Spectral wave-induced loads and fatigue life of ship structures for different sea states

• Wave-induced fatigue loads in naval structures were analyzed under different sea states using direct calculations. • Discrepancies were identified between standardized formulas and direct fatigue stress calculations. • Long-term satellite observations and severe sea state conditions were considered for analysis. • Fatigue performance of four steel materials (AH32, AH36, AISI 1020, and AISI 316L) was compared. • AH36 and AISI 1020 exhibited superior resistance to marine-induced stresses, while AH32 and AISI 316L demonstrated shorter lifespans. • Proposed the development of am onboard ship real-time monitoring tool to calculate instantaneous stress values and predict remaining fatigue life. Ensuring the fatigue reliability of large container ships is critical for preventing catastrophic structural failures. Wave–structure interaction plays a critical role in the long-term structural integrity of marine vessels operating under stochastic seas. This study analyzes the fatigue behaviour of container-ship structures exposed to wave-induced vertical bending moments by combining a spectral representation of irregular waves with material-specific fatigue models. Wave loads are described using the Pierson–Moskowitz spectrum for a range of representative sea states, and the corresponding structural responses are evaluated through spectral fatigue analysis. Fatigue damage is quantified using S–N curves and the Palmgren–Miner rule for four common marine steels (AH32, AH36, AISI 1020, AISI 316L). Results show that AH36 and AISI 1020 provide robust resistance to cyclic wave loads, while AH32 and AISI 316L exhibit significantly shorter fatigue lives under extreme sea states. The comparison with classification-society design formulations shows discrepancies of up to 23% relative to direct calculations, highlighting the inherent limitations of rule-based design methods. The study also outlines inspection intervals and monitoring strategies intended to mitigate early crack initiation and propagation in structurally sensitive midship regions. Collectively, these findings contribute to improving structural safety, operational reliability, and the long-term durability of ocean-going vessels. The findings enable the possible development of a tool that can be installed on ships to provide real-time insights. By utilizing the transfer function provided by the ship's designers and real-time sea conditions, the tool could calculate instantaneous maximum stress values experienced by critical structural components. This allows for immediate prediction of the remaining fatigue life if the material's fatigue limit is exceeded. The findings support the development of real-time fatigue monitoring tools, enabling ship operators to anticipate critical conditions and implement preventive maintenance before failure occurs.