The corrosion makes ships prone to structural degradation and the possibility of accidents increases with the aging of ships. Hull girder section modulus (HGSM) is a fundamental descriptor of the ship longitudinal strength and can be used to measure how the ship hull deteriorates over time because of corrosion. Traditional strength analysis uses simplified deterministic approaches to account for this time dependent deterioration process. Common practice is that nominal corrosion additions are to be predefined. Thus, the Classification Societies Rules, including the newly developed Common Structural Rules for Double-hull Oil Tankers, assume constant loss of HGSM during the whole service lifetime of a ship. Ship structural assessment, in the design phase, is performed assuming such \"net\" HGSM. Although \"the Rules\" approach is practical, it is obviously conservative one, as HGSM loss is actually a time-dependent non-linear function. Nowadays, there is a clear trend of adopting non-linear models in order to predict the long-term corrosion wastage degradation and the associated HGSM reduction as a function of time. Developing a direct approach accounting for the real corrosion degradation will be a very important and useful tool for the Classification Societies (CS) and ship owners in order to predict the long-term hull degradation and to decide whether the renewal of hull structures are necessary and when would be the optimal time for repairs. The time for repair of aged ships is empirically determined by CS, based on existing criteria of damage allowance. One of the most important criteria is the so-called \"10% rule\", which represents tolerated 10% loss from initial as-built HGSM. Recently, based on numerous studies and public pressure as well, the approach for longitudinal strength assessment through HGSM of new-built ships started to be complemented based on the hull girder ultimate strength (HGUS) assessment, as a physically justified criterion for entire capacity of the ship hull. The logical continuation of this approach in further research is that for the old ships HGUS is used as a criterion of structural adequacy. The results of such approach were used in this thesis. Although the longitudinal strength of the hull structure is in focus as the most important component of the ship strength, the results and findings presented in this thesis can be used in the calculations of other components of strength, primarily in the analyses of the primary transversal structure using the finite elements method (FEM).