Fracture control - Offshore pipelines - New strain-based fracture mechanics equations including the effects of biaxial loading, mismatch and misalignment

In this paper a framework for a strain-based fracture mechanics crack driving force methodology for pipes with surface cracks, are presented. The model addresses the effects of crack depth, crack length, pipe diameter, wall thickness and yield to tensile ratio. Based on FE simulations, an equation to calculate the applied crack driving force, either through CTOD or J, has been derived. The equation is intended for use in cases where global plastic deformation occurs. A general approach to introduce the effects of biaxial loading, yield stress mismatch, and misalignment on the driving force, through definition of an effective wall thickness and an effective crack ligament height, is outlined. Models to quantifY the effects of the different parameters are also derived. Finally, results are presented from comparison between 2D and 3D FE simulations and the predictions made by the proposed driving force equations.