-16 (reapproved 2023) and E2929-18 (reapproved 2022). The UK also publishes BS 9690 parts on guided wave testing. Owners can and do apply GWT outside the basic configuration (bends, coated lines, buried sections), but the procedure must then address those conditions explicitly, and the reliability of the screen falls as complexity rises.

Where GWT earns its place in an API 570 program

API 570 requires owners to inspect piping for the damage mechanisms that apply, with particular attention to locations where damage concentrates and that are hard to reach. GWT is most useful where conventional access is expensive or disruptive:

Insulated piping susceptible to CUI. A single collar location, with a small window of insulation removed, screens a length of insulated pipe in each direction. Call-outs show where to strip insulation for follow-up. See CUI inspection and the broader CUI monitoring guide.
Road crossings, casings and wall penetrations. Sections that cannot be reached without excavation or demolition.
Soil-to-air interfaces and short buried sections. Places where piping enters the ground and corrosion concentrates; range into soil is usually much shorter than in air.
Pipe racks and elevated runs. Long runs where scaffolding for spot UT would be costly.
Under supports. Contact-point corrosion at supports is a common screen target, though supports themselves generate reflections that the analyst must separate from damage.

GWT does not replace the CML program. It tells the inspector where to look; the CML readings and follow-up measurements are what feed the corrosion rate. For the circuit and CML framework, see piping circuit and CML inspection.

What limits range and sensitivity

Range quoted in sales literature is usually for clean, straight, uncoated pipe in air. Real plant piping rarely looks like that. The main factors that shorten range or blur the screen:

Condition	Effect on GWT	Practical response
Bends and elbows	Distort the wave mode; reduce sensitivity beyond the bend	Add test locations on each side; treat beyond-bend results with caution
Heavy or bonded coatings (bitumen, wraps, some epoxies)	Absorb energy; sharply reduce range	Shorter test spacing; consider other screening methods
Buried pipe and soil contact	Energy leaks into surrounding soil	Shorter range; excavate for test locations as needed
Flanges, valves, tees and branches	Strong reflections; little or no transmission past many fittings	Treat each as a test-section boundary
Clamped or welded supports	Reflections that can mask or mimic corrosion	Map supports in the field and compare with signals
Liquid contents and deposits	Can attenuate some wave modes	Note service conditions in the procedure
Small, isolated pits and gradual wall loss	Low cross-section change; hard to detect	Rely on CMLs and scanning, not GWT, for these mechanisms
Temperature	Equipment and collar temperature limits apply	Confirm limits with the system and procedure

The most important limitation is the last few rows: GWT responds to the loss of cross-sectional area around the circumference, so a small isolated pit can be invisible to it. That is why it is a poor primary tool for pitting mechanisms and a good one for general or area corrosion such as CUI.

From call-out to decision: the follow-up workflow

A GWT survey that stops at the report is only half done. A typical workflow, described qualitatively:

Plan. The owner's inspector identifies lines and damage mechanisms to screen. The NDE provider reviews isometrics, coatings, insulation, supports and access, and proposes test locations.
Collect. Collars are fitted at each location; data is collected in both directions with distance-amplitude reference from known features such as girth welds.
Analyse. The analyst identifies welds, supports and fittings, then flags anomalous reflections and grades them by relative severity under the procedure.
Report. Call-outs are reported with axial position, a severity category and confidence notes on range and coverage limits. Areas not screened (beyond bends, past flanges, out of range) are stated, not left blank.
Follow up. The owner's inspector decides which call-outs to verify. Verification uses quantitative methods: UT thickness grids, corrosion mapping, profile radiography or PAUT, often after insulation or coating removal at that spot.
Decide. Measured thickness feeds the API 570 calculations. If wall is below minimum, the owner decides on repair, replacement or an engineering assessment; Atlantis does not perform fitness-for-service assessments.
Baseline and repeat. Where permanently installed collars or repeated surveys are used, comparing signals over time can improve sensitivity to change. The owner decides whether and when to repeat.

A frequent program weakness is call-outs that were never verified. If a screen produces a call-out and no follow-up is recorded, an auditor will reasonably ask what the owner knew and when.

Worked example, described qualitatively

A refinery has an insulated Class 2 hydrocarbon line running along a pipe rack, with history of CUI on neighbouring lines. Stripping the full run for visual and UT would mean scaffolding and weeks of insulation work. The owner's inspector asks for a GWT screen. The NDE provider sets test locations at intervals that account for two elbows and several clamped supports, removing a small window of insulation at each collar location.

The screen identifies welds and supports as expected, and three anomalous reflections between supports. Two are graded as minor and one as significant. The inspector directs insulation removal at all three for follow-up. Corrosion mapping at the significant call-out shows an area of wall loss under a damaged jacket seam; UT grids at the two minor call-outs show light surface corrosion. The measured data goes into the circuit's corrosion-rate and remaining-life calculations. The report also states which segments, beyond one elbow, were screened with reduced confidence, so the inspector can decide whether to add spot UT there. Intervals and repairs are the inspector's decisions.

