Relative Material Loss—A Methodology for Approximating Material Loss on Structural Plating Separating Dissimilar Marine Environments

The use of non-destructive testing techniques such as ultrasonic pulse echo is typical for measuring remaining member thickness on structural plating for determination of remaining service life. However, ultrasonic methods are limited and can only determine total material loss across the structural member - it cannot distinguish the degree of material loss on each side of the member — especially when the marine environments are dissimilar. This paper presents a new maintenance inspection methodology called relative material loss (RML) for approximating material loss on each side of structural plating separating two or more dissimilar marine environments. The RML methodology leverages actual "at sea" environmental and operational conditions and the relationships between each to solve for material loss contribution on each plate side. The technique can determine the effectiveness of the structure's preservation and coating system in its real word operational and marine environments. Byproducts of RML include (1) reduction in precision error and (2) location of structural anomalies inside inaccessible structures such as sealed ballast tanks. The paper presents a case study on a sixty five year old "at-sea" structure; a dry dock caisson gate to demonstrate the RML methodology.