The development of fatigue design formulas for the outer brace SCFs in offshore three-planar tubular KT-joints

A set of parametric stress concentration factor (SCF) analyses is carried out in the present research for right-angle three-planar tubular KT-joints under three different axial loading conditions. Three-planar KT-joints are one of the most important joint types in the six-legged offshore jacket-type structures since they connect the bracing members to the main legs of the jacket template. The analysis results are used to present general remarks on the effect of geometrical characteristics and loading conditions on the SCFs at different saddle positions on the main (outer) braces. Based on the results of FE analyses which are verified against the experimental data, a complete set of SCF databases is developed. Afterwards, through nonlinear regression analysis, a new set of SCF design equations is established for the fatigue design of three-planar KT-joints under three different axial load cases. An assessment study of these equations is conducted against the experimental data and the acceptance criteria proposed by the UK Department of Energy (DoE).

► Parametric FE analyses were used to investigate outer brace SCFs in 3-planar tubular DKT-joints. ► Effects of geometrical characteristics and loading condition on the saddle SCFs were studied. ► Highly remarkable differences were observed among three-planar, two-planar, and uni-planar SCFs. ► For fatigue design purposes, parametric formulas were developed to calculate chord-side SCFs.