Fatigue Life Prediction of Ship Structure Using Spectral-based Fatigue Method: Case Study of 150 DWT Ro-Ro Ship

Abstract Ships experience continuous wave loading that drives cyclic stresses and fatigue damage. Since many ships in Indonesia fleet are aging, assessing their fatigue life is essential to ensure safety and continued operation beyond their design life. This research aims to predict the fatigue life of the bottom structure of a 150-DWT Ro-Ro ferry operating at full load on the Bali Strait. Spectral-based method is applied across the full wave-period range from local wave-scatter data. Hydrodynamic responses are computed with 3-D diffraction theory to obtain bending-moment RAOs, which are combined with the Pierson–Moskowitz spectrum to form short-term stress spectra across eight headings. The approach uses the global hull girder bending from wave elevation amplitude to determine RAO stress. Cumulative damage is evaluated with Class-F and the Palmgren–Miner rule for 20-year design life. The cumulative damage index is 0.26098, dominated by west wave direction. The predicted fatigue life is 76.63 years and the extreme stress range is 39 MPa.