Residual Strength of Cracked Tubular Joint Using Nonlinear Finite Element Analysis

Abstract

Fixed offshore platforms have been used for extraction of oil and gas. These platforms were primarily constructed using steel frames made of tubular members welded at joint or specially fabricated joints. The tubular joints are vulnerable to fatigue-induced cracks which initiate at joints and may propagate through its design life. If the platform life is extended depending upon oil and gas availability, the initial cracks may extend beyond acceptable limits. In recent times, the research on evaluation of residual strength of cracked tubular connection has been considerably increasing since the platforms in various oil and gas fields are ageing. To determine the residual capacity of cracked T-tubular joints, a nonlinear finite element analysis has been carried out. The FEM model of uncracked T joint was validated with experimental result available in literature. The benchmark study has also been made on uncracked T-joints with a specific d/D, t/T and D/2T and compared with the results obtained from empirical equations (API RP 2A). The possible crack locations have been identified using the maximum SCF at crown and saddle points for axial loads. The cracks are introduced in the maximum SCF locations of tubular joint. The study has been extended to range of d/D and D/2T. A correlation has been established between lengths of crack to the residual strength for various crack locations investigated. The residual strength obtained has been compared with reduction factor (BS 7910). It was also found that the residual strength of joints decreases with increase in D/2T.