Pipeline Fracture due to Compression-Tension Loading Caused by Foreign Object Impact

In areas frequented by fishing vessels, trawl equipment or anchors may interfere with pipelines and cause damage through impact, potential hooking, and ensuing release of the pipeline. This load sequence of denting followed by global bending and springback results in a complex stress and strain history. Experiments have shown that fracture in an impacted pipe typically arises along the bottom of the dent, where the material suffers high compressive strains in the impact and hooking phase, and a rapid change to tension during the rebound phase. High compressive strains may reduce the strain to failure significantly for a succeeding tensile phase. A common trait of ductile damage models is to account for damage through nucleation, growth and coalescence of voids, which traditionally is thought to occur during tension. In this study, an uncoupled phenomenological Cockcroft-Latham-type fracture model accounting for anisotropic damage is used. The fracture model is implemented in the explicit finite element programme IMPETUS Afea Solver, and calibrated using material tests. Simulations show that the proposed fracture model is able to account for the observed behaviour.