The extreme loading conditions, i.e. the large plastic bending combined with high internal pressure, and the potential cracks or flaws in the welds pose tremendous challenges to the structural integrity for offshore pipelines. In this paper, the fracture response of offshore pipeline with semi-elliptical surface crack under large plastic deformation is analyzed through 3-dimensional finite element simulations. Extensive parametric studies are performed to investigate the influences of the crack geometry, strain hardening coefficient and internal pressure on the evolutions of crack tip opening displacement (CTOD). Furthermore, by virtue of response surface methodology, the new strain-based CTOD estimation formulations are proposed to assess the fracture behavior of flawed pipelines subjected to pure bending load as well as biaxial load, respectively.