Stiffness and Yield Strength of Simple V-joint of Offshore Structure

Abstract

Most of offshore structures for oil exploitation are composed of deck and legs (pipe framed structure). The local failure of the leg may lead to overall collapse of the structure. Accordingly, it is very important to clarify the ultimate strength of the leg.For the analysis of the strength of the leg, an investigation on the strength of a joint has to be performed previously. However, the studies carried out so far are limited to the joints of which chord and braces exist in the same plane, such as Y (T) -joint and K (TY) -joint. While the loading condition of the K (TY) -joint is limited to brace-axial force.In the actual structures, most of joints are constructed three-dimensionally. The loads acting from the brace to the chord are not only brace-axial force but also in-plane and out-of-plane bending moments. The mechanical behavior of such three-dimensional joint may differ from the one of two-dimensional joint.In this study, simple (unstiffened) V-joint is investigated as one of three-dimensional joints, of which two braces exist in the plane perpendicular to the axis of the chord. The mechanical behavior under various loads (normal force and bending moment) are analyzed by PNM (Plastic Node Method) which is one of the very efficient and accurate FEM for elastic-plastic analysis.Based on the results, the characteristics of the stiffness and strength of the V-joint are clarified, comparing with those of the T-joint which has the same dimensions. The mechanisms which vary the stiffness and strength depending upon the loading condition are shown. One of the most important result is that the stiffness and strength of V-joint is reduced to 70 % of those of T-joint in severe loading condition. So the reinforce method by using a stiffener is discussed.