Start hereRules checked 20 October 2026
A crack in a hull weld rarely announces itself. It may start at the toe of a bracket under coating, grow a little every time the ship works in a seaway, and show only as a rust line or a weep of ballast water. When the surveyor sees that line, the question is no longer whether to test but how: which non-destructive testing (NDT) method will find the end of the crack, and whose result will class accept?
The answer depends on where the defect sits (surface or inside), the material (magnetic steel or not), the thickness, and who holds the probe. IACS answers the first three for new construction in Unified Requirement (UR) W33, and the last one in UR W33 and UR W35, with a change coming in 2028. Start by sorting the jobs below.
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In this lesson
- Match each job to the NDT method that suits it best
- In plain words
- In detail
- What the rules say
- Flag and class notes
- Key numbers
- What inspectors find
- Quiz: 6 questions
Try it
Match each job to the NDT method that suits it best
In plain words
NDT means checking a part without damaging it. There are five everyday methods on ships.
- Visual testing (VT): looking, with good light, a mirror, a magnifier or a camera. Cheap and always first.
- Penetrant testing (PT): a coloured or fluorescent liquid soaks into surface cracks; a white developer draws it out so the crack shows as a line. Works on any non-porous metal, but only for cracks open to the surface.
- Magnetic particle testing (MT): the steel is magnetised and fine iron particles are sprinkled or sprayed on. A crack disturbs the magnetic field and the particles gather along it. Faster and more sensitive than PT on ordinary hull steel, and finds cracks just below the surface too, but useless on aluminium or austenitic stainless steel.
- Ultrasonic testing (UT): high-frequency sound sent into the steel reflects from flaws or from the far side. It finds cracks and lack of fusion inside welds and measures thickness.
- Radiographic testing (RT): X-rays or gamma rays pass through the weld onto film or a digital detector. Gas pores and slag show as dark spots. It needs radiation safety controls and access to both sides.
Each method has a blind spot, so the choice follows the defect you expect. Cracks at the surface: MT or PT. Flat defects inside: UT. Round defects inside: RT.
Who does it matters as much as how. NDT people are certified at three levels, usually under the international standard ISO 9712. Level 1 can carry out a test and collect data under supervision. Level 2 can set up the test, carry it out and judge the result. Level 3 writes and approves procedures and supervises. IACS expects a Level 2 to interpret every result used for a hull weld.
In detail
New construction: UR W33 and W34
UR W33 sets the minimum NDT methods and quality levels for hull structure welds at newbuilding. It applies to ships contracted for construction on or after 1 July 2021, and replaced IACS Recommendation 20, which was deleted on that date. Key points:
| Topic | UR W33 requirement |
|---|---|
| Who tests | Normally the shipbuilder or its subcontractors; the class surveyor may witness |
| Plan | The shipbuilder submits an NDT plan for class approval: locations, extent, quality levels, procedures |
| Checkpoint priorities | High-stress and fatigue-sensitive areas, important elements, welds hard to inspect in service, field welds, suspected problem areas, block joints |
| Visual | All welds, full length |
| Timing | After cooling to ambient; for steels with yield 420 to 690 N/mm² not before 48 hours, above 690 N/mm² not before 72 hours |
| Testing volume | Weld plus at least 10 mm of parent metal each side, or the heat-affected zone if wider |
| Standards | PT to ISO 3452-1, MT to ISO 17638, RT to ISO 17636-1, UT to ISO 17640 (procedure), ISO 23279 (characterisation) and ISO 11666 (acceptance) |
| RT length | Typically at least 300 mm per checkpoint for hull welds |
| Escalation | If an unacceptable level of indications is found, the extent of testing is increased |
Methods by joint type (UR W33 Table 1):
| Weld joint | Plate under 8 mm | Plate 8 mm and over |
|---|---|---|
| Full-penetration butt | VT, PT, MT, RT | VT, PT, MT, UT, RT |
| Full-penetration tee, corner, cruciform | VT, PT, MT, RT (limited) | VT, PT, MT, UT, RT (limited) |
| Partial penetration and fillet | VT, PT, MT, UT (penetration check), RT (limited) | same |
Advanced methods, such as phased array UT (PAUT), time-of-flight diffraction (TOFD) and digital or computed radiography, are covered by UR W34, which also applies to ships contracted on or after 1 July 2021. A revised W34 (Rev.1) comes into force on 1 January 2028.
Ships in service
In service there is no fixed NDT plan. UR Z7 paragraph 5.3.2 simply says that one or more fracture detection procedures, radiographic, ultrasonic, magnetic particle or dye penetrant, may be required if the surveyor considers it necessary. In practice that means:
- a crack found during a close-up survey is traced to its ends with MT (or PT on non-magnetic material) before a repair is designed;
- after a crack is gouged out and rewelded, the repair is tested to the extent the surveyor requires, usually by MT and, for full-penetration repairs, UT or RT;
- damage that affects structural, watertight or weathertight integrity must be promptly and thoroughly repaired (UR Z7 paragraph 1.3.1).
