Ultimate strength and mechano-electrochemical investigations of steel marine structures subject to corrosion

It is well known that ageing steel marine structures are susceptible to corrosion in its all manifestations. The most critical areas are cargo and ballast tanks of merchant ships. However, due to the regulations such as the Performance Standard for Protective Coatings, which requires a 15-year target life of coating in ballast tanks, plus the cathodic protection systems, the internal structures within cargo holds have become more problematic but poorly studied. In the underdeck area and bottom plating, the structures are not normally fully protected. Also, the complex structural arrangement may place difficulties in inspection and repair. In extreme cases, it has been reported that the corrosion rate in these areas could be 5 to 7 times higher than a normal value, and has led to catastrophic structural failures. Currently, the classification societies apply both visual and gauging methods for corrosion inspection during ship surveys. However, it is time consuming especially for large vessels and is highly experience dependent. Therefore, to improve the survey efficiency, facilitate economical maintenance decisions, and even extend the structural life, it is essential to investigate the ultimate strength of such aged and corroded steel structures. Based on the identification of existing corrosion issues in cargo tanks of oil tankers and bulk carriers and the state-of-art of corroded marine structural strength assessments, a nonlinear finite element method was adopted to investigate the influences of pitting and grooving corrosion on the structural integrity. Two full-field experimental techniques were used for a complete validation of the numerical models. Based on the repair conditions provided by classification societies, the numerical results showed that 25% locally corroded area of a plate (800 mm × 800 mm × 15 mm) with 3.75 mm remaining thickness may reduce the ultimate strength by up to 20% compared to a uniformly corroded plate. The weld-induced grooving corrosion of a width of 59 mm and a remaining thickness of 3.75 mm would cause up to 26% strength capacity reduction for a stiffened plate (4750 mm × 950 mm × 15 mm). Moreover, it was shown that the corrosion depth had a greater influence on structural performance compared to corrosion area for the same volume/material loss. By combining mechano-electrochemical protocols with the stress and strain results obtained from the modelling, it enables predictions of the ‘hot spot’ locations of mechanically-induced corrosion acceleration. Results showed that the anodic current density inside grooving corrosion damage (24 mm in width and 3.75 mm in depth) was 7 times greater vis-a-vis the unstressed condition for the stiffened plate at its ultimate strength state. The results, which are closely related to the industrial corrosion inspection and repair requirement, will not only benefit the shipping industry, but are also applicable to a whole range of marine structures (offshore platforms and steel bridges)