Impact against offshore pipelines
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In 2007 a pipe at the Kvitebjørn oil field was impacted by an anchor and dragged out of its position. After the accident it was decided safe to run production, but later a leakage was found and the production was shut down. This gave rise to a need for further knowledge of the residual strength of impacted pipes. This thesis is part of an ongoing research program and a continuation of previous works. In 2010, scaled pipes were initially impacted in the pendulum accelerator at SIM lab, before they were stretched almost straight in a stretch rig at Statoil's laboratory in Trondheim. The test setup was an attempt to recreate the hooking scenario from the accident. Numerical simulations applying the Johnson-Cook isotropic hardening model showed good agreement with the experiments for the impact, while a deviation was seen for the stretching. In the present work a combined isotropic/kinematic hardening model was calibrated from uniaxial tension-compression tests. This was done in an attempt to improve the Finite Element Analyses of the stretch step, assuming the kinematic properties of the material to be important due to the reversed loading experienced by the pipes. The material tests revealed a kinematic behaviour, as the reversed yielding initiated at a lower stress than the initial yielding. However, the simulations of the stretching were not considerably improved by applying combined hardening in Abaqus/Explicit. Impact experiments were conducted on water filled pipes to evaluate the effect of contents on the impact behaviour. Tests performed on open pipes indicated that the mass effect of the water might be neglected, since the global behaviour was similar to that of the empty pipes. However, a small change in the local to global deformation ratio could be seen. Tests on pipes having a membrane welded to the end revealed a difference in the force-displacement curves, as well as on the deformation ratio, and these results were ascribed to the build-up of pressure. This pressure was seen to limit the local indentation, but not the total deformation. Although further experiments are recommended, pressure effects seem to be more dominant than mass effects.