The Study Of Stress Concentration Factor For Multi-planar Tubular DT-joints In Jacket Platform Under Axial Loads

For the elastomer with hole is under loads,the stress around the hole is much greater than the stress without hole and the stress away from hole.This phenomenon is named as stress concentration.In this research,the stress around the hole of a infinite plate under uniform loads is calculated by polar coordinates method.the stress around the elliptical hole of a infinite plate under various loads and the stress around crack tip is calculated by complex variable function method.Stress concentration factor(SCF)plays an important role in the fatigue life assessment of tubular joints.There have been many SCF parametric equations proposed for the uni-planar tubular joints,however,few was focused on the multi-planar tubular joints.In the present study,finite element method was employed to investigate the SCF distributions along the chord-brace intersection of multi-planar DT-joints.In order to guarantee good quality and appropriate quantity of FE meshes,a method of sub-zone mesh generation was developed.The effects of geometrical parameters on the SCF distributions were investigated.A set of equations was proposed based on the results of the finite element analysis.Two types of error analysis were used to verify the reliability of developed equations.Stress concentration factor is considered as one of the random variables which greatly affect the results of fatigue reliability assessment for the tubular joints and offshore jacket-type platform structures.Several probability models commonly used in the fatigue reliability analysis are fitted to the density histograms for statistical samples of maximum SCFs.The values of parameters in each probability model are estimated by maximum likelihood(ML)method.After the chi-squared goodness-of-fit test is performed,the Birnbaum-Saunders distribution is found to be the best fitted one.A set of probability density functions(PDFs)following Birnbaum-Saunders distribution are proposed for the maximum SCFs on the chord and the brace sides under two kinds of axial loading.