A fatigue damage evaluation using local damage parameters for an offshore structure

The fatigue design of offshore structures normally uses wave and wind fatigue loading. Currently, fatigue analyses for fatigue damage accumulation assessments of this type of structure are based on signal–noise (S–N) curves for welded structural components, the hot-spot stress approach and the Palmgren–Miner law – according to design codes. Fatigue analyses in design codes and/or recommended practices are supported by global structural analysis. In this paper, a global–local fatigue methodology applied to an offshore jacket-type platform using a local approach through the notch strain damage parameter is proposed. The local approach is based on Neuber's rule combined with the Ramberg–Osgood description. Then, the Coffin–Manson strain–life relation together with the Palmgren–Miner linear damage rule are used to evaluate the fatigue damage accumulation for the critical tubular welded joint. For application of Neuber's rule, the stress concentration factor values are calculated, according to Efthymiou's analytical equations, for the connection under consideration. The proposed methodology is compared with the simplified fatigue analysis presented in the Det Norske Veritas (Norway) and Germanischer Lloyd (Germany) DNVGL-RP-C203 recommendations. These analyses were performed using wave loads from the scatter diagram collected in the North Sea, which were computed through the fifth-order Stokes wave theory and the Morrison formula.