A Study on the Degree of Bending (DOB) in Two-Planar Tubular DYT-Joints of Offshore Jacket Structures: Geometrical Effects and Fatigue Design Formulation

Abstract

Abstract Through-the-thickness stress distribution in a tubular member has a profound effect on the fatigue behavior of tubular joints commonly found in steel offshore structures. This stress distribution can be characterized by the degree of bending (DoB). Although multi-planar joints are an intrinsic feature of offshore tubular structures and the multi-planarity usually has a considerable effect on the DoB values at the brace-to-chord intersection, few investigations have been reported on the DoB in multi-planar joints due to the complexity of the problem and high cost involved. In the present research, data extracted from the stress analysis of 243 finite element (FE) models, verified based on available parametric equations, was used to study the effects of geometrical parameters on the DoB values in two-planar tubular DYT-joints. Results showed that it is quite common for an axially loaded two-planar DYT-joint to have a low DoB. Therefore, when the current standard HSS-based S-N approach is used for the fatigue analysis of axially loaded two-planar DYT-joints, results should be modified to include the effect of the DoB in order to obtain more accurate fatigue life prediction. It was also concluded that for axially loaded two-planar DYT-joints, the parametric formulas of uniplanar YT-joints are not applicable for the DoB prediction, since such formulas may lead to under-/over-predicting results. Parametric FE study was followed by a set of nonlinear regression analyses to develop six new DoB parametric equations for the fatigue analysis and design of axially loaded two-planar DYT-joints.