Impact against offshore pipelines
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In this thesis impact loads against offshore pipelines are studied. Impact loads are often associated with accidents, such as an anchor hooking. A pipeline subjected to an anchor hooking may experience severe deformations and a complex stress and strain history. Local deformation will occur when the anchor hits the pipe. Then the pipe may be dragged along the sea bed until the anchor is released or the anchor chain snaps. Axial forces, which increase as the pipeline deflects, will drag the pipe back against its initial position. Fracture may occur in the area where the anchor hits. Experiments and numerical analyses are applied to investigate the problem. The experiments consisted of two tests, impact and stretching. Scaled pipes were first impacted in the pendulum accelerator at Department of Structural Engineering, NTNU, Trondheim.Stretching of the dented pipes was performed at Statoil’s laboratory in Trondheim. Fracture occurred in all the pipes during the stretch tests. Material test specimens were taken from the pipe wall and tested in the laboratory at NTNU. A material model, including a constitutive relation and a fracture criterion, were calibrated for use in numerical analyses. Strain hardening and strain rate sensitivity was accounted for in the constitutive relation, and isotropic hardening was assumed.Notched test specimens were tested to capture the effect of stress triaxiality on the fracture strain. The pipes were modeled using Abaqus/Explicit and the experiments were recreated numerically.Both shell and volume element models were used. The global response in the experiments and the numerical simulations agreed very well. A mesh sensitivity study revealed that a large number of elements is required to accurately describe the plastic strains in the critical area. A study of the stress situation in the critical element indicated that isotropic hardening may underestimate the strains and that the cyclic hardening properties of the material should be further investigated.