End-of-Life Corrosion Estimation and Profile of Ship Hull Structure: Non-Parametric Statistical Analysis of Medium Endurance Cutters

Abstract

Corrosion in hull structure of Coast Guard cutters is a primary degradation mode that accounts for a significant portion of depot budgets and the occasional unavailability of ships in general. Corrosion exhibits great variability spatially and temporally. This paper presents, summarizes, and analyzes a one-of-a-kind data set for end-of-life corrosion estimation and profile of ship hull structure. The data set was created over several years and on several vessels, and collected by maintenance personnel at several geographic locations. This study analyzes wastage data due to corrosion that were systematically collected in 2007 to 2008 from twelve 210-foot Medium Endurance Cutters, commissioned in 1964 to 1969, in the form of thickness measurement using visual inspection and ultrasonic testing methods. A total of 76,091 thickness measurements were analyzed at positions covering the entire hulls. The measured corrosion levels mean is about 0.02 to 0.04 inches (1 in. = 25.4 mm), i.e., 6 to 14% of the as-built thicknesses after no more than 43 years of use of these 12 cutters as of 2007; however, the analysis of outliers indicates that the average wastage values can be misleading in predicting extreme corrosion. A method is proposed for estimating the counts and intensity of outliers. Examining geographic locations of the operations of these cutters and corrosion revealed that southern warm water led to appreciably larger corrosion compared to the northern colder waters, at a ratio of about 1.25 to 1.5.