An Analytical Study of Stress Concentration Effects in Multibrace Joints Under Combined Loading

Abstract

ABSTRACT This paper summarizes the results of a finite element study of stress concentration effects in multi-branch tubular joints subjected to combined in-plane loading. The joint configurations analyzed include T, K, non-overlapping and overlapping TK and overlapping TKT. Simple joints of the T and K type subjected to axial load and/or in-plane bending were studied first. The interaction of various branches of a multi branch joint were then investigated by introducing additional members in the neighborhood of simple joints. The study also compared the computed hot spot stresses for a T joint under combined loading with those estimated by the use of parametric equations and various superposition schemes. INTRODUCTION Fatigue of structural elements is a complex process involving a large number of influencing parameters. In addition to the magnitude and nature of cyclic loading, the parameters that influence fatigue life include the stress concentration effects, residual and mean stress levels, the metallurgical structure of the material and the corrosiveness of the environment. The common fatigue design practice utilizes a semi-empirical approach that expresses the fatigue life of structural elements as a function of the analytically or experimentally determined stress ranges and accounts for the effects of the other parameters through empirical relationship denoted by S-N curves. Current Design Practice The current offshore design practice concerning the fatigue life evaluation of tubular structures is based on the proven engineering approach depicted above. These applications are, however, particularly severe, both from the viewpoints of complexity and diversity of structural configurations and the complexity of loadings. A typical joint from the fatigue prone areas of a jacket structure is illustrated in Figure 1. The loads acting on the member ends of the multi planar joint were obtained from a global analysis of the jacket. A typical North Sea wave was used for this purpose. The nature of member end loads under changing magnitude and direction of the impinging wave is expected to be similar to that shown in Figure 1. Fatigue life evaluation of this joint requires an estimation of the hot spot stress history under combined loading and the provision of an applicable S-N curve. The common tubular joint fatigue evaluation procedure utilizes parametric equations 1, 2, 3 to estimate the hot spot stresses at selected locations (crown and saddle points) of the tubular joint interface. These parametric equations are based either on a finite element analysis utilizing thin shell theory or on laboratory testing of simple joints subjected to simple loadings. The parametric formulation of stress concentration effects, to date, is limited to X, T, K and TK joints subjected to simple loadings of the axial, in-plane bending and out-of-plane bending types, applied separately to the joints.