NDT in Shipbuilding and Ship Repair

What can go wrong at sea

Ships are subject to constant cyclic loading from waves, corrosion from seawater and cargo, vibration from machinery, and occasional impacts. The main threats are weld defects introduced during construction, fatigue cracks that develop at structural details over years of service, corrosion that thins plating and stiffeners, and damage to machinery such as propulsion shafting, rudders and pressure systems.

The history of welded ships includes a hard lesson: the brittle fractures suffered by some welded steel ships built during the Second World War, which helped launch the modern study of fracture mechanics and led to better steels, better welding practice and better inspection. Today's consequences of structural failure are just as serious: loss of life, environmental damage, loss of the vessel and cargo, and for naval ships, loss of mission capability.

Where NDT is used

• Visual testing (VT) of every weld during construction, performed by qualified inspectors, is the foundation. It catches fit-up problems, undercut, cracks and profile defects before they are buried under paint and outfitting.
• Ultrasonic testing (UT) and radiographic testing (RT) examine the volume of hull butt welds and other critical joints. The extent of examination is usually set by the governing rules and increases for highly stressed areas.
• Magnetic particle testing (MT) is used on steel welds and castings to find surface cracks, including in repair work and at structural details prone to fatigue.
• Liquid penetrant testing (PT) serves non-magnetic materials such as aluminum superstructures, stainless steel and copper-nickel piping.
• Ultrasonic thickness gauging measures remaining plate and stiffener thickness during surveys, so corrosion can be compared against allowable limits and steel renewed before it becomes weak.
• Eddy current testing (ET) inspects condenser and heat exchanger tubing, and helps detect surface cracks through coatings.

Propulsion shafts, rudder stocks, propellers and machinery components are examined during drydocking and overhaul with UT, MT and PT as the rules and manufacturer require.

How inspection fits into a ship's life

New construction. Shipyards follow approved welding procedures and inspection plans, with NDT extent and acceptance criteria agreed with the classification society or, for naval vessels, set by the Navy's requirements. Surveyors witness and review inspection records.

In service. Commercial ships undergo periodic class surveys, with more extensive inspections as they age. Thickness measurement programs and close-up surveys of structure are key parts of these, and their scope generally increases with the ship's age and type. Where the rules and the ship's condition allow, some hull inspections are carried out in the water, with divers or remotely operated vehicles performing visual inspection and thickness measurement instead of taking the ship out of service for drydocking.

Machinery and systems. Beyond the hull, boilers, pressure vessels, piping and heat exchangers are inspected on their own schedules. A leaking condenser or heat exchanger tube can take a system out of service, and eddy current inspection during planned maintenance helps decide which tubes to plug or replace before they leak.

Repair and conversion. Every repair weld is a new weld with the same inspection needs. Ship repair yards work under intense schedule pressure, which makes clear procedures and qualified inspectors even more important.

Codes, rules and requirements

• Classification society rules — for example those of ABS, DNV and Lloyd's Register, many harmonized through IACS — govern construction, welding, NDT and surveys for most commercial ships. Flag states such as the United States, through the U.S. Coast Guard, set further requirements.
• NAVSEA requirements govern U.S. Navy ships. Commonly referenced documents include NAVSEA Technical Publication T9074-AS-GIB-010/271 for NDT methods and personnel certification, MIL-STD-2035 for NDT acceptance criteria, MIL-STD-1689 for fabrication, welding and inspection of ship structure, NAVSEA S9074-AR-GIB-010/278 for machinery, piping and pressure vessels, and MIL-STD-22 for welded joint design. The Navy's personnel requirements build on ASNT SNT-TC-1A with changes of their own, so a certification that satisfies commercial work does not automatically satisfy naval work.
• AWS codes, such as D3.6 for underwater welding, apply to specific situations.
• For commercial work, personnel are often qualified to ASNT SNT-TC-1A (Level I, II and III), ISO 9712 or the scheme a classification society accepts.

Naval work and commercial work differ in their paperwork and acceptance criteria, and a shipyard doing both needs to keep the two sets of requirements clearly apart.

What this means for owners and shipyards

For shipowners and operators, inspection data is part of the ship's value. Thickness records, repair histories and NDT reports support class surveys, insurance, charter vetting and resale. Keep them organized and complete.

For shipyards and repair yards, the recurring issues are practical: making sure the right procedure revision is used for the right contract, that inspectors are certified in the methods and techniques they perform, that calibration blocks and instruments are current, and that records reach the customer's surveyor or representative without delay. Many yards also rely on subcontracted NDT; auditing those subcontractors is part of owning the result.

How NDT LLC can help

NDT LLC supports shipyards, repair facilities and owners with NDT program development, procedure writing, personnel training and qualification, and independent quality assurance for both commercial and NAVSEA work. Contact us to discuss your next build, overhaul or audit.