Tank testing is not NDT but sits beside it. At each special survey, boundaries of double-bottom, deep, ballast, peak and other tanks are tested with a head of liquid to the top of the air pipes (UR Z7 paragraph 2.2.9). Fuel, lube oil and fresh water tanks are tested to the highest point liquid can rise in service, or may be specially considered on the master’s confirmation and an external examination.
Who may do it: personnel and firms
Today (UR W33 Rev.1 section 3): the shipbuilder or subcontractor qualifies its NDT supervisors and operators to a scheme based on ISO 9712:2012, preferably with third-party certification. Employer-based schemes such as SNT-TC-1A or ANSI/ASNT CP-189 may be accepted if class reviews the written practice. At least one supervisor certified to Level 3 in the methods used must be employed full-time (an external Level 3 may cover methods the in-house Level 3 does not hold). Level 3 cannot simply be appointed: it must be certified by an accredited certification body. Operators who interpret results must hold Level 2; Level 1 may only gather data. The supervisor re-evaluates operators every year.
NDT service suppliers (UR W35): since 1 July 2020, independent NDT companies and yard NDT departments working on new construction must meet UR W35, which (in its current Rev.1) references ISO 9712:2021 and requires a quality system, written procedures, qualified personnel and suitable equipment. Rev.1 has applied since 1 January 2025; Rev.2 comes into force on 1 January 2027. Under W35 such firms are verified by class but are not part of UR Z17.
Coming 1 January 2028: UR Z17 Rev.23 adds “Firms engaged in NDT and ANDT services on the new construction of ships and offshore structures” as a new Z17 approval category (Annex 1, section 19), with the detailed requirements in UR W35. External NDT firms must be independent of the manufacturer and shipyard. On the same date UR W33 Rev.2 moves its personnel rules into UR W35.
Other service suppliers that do NDT: an in-water survey firm must document NDT competence to a recognised standard only if it actually performs NDT, and firms examining ro-ro bow, stern and side doors must use NDT operators qualified to a recognised standard (UR Z17 Annex 1, sections 3 and 8).
What the rules say
- UR W33 paragraph 1.1 sets minimum NDT methods and quality levels for hull structure welds at newbuilding; it applies to ships contracted for construction on or after 1 July 2021.1
- UR W33 paragraph 5.4 requires visual testing of all welds over their full length, and 5.5 requires MT for ferromagnetic welds unless otherwise agreed with class.1
- UR W33 paragraph 5.7 states RT is generally most effective for volumetric defects and UT for planar defects such as lack of fusion and cracks.1
- UR W33 paragraph 2.4.2 forbids NDT of high-strength steels with yield 420 to 690 N/mm² until 48 hours after welding, and above 690 N/mm² until 72 hours.1
- UR W33 paragraphs 3.2 and 3.3 require a full-time Level 3 supervisor and operators certified to at least Level 2 to interpret results, with Level 1 limited to data gathering.1
- UR W33 Rev.2, in force for ships contracted on or after 1 January 2028, refers to UR W35 for personnel qualification.2
- UR W34 covers advanced NDT (PAUT, TOFD, digital and computed radiography) of materials and welds at newbuilding for ships contracted on or after 1 July 2021.3
- UR W35 paragraph 1.1 applies to independent NDT companies and yard NDT departments on new construction, and references ISO 9712:2021 for personnel certification.5
- UR Z17 Rev.23, in force 1 January 2028, adds NDT and ANDT firms on new construction to the Z17 approval scheme and requires external firms to be independent of the manufacturer and shipyard.6
- UR Z7 paragraph 5.3.2 lets the surveyor require radiographic, ultrasonic, magnetic particle or dye penetrant fracture detection in service, and 2.2.9 sets tank testing with a head of liquid at special surveys.4
- UR Z17 Annex 1 paragraph 3.2 requires an in-water survey firm to document NDT competence to a recognised standard only if it performs NDT, and paragraph 8.4 requires NDT operators of ro-ro door examination firms to be qualified to a recognised standard.8
- IACS Recommendation 20 on NDT of hull welds was deleted on 1 July 2021, on implementation of UR W33.7
Flag and class notes
- UR W33 leaves the extent of testing and number of checkpoints to agreement between the shipbuilder and the class society (paragraph 1.5), so the NDT plan approved by your class is the document to check at the yard.1
- UR W35 paragraph 1.1(iii) leaves the method of verifying NDT service suppliers to each society until UR Z17 Rev.23 takes over in 2028. Ask the society which list or verification letter it issues.5
- In service, the surveyor decides when NDT is needed (UR Z7 5.3.2). Agree the method, extent and acceptance standard with the surveyor before the NDT firm starts, and keep the operator’s certificate copy with the report.4
Key numbers
| What | Value |
|---|---|
| UR W33 and W34 apply to ships contracted | on or after 1 July 20211 |
| Delay before NDT, yield 420 to 690 N/mm² | 48 hours1 |
| Delay before NDT, yield above 690 N/mm² | 72 hours1 |
| Testing volume each side of weld | at least 10 mm, or the HAZ if wider1 |
| Typical RT length per hull checkpoint | 300 mm1 |
| Plate thickness from which conventional UT is listed | 8 mm1 |
| Minimum level to interpret results | Level 21 |
| UR W35 Rev.2 in force | 1 January 20275 |
| NDT firms join UR Z17 (Rev.23) | 1 January 20286 |
What inspectors find
- Cracks traced only by eye: repair designed to the visible length, then the crack reappears beyond the weld because its tips were never found with MT or PT. UR Z7 5.3.2 allows the surveyor to require fracture detection for exactly this reason.4
- Reports signed by an operator without the required level for interpretation, or without a copy of the certificate; UR W33 requires Level 2 to interpret.1
- RT or UT done on high-strength steel welds before the 48 or 72 hour delay, so delayed cracking is missed.1
- Shipyard NDT plans followed by rote while the rate of rejected indications rises; UR W33 5.2 requires the extent to increase.1
FAQ
Is ISO 9712 the only acceptable certification?
