Stress Concentration Factors of DK Square-to-Square Tubular Joints

The fatigue life of tubular joints with many braces, e.g., DK and DTK joints, are commonly determined by treating them as tubular joints with less braces, e.g., T/Y, K, and X joints, based on the joint classification approach recommended by the American Petroleum Institute. The DK square-to-square tubular joint type was selected to verify the reliability and accuracy of such an approach. A parametric stress analysis of DK square-to-square tubular joints subjected to axial loads, in-plane and out-of-plane bending moments has been performed using the finite element technique. The results of this analysis are presented as a set of formulas expressing the stress concentration factor as a function of the relevant geometric parameters for various loading conditions. A comparison is made between the results obtained for DK square-to-square tubular joints and those obtained for X and K square-to-square tubular joints, which are commonly employed to simulate the former when the joint classification approach is adopted. In general, the stress concentration factors for DK joints are significantly higher, which shows that the recommended approach may not be reliable and accurate in dealing with DK joints.