In ship repair, quality is not proven only during execution. It is proven afterwards—when you must demonstrate that a weld is sound, that hull thickness is sufficient, or that a repaired pipe has no internal defects.

Non-destructive testing (NDT) makes that possible without damaging the inspected material. It is a key tool to close work with the classification society, document repair integrity and provide real assurance to the shipowner.

This article explains what NDT is, which methods are most common in ship repair, when they are required, and how they should be integrated into a traceable quality system.

What NDT is and why it matters

NDT refers to inspection methods used to evaluate the integrity or quality of a material or welded joint without altering it. In ship repair, NDT is used to detect defects such as cracks, porosity, lack of fusion, inclusions or thickness loss—issues that can compromise structural safety.

NDT is not optional or “nice to have”. Classification societies (DNV, ABS, Lloyd’s Register, Bureau Veritas, RINA, ClassNK, among others) define which methods must be applied, in what percentage and with which acceptance criteria, depending on the scope and the area involved.

For shipowners, NDT provides objective evidence that the work was executed correctly. For contractors and shipyards, it is the technical basis to demonstrate compliance and close the job without ambiguity.

Most common NDT methods in ship repair

Visual testing (VT). The first level of control and the most frequently applied. It identifies visible surface issues (undercut, spatter, misalignment, weld profile defects). It may look basic, but rigorous VT by qualified personnel removes a large share of defects before moving to more complex methods.

Penetrant testing (PT / dye penetrant). A portable method to detect surface-breaking discontinuities. Particularly useful on non-ferromagnetic materials (stainless steel, aluminium, CuNi), where MT does not apply.

Magnetic particle testing (MT / MPI). Applicable to ferromagnetic materials (mainly carbon steel). Sensitive to surface and near-surface discontinuities and widely used for hull/structural and piping weld inspections.

Ultrasonic testing (UT / UTM). In ship repair, this has two major applications. Thickness measurement (UTM) is often the most demanded: it confirms remaining thickness in hull, tanks, bulkheads, piping and structural members without dismantling, and it drives many class decisions. Conventional UT can also detect internal weld defects (lack of fusion, inclusions, internal cracking), sometimes as an alternative or complement to RT.

Radiographic testing (RT). Uses X-rays or gamma rays to produce an internal image of a weld. It is strong for volumetric defects (porosity, slag inclusions, etc.), but can be operationally challenging on a crewed vessel due to radiological safety constraints. It is typically reserved for high-responsibility joints or when the project/specification requires it.

When NDT is required in ship repair

NDT is not applied “ad hoc”. Requirements are defined in the ITP (Inspection & Test Plan), established before work starts and agreed with class and, often, with the shipowner.

Typical cases include:

• Welds in steel renewal on hull and structure.
• Piping joints in class-related systems (fuel, ballast, bilge and other critical systems).
• Repairs in areas subject to cyclic loads or fatigue.
• Thickness verification for class surveys (annual, intermediate, special).
• Welds defined as critical by the project or by class.

The method, inspection percentage and acceptance criteria depend on the applicable standard, vessel area and class requirements. For structural steel renewals, for example, VT at 100% and complementary testing (MT, UT or RT) on a percentage of welds is common, with the percentage driven by criticality.

Who can perform NDT?

NDT must be executed by qualified and certified personnel. In practice, this typically means Level II technicians for execution and interpretation, supported by Level III for procedures and validation when required (under frameworks such as ISO 9712 or equivalent).

Signed NDT reports are part of the project close-out documentation and are essential for class acceptance.

NDT within a QA/QC system: where it truly adds value

A standalone NDT result has limited value if it is not part of a documented quality framework. Traceability is what turns an inspection result into valid evidence for class and the shipowner.

A robust QA/QC system typically integrates NDT with:

• Approved welding procedures (WPS) and qualification records (PQR/WPQ).
• ITP defining inspection points, NDT methods and acceptance criteria.
• Material traceability (MTR): certificates for materials and consumables.
• Signed NDT reports by qualified personnel.
• A consolidated close-out dossier for owner and class.

Without this framework, a test may “pass” and still fail to close a class item due to missing traceability across the full process.

Can NDT be performed onboard or only in a shipyard?

Most common NDT methods are portable and can be performed onboard, whether alongside, at anchorage or even during transit:

• VT, PT, MT and UTM/UT are fully feasible onboard when surface preparation and safety conditions are met.
• RT is more complex due to radiological safety, but it can be performed with proper coordination (typically in port with controlled access).

This means, for example, that an in-navigation repair can be completed with onboard NDT and full documentation delivered at the next port of call—without waiting for shipyard entry.

How SYM Naval integrates NDT

At SYM Naval, NDT is embedded in the QA/QC approach applied to each intervention. When required, inspection points (ITP) are defined, testing is coordinated (MT/MPI, PT, UTM/UT and RT where applicable), and verification is executed in line with class requirements.

Technical documentation (WPS, PQR, WPQ, ITP, MTR and NDT reports) is compiled and delivered to the shipowner and the classification society to ensure full traceability. This approach applies in shipyard projects and also in afloat, in-navigation or anchorage interventions.

Frequently asked questions

What NDT does class require for steel renewal?
It depends on area and criticality. As a common reference: 100% VT and complementary testing (MT, UT or RT) on a defined percentage according to the ITP agreed with class.

Can ultrasonic thickness measurements be performed without dismantling?
Yes. UTM is applied from the outside of plating or piping and only requires access to the surface and minimal preparation.

Does NDT delay the project?
If integrated into the ITP, no. It is executed within the normal work sequence. What causes delays is discovering late a defect that should have been detected earlier.

If you need ship repair with traceable quality control and class-compliant testing, we can help. Contact our technical team.