The effects of corrosions and fatigue induced cracks on strength degradation in ageing ships

Abstract

Over the past decades there have been many losses of the merchant vessels due to either accidents or exposure to large environmentally induced forces. The potential for the structural capability degrading effects of both corrosion and fatigue induced cracks are profoundly important and must be fully understood and reflected in vessel’s inspection and maintenance programme. Corrosion has been studied and quantified by many researchers, however its effect on structural integrity is still subject to uncertainty, particularly with regards to localized corrosion. The present study is focused on assessing the effects of corrosion and fatigue induced cracks on the strength degradation in marine structures. Various existing general corrosion models for tanker structures have been studied and compared for time variant neutral axis, section modulus at deck and section modulus at keel based on various years of service. Simplified formulae to estimate time variant vertical/horizontal section modulus degradation and stress change at upper deck and keel are developed based on the double hull tanker. A fatigue assessment study which considers the new corrosion degradation model has also been carried out for the side shell stiffened plates of a North Sea operating shuttle tanker and of a world wide operating tanker. In addition, over 265 non-linear finite element analyses of panels with various locations and sizes of pitting corrosion have been carried out. The results indicate that the length, breadth and depth of pit corrosion have weakening effects on the ultimate strength of the plates while plate slenderness has only marginal effect on strength reduction. Transverse location of pit corrosion is also an important factor determining the amount of strength reduction. When corrosion spreads transversely on both edges, it has the most deteriorating effect on strength. In this study, The multi-variable regression method and the Artificial Neural Network (ANN) method are applied to derive new formulae to predict ultimate strength of both uncorroded and locally corroded plate. It is found out that the proposed formulae can accurately predict the ultimate strength of both uncorroded and locally corroded plate under uni-axial compression. It is certain that undetected defects and developing cracks may lead to catastrophic fracture failure. Fracture control is necessary to prevent the ship’s structure safety not to fall down below a certain safety limit. It is very important to calculate how the structural strength is affected by cracks and to calculate the time in which a crack growth to the unacceptable limits. Fatigue analysis can estimate the elapsed time and locations where cracks could develop, whereas fracture mechanic approach can estimate crack growth times and response of structural strength as a function of crack size. In this study, the linear elastic fracture mechanics (LEFM) method based on stress intensity factor (K) and the elastic plastic fracture mechanics (EPFM) approach based on J-Integral and crack tip opening displacement (CTOD) have been investigated under different loads and crack sizes and material properties by using finite element analyses method. The finite element modelling and calculation for stress intensity factor (K) and J-computation are not easy tasks for most of engineers and researchers who do not have enough experiences. Accordingly some useful macro programs are developed for automatic creation of geometry, mesh details, boundary condition and applying loads, for automatic calculation of stress intensity factor (K) and computation of J-integral value. Proposed formulae based on multi-variable regression method and ANN might be useful to assess structural integrity during the initial design, on-site inspection and maintenance. In addition the developed macro programs for stress intensity factors (K) and J-computation could save time and efforts from time consuming finite element analyses.