Ships Hull Corrosion Diagnosis From Close Measurements of Electric Potential in the Water

Abstract

We present here an original method to search the corroded zones of an underwater steel structure. Nowadays, after a defined navigation period, a vessel is placed in dry dock, to examine its hull state, find the damaged areas and then paint them. This step of identification, very long and relatively inefficient, could be replaced by a series of electrical measurements which would be processed to get clues about the state of the hull. Those results are obtained thanks to the study of the cathodic protection system equipped on the hull. This new method allows a great timesaving but also a precision never reached before.

Corrosion Diagnosis of a Ship Mock-Up From Near Electric-Field Measurements

“…This diagnosis method globally succeeds in locating the corroded areas of an immerged part of a hull with real electric-field measurements. Previous work was based on an electric potential study, giving also good theoretical and empirical results [2]. These two approaches of the work do not have the same advantages, because of their own decreasing behavior in a conducting region.…”

“…When directly inversing (5), the solution obtained is charges and dipoles decorrelated, which do not guarantee the third Green's identity. A good way to solve the physical difficulty is to add Green's equations from (2), linking all unknowns on the boundaries. This ensures the solution to fit to the 3rd Green's identity.…”

“…Then, a point matching approach of (1) is provided, setting the unknowns at the barycenter of each element. This numerical stage leads to write (1) as the following matrix system: (2) Making this approximation is known to not be very precise, but since the main goal is the development of an inverse method, it is acceptable for us. Of course, this system has more unknowns than equations.…”

“…This system is then usually solved by an iterative Newton-Raphson process. Some results are presented in [2] on a simple ship mock-up: with parts of the boundary conditions on the structure (potential of SACP anodes, current densities of ICCP anodes, isolated parts, and polarization laws of cathodes), and this algorithm finds the missing ones.…”

“…Then, the problem will be inversed and corrosion diagnosis will be deducted, starting from near electric-field measurements and leading to the corroded areas localization. This paper, based on electric fields and applied to real measurements on a mock-up, extensively improves a method based on electric potential measurements, previously introduced [2].…”