Fatigue strength analysis of offshore tubular welded joints under constant amplitude loading - local strain and fracture mechanics approach

The main components of steel offshore structures, whether fixed or floating, are generally tubular members. Large stress concentrations arise1 due to the abrupt geometric discontinuities at the intersections of these welded tubular members, called joints or nodes. The varying environmental loads acting on these joints cause fatigue crack initiation, growth and their final catastrophic failure. This thesis presents a numerical study of the total fatigue life of offshore tubular welded joints under the action of axial, in-plane and out-of-plane bending loads, using local stress-strain and linear elastic fracture mechanics approaches. The study includes the development of a computer program for the automatic generation of meshes for tubular joints and a contact program for the prevention of crack surface penetration. -- Stress analysis to determine the possible location of the crack initiation op the tubular joint has been carried out using eight noded degenerate isoparametric shell elements. The influence of geometric parameters on the stress distribution around the joint as well as through the joint thickness has been investigated, and the results obtained therein compared with experimental results: they also have been compared with established parametric equations. Good comparisons have been obtained with the experimental values. -- The local stress-strain approach, using the Mansoii-Coffin rule, is utilized for the study of the crack initiation. Using experimental investigations on crack initiation life of tubular welded T-joints, fatigue strength exponent b and fatigue ductility exponent c have been determined empirically and used to compute crack initiation life of the tubular joints analyzed in this study. -- The weld toe crack influence on the through thickness as d surface stress distribution has been studied using the line spring element in conjunction with the degenerate eight node shell elements to model the crack. The stress intensity factors determined, from this study, were compared with available theoretical studies and found to give good results. -- Using the stress intensity factors obtained from the line spring model, the through-thickness crack propagation lives of the tubular joint under consideration were predicted using Paris crack growth law. The propagation lives for each loading conditions were predicted, on an incremental cycle basis, up to 90% of the chord thickness cracking. The estimated fatigue lives were compared with experimental investigations carried out at Memorial University of Newfoundland St. John's (for axial loading) and University of Waterloo (for in-plane loading) under the Canadian Cooperative Offshore Tubular Joint Program and found to give good results