Personnel qualification and procedures

Guided wave testing is recognized as its own method in ASNT's SNT-TC-1A, so employers can qualify and certify GW personnel under their written practice. Because interpretation depends heavily on the analyst's skill, owners commonly ask for the analyst's GW certification and for evidence of training on the specific equipment. A good procedure states the equipment, the wave modes and frequencies used, the pipe configurations covered, the reference method for amplitude, how call-outs are graded, how unscreened zones are reported, and how follow-up is recommended. The procedure should be reviewed by a Level III with GW and UT knowledge. For training under SNT-TC-1A, see ASNT Level III training.

GWT compared with other piping screening and measurement methods

Method	Measures thickness?	Coverage per setup	Best role
Guided wave testing	No (screening)	Long lengths from one location in good conditions	Find where to look on insulated, elevated or inaccessible runs
UT spot thickness	Yes	A single point	CML readings for corrosion rates
Corrosion mapping (UT scanning)	Yes	An area grid	Quantify localized corrosion found by screening
Profile radiography	Yes, by image measurement	A short section per exposure	Small-bore and insulated lines; verify call-outs without full stripping
Pulsed eddy current	Average wall over a footprint	Point-by-point through insulation	Screening insulated pipe for general wall loss

What to include in a guided wave scope request

A clear scope makes the difference between a screen the inspector can act on and a report full of caveats. Before requesting GWT, owners usually assemble the following:

Line list and isometrics. Line numbers, sizes, schedules, materials and routing, with bends, fittings, branches and supports marked.
Condition information. Insulation type and jacketing, coating type (especially any bonded or heavy wrap), buried or cased sections, and known damage history.
Service conditions. Operating temperature at the collar locations, contents, and whether the line can stay in service during the survey.
Damage mechanism of concern. CUI, under-support corrosion, soil-side corrosion and so on, so the procedure and test spacing suit the target.
Follow-up expectations. Whether follow-up UT or corrosion mapping should be done in the same mobilization, and who authorises insulation removal at call-outs.
Reporting format. How call-outs should be referenced to isometrics or CML numbering so they can be traced in the owner's inspection records.

Doing screen and follow-up in one mobilization usually gives the inspector a decision-ready package; splitting them across months tends to leave call-outs unverified.

How Atlantis supports guided wave programs

Atlantis performs guided wave screening on piping and the quantitative follow-up that makes it useful: UT thickness grids, corrosion mapping, profile radiography by crews licensed where the work is done, and PAUT. Work is done by ASNT-certified technicians under ASNT Level III oversight, to written procedures that state coverage limits. We deliver results to the owner's API 570 inspector, who remains inspector of record and decides follow-up, intervals and repairs. Request a quote for GWT with follow-up; we respond within 24 hours.

Guided wave testing FAQ

What is guided wave testing (GW) in NDT?

It is an ultrasonic method that sends low-frequency waves along a pipe from a transducer ring at one point, detecting changes in cross-section over long distances. It is used as a screening tool.

Does guided wave testing measure wall thickness?

No. ASTM E2775 and E2929 describe it as screening that does not directly measure wall thickness. Follow-up with UT or other quantitative NDT is normally required.

What pipe sizes does guided wave testing cover?

The ASTM practices cover carbon and low-alloy steel pipe from NPS 2 to 48 with wall thickness from 0.15 to 1 inch, in a basic straight configuration.

Can guided wave testing inspect through insulation?

Yes. Only a small window of insulation is removed at each collar location, which is why it is widely used for CUI screening.

How far can guided waves travel in a pipe?

It depends heavily on conditions. Bends, coatings, soil contact, fittings and supports all shorten range. Expect much shorter range on coated or buried pipe than on clean pipe in air.

Can guided wave testing find pitting?

Small isolated pits are hard to detect because they change the cross-section very little. Use CML readings and scanning for pitting mechanisms.

Is guided wave testing accepted under API 570?

API 570 lets the owner choose inspection methods suited to the damage mechanisms. GWT is commonly used for screening within API 570 programs, with quantitative follow-up feeding the code's calculations.

Who is qualified to perform guided wave testing?

Personnel qualified and certified in the GW method under the employer's written practice, typically to ASNT SNT-TC-1A, with training on the specific equipment.

Planning a CUI or pipe-rack screen? Tell us about the lines or ask about road crossings and casings. Related: guided wave for pipeline inspection, NDE method selection by damage mechanism.

Sources: ASTM E2775 and ASTM E2929 scope statements (astm.org); ASNT SNT-TC-1A method list (asnt.org); API 570 product information (api.org). Summary only; confirm against current editions.