Today’s merchant and passenger vessels are complex structures, which operate in harsh environments with respect to static, cyclic loads and corrosion, which in turn lead to the creation and propagation of defects. Among the most common defects are cracks at stress concentration areas and the most common rectification practice in areas where cracks are met is to introduce an insert plate on top of the damaged section in order to increase the thickness of the section and therefore increase fatigue resistance. In the EU FP7 project “MOSAIC”, one of the goals is to substitute the conventional steel (e.g. AH36) used in these prone to crack initiation areas, with equal thickness HSLA steels which can offer increased Fracture Toughness (FT) as well as Fatigue Crack Growth (FCG) Performance. In this way the overall weight of the structure is reduced by avoiding the introduction of insert plates and furthermore the FT and FCG behavior of the structure is improved. In this framework the substitution of the AH36 steel with an S690 HSLA steel will be studied, using the Finite Element Model (FEM) of an Aframax Tanker. Comparisons between the FT and FCG behaviour accounting for the two steels will be accomplished.