Spectral fatigue assessment of offshore tubular joints under wave loading with consideration of corrosion effects

The design of offshore jacket structures requires reliable fatigue assessment, as fatigue life is strongly influenced by joint geometry and wave loading. This thesis investigates the fatigue behaviour of tubular joints under wave loading using a local spectral-fatigue approach, following the fatigue design recommendations of DNV-RP-C203 A MATLAB-based procedure is developed to derive stress power spectral densities from Morison wave loading, apply stress concentration factors (SCFs) from DNV hot-spot formulas, and compute long-term damage using DNV-RP-C203 S–N curves and Miner’s rule. A sensitivity study is performed on joint type (T and K), structural damping, inertia coefficient ��, hot-spot elevation, wave spectrum (JONSWAP vs Pierson–Moskowitz), and progressive wall-thickness reduction due to corrosion in the splash zone. Results are compared qualitatively with spectral-fatigue outputs from SACS v11.3. The analysis shows that fatigue life is mainly governed by SCF and hot-spot location: more severe SCFs and K-joints significantly reduce life, while moving the hot-spot from EL −1.0 m to −10.0 m increases life by roughly one order of magnitude. Corrosion can reduce life at the governing hot-spot by up to about 50%, particularly when the uncorroded life is long. Increasing damping to about 2% of critical improves life noticeably, whereas variations in �� have only a modest effect. Across all cases, the IPB brace-crown hot-spot is consistently the most critical, while chord hot-spots remain well above the 25-year design life. Despite differences in absolute values, MATLAB and SACS show consistent trends, and the MATLAB tool provides a practical, conservative framework for local spectral-fatigue assessment and parametric studies in line with DNV-RP-C203 practice.