Elastic-plastic fracture analyses for misaligned clad pipeline containing a canoe shape surface crack subjected to large plastic deformation

Offshore pipelines are subjected to large plastic deformation during the reeling stage or in-service operation. These pipelines are joined together by a welding process, and defects and weld misalignment are frequently introduced into the pipelines, posing tremendous challenges to the integrity of these pipelines. In this study, the fracture responses of misaligned clad pipeline containing a canoe shape surface crack located in weld centre line (WCL) and fusion line (FL) were investigated using 3-D elastic-plastic finite element (FE) analyses. The influences of crack depth ratio, crack length to perimeter diameter ratio and centerline offset ratio on the crack tip opening displacement (CTOD) were analyzed in detail. The relationship between the CTOD and the global strain (εg) was built up, and a linear relationship was observed for εg ranging from 0.6% to 2%. The fracture assessment equation of British Standard (BS) 7910 predicted an over-conservative result in comparison with that obtained by FE analyses. Therefore, a new strain-based failure assessment curve was developed to assess the fracture behaviour of misaligned clad pipeline subjected to large plastic deformation.