No. UR W33 Rev.1 accepts certification based on ISO 9712:2012 and, subject to class review of the written practice, employer-based schemes such as SNT-TC-1A or ANSI/ASNT CP-189. UR Z17 refers to “a recognised national or international industrial NDT standard (e.g. EN 473 … or ISO 9712)”. EN 473 has since been replaced by EN ISO 9712.
Can the ship's crew do MT or PT for class?
For a result used by the surveyor to decide on class, the work must be done by qualified, certified NDT personnel acceptable to the society. Crew can use penetrant kits for their own monitoring, but class will normally expect a qualified operator and a report.
Why does UR W33 not apply to my 2015-built ship?
It applies to ships contracted for construction on or after 1 July 2021. Older ships were built to the society’s rules and Recommendation 20 at the time; repairs today follow the society’s current rules and the surveyor’s instructions.
Does a phased array report count as UT?
Phased array UT is an advanced method covered by UR W34, with its own procedure and qualification requirements. It is accepted when the procedure is agreed with class.
Quiz6 questions
Sources
- 1
IACS Unified Requirement W33, Non-destructive testing of ship hull steel welds, Rev.1 Corr.1 (August 2021; ships contracted for construction on or after 1 July 2021), paragraphs 1.1 to 1.6, 2.4, 2.5 and Table 1, 3.1 to 3.3, 5.1 to 5.10, 6.1 to 6.6. https://iacs.s3.af-south-1.amazonaws.com/wp-content/uploads/2022/05/18145127/ur-w33rev1corr1.pdf
- 2
IACS UR W33 Rev.2 (July 2026; ships contracted for construction on or after 1 January 2028), section 3.1. https://iacs.s3.af-south-1.amazonaws.com/wp-content/uploads/2022/05/16133333/UR-W33-Rev.2-July-2026-CLN.pdf
- 3
IACS Unified Requirement W34, Advanced non-destructive testing of materials and welds (December 2019; ships contracted on or after 1 July 2021; Rev.1 in force 1 January 2028), paragraphs 1.1 to 1.5. https://iacs.s3.af-south-1.amazonaws.com/wp-content/uploads/2022/05/18145233/ur-w34.pdf
- 4
IACS Unified Requirement Z7, Hull Classification Surveys, Rev.29 Corr.1 (May 2024), paragraphs 1.2.3, 1.3.1, 2.2.9, 5.3.1 and 5.3.2. https://iacs.s3.af-south-1.amazonaws.com/wp-content/uploads/2024/05/21130428/UR-Z7Rev.29Corr.1.pdf
- 5
IACS Unified Requirement W35, Requirements for NDT Service Suppliers, Rev.1 (October 2023, implemented from 1 January 2025), paragraphs 1.1 and 1.4; Rev.2 (February 2025) in force 1 January 2027. https://iacs.s3.af-south-1.amazonaws.com/wp-content/uploads/2024/01/02143105/UR-W35-Rev.1-Oct-2023-CLN.pdf and https://iacs.s3.af-south-1.amazonaws.com/wp-content/uploads/2025/02/13122835/UR-W35-Rev.2-Feb-2025-CLN.pdf
- 6
IACS Unified Requirement Z17, Procedural Requirements for Service Suppliers, Rev.22, 23 and 24 (Rev.23 in force 1 January 2028), Rev.23 paragraphs 4.1.2 and 4.2 and Annex 1, section 19. https://iacs.s3.af-south-1.amazonaws.com/wp-content/uploads/2023/05/05122440/UR-Z17-Rev.22-23-24-CLN.pdf
- 7
IACS Recommendation No. 20, Non-destructive testing of ship hull steel welds, deletion notice (“Deleted 1 July 2021, on implementation of UR W33”). https://iacs.s3.af-south-1.amazonaws.com/wp-content/uploads/2022/05/20081450/rec20del.pdf
- 8
IACS UR Z17 Rev.21 (January 2025, implemented from 1 July 2026), Annex 1, paragraphs 1.2, 3.2 and 8.4. https://iacs.s3.af-south-1.amazonaws.com/wp-content/uploads/2023/05/14135618/UR-Z17-Rev.21-Jan-2025-CLN.pdf
Rules checked 20 October 2026. VSL Safety is not an approved training provider and doesn't issue certificates. These lessons don't replace STCW training, or the manufacturer's training and certification that service technicians need.
