Comparison of Spectral Fatigues Methodologies Using Equivalent Stresses Obtained by Mises and Battelle Applied to a Semi-submersible Platform

Abstract

Offshore structures require accurate fatigue analysis since the preliminary design phases. However, the arbitrariness of the loading conditions to which such structures are submitted suggests a spectral analysis to determine the useful life in the initial stages of the design process. This approach can result in several different results accordingly with the spectral fatigue analysis methodologies or the ocean spectra used or even the stresses amplitude’s RAOs. To the first one, several analytical, semi-analytical and empirical propositions have been made over the years. The ocean spectra depend on the world’s region analyzed, although, there is well-developed formulation to each site. To the stresses amplitude’s RAOs there are two main methodologies, the use of Mises’ equivalent stress or the use of Battelle’s equivalent stress [1]. Here we will analyze the fatigue life of a node of a semi-submersible platform with 85.0 m of length and beam and 27.5 m of draught, operating in Campos Basin, Brazil. The ocean spectra used was the modified JONSWAP to Brazilian waters. A global model has been generated to obtain the stress amplitude’s RAO considering eight directions of wave incidence and the range of peak periods between 4.0 and 24.0 s. To get the equivalent stresses the Mises’ equivalent stress and the Battelle’s equivalent stress approaches were used. The last one used only to analyze the weld joints, but with the results obtained in the global simulation. Moreover, some of the main and most used methodologies of fatigue spectral analysis as the narrowband approach [2] and the methods proposed by Dirlik [3], Wirsching & Light [4], Ortiz & Chen [5], Benasciutti & Tovo [6], Larsen & Lutes [7], Zhao & Baker [8] and an analytical approach of the method proposed by Petrucci & Zuccarello [9] were used to obtain the one-year fatigue damage and extrapolated to get the fatigue life estimation. The resulting fatigue damages are compared for the different fatigue spectral analysis methodologies using the two different RAO’s approaches.