Underwater NDT Inspection Methods: A Practical Guide

Most underwater inspection programmes begin with visual methods, and the two you will see most often are General Visual Inspection (GVI) and Close Visual Inspection (CVI). They differ in scope, proximity and the level of cleaning required, and a well-planned survey usually uses both in sequence.

A GVI is a broad overview pass. The diver or camera moves over the structure at a working distance to confirm general condition, locate obvious damage, mechanical deformation, missing components or heavy marine growth, and to flag areas that need a closer look. It typically needs little or no cleaning and gives a fast picture of overall integrity.

A CVI is a detailed, close-up examination of a specific area, carried out within touching distance and almost always after the surface has been cleaned. CVI is used to characterise a defect found during the GVI, to inspect welds and high-stress connections, and to support measurements such as crack sizing or pit depth. Our divers record both stages with HD video so findings are traceable back to a precise location.

Beyond visual inspection, ship hull NDT methods quantify what the eye cannot. The most common contact measurement underwater is ultrasonic thickness (UT) gauging, which sends a sound pulse through the steel and times its return to calculate remaining wall thickness. UT lets us measure plate, shell and appendage thickness in place, map corrosion and wastage, and compare readings against original or class-required minimums without dry-docking the vessel. Surfaces are cleaned to bare metal at each test point so the probe couples cleanly.

Crack detection on hulls and welded connections is typically carried out with magnetic particle inspection (MPI) or alternating current field measurement (ACFM). MPI magnetises a ferrous component and uses fluorescent particles to reveal surface-breaking cracks, while ACFM is an electromagnetic technique that can detect and size surface cracks, often through coatings and with less cleaning than MPI. Both are well-established industry techniques for fatigue-sensitive welds and stress concentrations.

We provide diver-led UT thickness gauging as a core capability, with results logged point by point and tied to a structure diagram. Where a programme calls for MPI or ACFM crack detection, the technique and qualified personnel are scoped to the specific job and class requirements.

Steel structures in seawater are protected from corrosion by a cathodic protection (CP) system, usually sacrificial anodes or an impressed current system. A cathodic protection survey confirms that this protection is actually working and tells the owner how much anode life remains.

The survey has two main parts. First is a physical assessment of anode condition: divers record how far each anode has depleted, whether it is consumed, passivated or detached, and whether the connections remain sound. Second is electrical measurement, where a half-cell reference electrode is used to take structure-to-electrolyte potential readings at defined points across the structure. Those potential readings show whether the steel is sitting within the protective range, under-protected and at risk of corrosion, or over-protected.

Read together, anode condition and half-cell potentials let owners plan anode replacement before protection is lost and feed directly into class and integrity records. Our divers carry out CP surveys as a standard part of structural and hull inspection work.

A marine growth survey assesses the biological fouling, from algae and weed to hard-shell organisms such as barnacles and mussels, that accumulates on submerged structures. The survey records the type, distribution and thickness of growth across the hull or structure.

Marine growth matters for two reasons. On ship hulls and propellers it increases drag, which raises fuel consumption and emissions and reduces speed, so a fouling assessment supports decisions about hull cleaning and propeller polishing. On fixed structures, heavy growth adds weight and hydrodynamic loading and, just as importantly, hides the steel underneath, so it must be assessed and removed before reliable visual inspection, UT gauging or CP measurement can take place.

Because growth controls inspection access, we treat the fouling assessment as the first step that shapes the cleaning plan for the rest of the NDT scope, and we document it with measurements and HD video.

A common question is whether an inspection should use a remotely operated vehicle (ROV) or a diver. They are complementary rather than interchangeable, and the right answer depends on depth, scope and the type of data required.

Divers are the right choice for hands-on NDT and contact measurement. A diver can clean a surface to bare metal, place a UT probe accurately, run MPI or ACFM, handle anodes during a CP survey and make judgement calls in real time, all within the surface-supplied range. For class-driven hull and structural inspection, that hands-on contact capability is exactly what most scopes need. Our diving is surface-supplied to a maximum of 50 metres, which covers the great majority of ports, jetties, hulls and shallow-water structures in our region.

An ROV is most useful for deeper work, long pipeline or cable runs, or situations where prolonged or higher-risk diver exposure is best avoided, and it excels at visual coverage and video. Almancy does not own or operate an ROV fleet; where a project's depth or scope genuinely calls for ROV support, we arrange it through specialist partners to augment a diver-led programme rather than replace it.

NDT data is only useful if it is documented clearly. Our deliverables pair findings with evidence: annotated photographs and HD video, UT readings tied to a structure diagram, anode and half-cell results, marine growth records and a written assessment with recommended actions. Reports are structured to support class-compliant documentation, so the classification society and the owner can act on them directly.

Location is a practical advantage in this part of the world. Almancy is based at Port Tawfiq in Suez, at the mouth of the Suez Canal, which lets our mobile teams reach vessels and structures across the Suez Canal, the Gulf of Suez, the Red Sea, the Mediterranean and Egyptian ports quickly from a single office. Fast mobilisation matters when a vessel is on a tight canal transit schedule or a structure needs assessment before it can return to service.

Almancy has operated since 2008, holds ISO 9001:2015, ISO 14001:2015 and ISO 45001:2018 certification, and is a member of IMCA and ADCI. Across more than 500 projects and 18-plus years we have maintained a zero lost-time incident record, which underpins how we plan and document every underwater NDT scope.