All ships must undergo periodic mandatory inspections according to their class. These inspections aim at assessing the condition of the ship and assessing damage to plan repairs. In many cases, ultrasonic testing of the hull thickness from the inside of the ship provides an accurate assessment of the hull plate’s remaining thickness. The inspection identifies areas of concern, and when required by class, repairs are performed.
When accessing the hull from inside the ship is not possible, measurements can be performed from the outside. This process may involve work at heights, endangering workers’ safety. Alternatively, for the submerged part of the hull, UT can be deployed using Remotely Operated Vehicles (ROV) underwater. In both cases, single-channel UT probes are used. Typically, a coarse grid spacing is used, 150 millimeters by 150 millimeters (6 inches by 6 inches), for example. The coarse grid may leave smaller areas of critically severe corrosion unnoticed.
The Challenge Corrosion Presents in Ship Hulls
Corrosion is often found above longitudinal stiffeners in aging ships. With time, the drain holes designed to let the water flow down to an area where it can be collected tend to get clogged with debris. With the passive protection from the drain hole compromised, water gets trapped above the longitudinal stiffeners which creates a corrosion prone area. Corrosion grows just above the stiffener, slowly weakening the hull plates. This damage mechanism typically spans only a few centimeters above the structural members, making it too small to ensure detection by a large UT grid from outside the hull. Furthermore, surface conditions on the outside of the hull such as a coating disbonding from the steel hull or highly attenuating coatings may render ultrasonic measurements simply impossible without surface preparation.
Figure 1: Internal Corrosion in Bilge Area of an Aging Ship
The Pulsed Eddy Current Technology Solution for Hull Inspection
The multiple challenges associated with traditional corrosion assessment in hulls have led to considerations for a new approach to address this problem. With the recent introduction of a more sensitive Pulsed Eddy Current sensor and array (PECA) sensors, the technology is increasingly raising more interest in the naval and shipping industries. With high coverage and productivity —but even more importantly— high resilience to harsh surface conditions and liftoff, PEC is the ideal screening solution to identify areas of corrosion in ship hulls.
Tests were performed with national navies to assess the detection capabilities of PEC on real ship hull corrosion damage. Various probes were tested to establish the best combination of speed and accuracy required for these specific applications. The standard PECA-6CH-MED probe with 457 millimeters (18 inches) enables fast scanning for detection, and smaller more sensitive probes such as the PECA-HR-SM provide better detection of small flaws at the expense of productivity.
Figure 2: Corrosion Mapping on Ship Hulls using Standard PECA Probes
Working with a NATO country navy, Eddyfi Technologies has developed a probe optimized to provide the best combination of sizing and accuracy. The 311-millimeter (12.25-inch) wide PECA sensor is fully watertight to permit underwater inspection and is packaged to facilitate integration on magnetic crawlers. This custom solution is typically deployed using a resolution of 45 millimeters by 22.5 millimeters (1.75 inches by 0.875 inches), much finer than traditional a UTM grid to ensure that no corrosion is left undetected.
Figure 3: Custom PECA Probe for Ship Hull Inspections
Vertical scan lines proved to offer the best data quality when assessing corrosion just above the longitudinal stiffeners. This pattern minimizes influence from the structural members themselves and highlight area of corrosion in green, yellow, and red from the surrounding nominal wall thickness shown in blue in the following picture.
Figure 4: Corrosion Mapping with PEC
Robotic Deployment of PEC
With an optimized NDT modality on hand, the next inevitable step is to integrate it with magnetic crawlers able to carry out the inspection remotely, without any surface preparation or work at heights endangering personnel. Extensive testing ruled out any interference between the PEC electromagnetic sensors and the powerful magnets and motors in our standard robotic crawler.
Figure 5: Inuktun Magnetic Crawlers Can Climb Vertical Surfaces
With a depth rating of 60 meters (200 feet), Magg™ magnetic crawlers can perform an inspection underwater while the ship is not yet in a dry dock. Early assessment of the corrosion damage allows naval engineers to plan corrosion repairs long before the ship enters the shipyard, reducing unplanned corrosion repairs cost and refit schedule disruption.
The future is looking bright for the marine and naval sectors keen on detecting corrosion early thanks to easily deployable and robust NDT enabled robotics. We invite you to contact our experts to discuss how our advanced solutions can keep you Beyond Current today!