Fatigue analysis of tubular joints is an important part of the design process of offshore structures. Tubular joints are randomly subjected to combined loadings with variable amplitude involving axial force and bending moments. In this paper using the line-spring element method the stress intensity factors of the surface cracks located at saddle and crown positions of tubular T-joints are calculated for loading conditions of axial forces, in-plane bending and out-of-plane bending applied on branches. Polynomials are introduced to describe the relationship between non-dimensional stress intensity and crack depth for cracks with various shapes and loading conditions. Thus, using linear elastic fracture mechanics and the Paris" Law fatigue crack growths can be computed cycle by cycle leading to fatigue life or remaining fatigue life calculations. The Monte Carlo method is introduced to simulate random loadings and the incredibility of the method is estimated in statistical means. Several combinations of stochastic loadings are considered and the obtained lives are compared.
The traditional method in estimating fatigue life is based on the SoN curves and the Damage Cumulation Principle referred to as the Palmgren-Miner Rule. Fracture Mechanics as an alternative method for fatigue analysis is also used since the 1970"s, where fatigue life estimates are obtained via calculation of crack growth rates. This latter method is suitable for determining residual lives for structures such as offshore platforms. In the present paper, Fracture Mechanics is used to estimate residual life of tubular T -joints with initial surface cracks when combined random loads of axial tension, in-plane bending and out-of-plane bending are applied. Crack growth is predicted by Paris" Laws (1963) and stress intensity factors are calculated by line-spring fInite element method. The random loads are supposed to, l have Rayleigh distribution style and simulated by Monte Carlo method.