FINITE ELEMENT FATIGUE ASSESSMENT OF STEEL TUBULAR JOINTS IN OFFSHORE STRUCTURES

Background: Fatigue analysis is a very complex process, which is still today not fully under-stood. This thesis numerically investigates the advantages and limitations of the employment of the Finite Element (FE) method in actual standard fatigue design procedure. Two studies are conducted to validate the SCF calculation and the application of Hot-Spot Stress (HSS) method to fatigue design. Study I: The Stress Concentration Factor is a magnification factor introduced to account for geometrical or material discontinuities in structural joints. In these locations the stress may reach values several times higher than the nominal stress and may lead to the premature collapse of the structure when subjected to fatigue loading. However, current methodologies to compute the SCF cover only a limited number of joint types and configurations. The aim of this study is to investigate the SCFs in tubular joints through the application of empirical formulas and an extensive numerical analysis campaign. SCFs are assessed for several types of tubular joints and loading conditions according to current standards and literature. Obtained results are compared together to highlights the weaknesses and strengths of each approach and to investigate their implications for the design of tubular joints. Study II: Fatigue is one of the most frequent causes of steel structures failures, particularly in offshore platforms. Thus, the fatigue life assessment is one of the most relevant procedures for the comprehensive evaluation of both the load-carrying capacity and service life of the structure. The standard fatigue procedure consists of the method of local stress, the so-called Hot-Spot Stress (HSS) applied on a global analysis model. HSS method consists in calculating the stress occurring in the cross sections of the whole elements and applying them a multiplying factor to account for the geometry of the detail. At present, the use of Finite Element makes possible looking for more accurate fatigue stress ranges calculation. This study presents fatigue assessment of representative types of tubular joints in welded offshore platform. Joint fatigue damages are calculated performing global and local FE analyses. The obtained results are com-pared together to evaluate the limits and advantages of the different approaches and model types.