Stress concentration factor parametric equations for tubular X and DT joints

During the design stage, the peak stress is usually needed for estimating the fatigue life of offshore tubular welded joints by an S-N approach. However, for fracture mechanics calculations of remaining life, on cracked joints in service, information is required on the magnitude and distribution of the stress acting in the anticipated crack path, not just the peak stress at one location. Fatigue crack propagation rates are important to reliability-based inspection scheduling: hence the need for this information is becoming more pressing. Parametric equations are available for Y and T joints in terms of peak stress, stress distribution and bending-to-membrane ratio. However, for X and DT joints, there are no parametric equations for stress variation through the thickness and around the intersection. Even for stress concentration factor (SCF), so far there is no full set of parametric equations especially for single-brace loading. Thin-shell finite element analyses have been conducted for 330 X and DT joints typical of those used in offshore structures, subject to six modes of loading. The results from this work have been used to produce a new set of parametric equations as a function of non-dimensional joint geometric ratios α, β, γ, τ and θ by carrying out regression analysis. These equations can be used to predict SCFs at the crown toe, saddle, crown heel and hot-spot positions for each mode of loading, for both chord and brace, as well as the angular location of the hot-spot stress site around the intersection. This set of SCF parametric equations has been assessed by comparing the predicted values with results from steel and acrylic model tests and also with the predictions from existing parametric formulae given in the literature. The degree of bending data, and stress distribution data, will be reported in other publications.