Mitigating the Cost of Not Knowing in Ships

Map corrosion in ship deck plates quickly without removing coatings or linings.

Deck plates are essential parts of ships because they are vital to their structural integrity and the health and safety of their personnel. They are plagued by corrosion that remains unseen until it is usually too late.

The plates are lined with a non-slip epoxy coating and/or a variety of materials from ceramic tiles to concrete by way of wood that conceal corrosion. Linings must be removed before plates can be inspected to find out whether they need repair or not—very expensive and, arguably, quite destructive. Inspection/repair service providers, however, often quote jobs based on surface area, without usually knowing exactly what will actually have to be repaired. Once linings are removed and deck plates are inspected, the overall portrait of their state often drives the price up significantly. You can see that beginning work without previous knowledge of what lies underneath is a costly mistake—the cost of not knowing—hence the need for a solution that can assess this before removing anything.

Pulsed eddy current solution enabling scans through coatings and linings.

Corroded plate used in testing
Corroded plate underside
First scan, sides clamped, along stiffeners
Second scan, no clamps, along stiffeners
Third scan, no clamps, perpendicular to stiffeners

In recent years, Eddyfi Technologies has spared no effort in evolving Pulsed Eddy Current (PEC) technology. It has the demonstrated advantage of being able to scan through non-magnetic coatings and insulation to discover the corrosion hiding underneath, without having to strip said linings.

All this research and development gave birth to the Lyft^® PEC solution. It features a state-of-the-art portable instrument; real-time C-scan imaging; fast data acquisition (up to 15 readings per second) with grid and dynamic scanning modes.

The solution also features a range of PEC probes for a variety of applications.

In this particular application, the carbon steel deck plate extracted from a decommissioned ship and targeted for inspection was approximately 5.4 mm (0.213 in) in nominal thickness and the lining 20.0 mm (0.787 in). The far side of the plate featured several other elements such as welds and stiffeners. Corrosion on the sides of the plate pushed the lining upward, separating it from the steel plate by close to 7 cm (2.8 in) on the left side. Bulges in the lining were also visible, the result of ferrous oxide buildups.

Considering the thickness of the deck plate, lining, and corrosion product in this application, our engineers used a small PEC probe. All our scans were performed at a grid resolution of 25 × 25 mm (1 × 1 in). For the first dynamic scan, clamps were used to close the gaps between plate and lining.

This scan raised several questions: How will the data be affected if the left-most gap is left open? Will the stiffener signals be affected by a change in the scan axis? How repeatable are scans on the same plate?

To answer these questions, our engineers re-scanned the plate without the clamps to leave the gap between it and the lining open. The second scan of the plate revealed that, when compared to the first, data was not significantly affected by the gap or oxide buildup.

A third scan was per formed perpendicular to the stiffeners. Based on this scan, our engineers concluded the following:

• When scanning along the stiffeners, they are visible in a 50–75 mm (2–3 in) wide line.

• When scanning perpendicularly to stiffeners, their signals are weaker, but spread across a wider area.

We could therefore conclude that scanning along stiffeners will help better position them, while maximizing the detection of corrosion in close proximity to them.

Similar inspections were performed on other ships with the same success, conclusively demonstrating that PEC was able to assess the presence and relative extent of corrosion in deck plates before undertaking destructive repair work.

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