NUMERICAL SIMULATIONS OF BURST OF CORRODED PIPES WITH THERMALLY INDUCED COMPRESSIVE AXIAL STRAIN

摘要

Some onshore pipelines conduct fluids that are too viscous to be conducted at ambient temperature; they must be heated to enable efficient pumping and flow. These pipelines present a failure rate that is many times larger than those that operate at ambient temperature. The prevailing failure mode for these pipelines is external corrosion: the external thermal insulating coating can give rise to a very severe corrosion process. Although corrosion is a significant threat for pipelines that operate with heated fluids, the available corrosion assessment methodologies might not be appropriate for this situation. Several studies have been conducted considering a pipeline with a corrosion flaw with axial stress (or load) plus pressure. But a heated pipeline with axial restraint - as caused by the soil friction in a buried pipeline - imparts a compressive axial strain, not a stress. Although in the elastic regimen the thermally induced axial strain generates significant axial stress, it can be expected some level of decrease in the axial stress after yielding, due to the large reduction in the material stiffness and the increase in Poison's ratio. Since localized yield in the flaws is allowed in the assessment of a corrosion flaw, it seems too conservative to use the elastic axial stress in this assessment. In this article a numerical study of the effects of the temperature in the burst pressure of a pipeline with axial restraint and thermal expansion is presented. Finite element simulations were conducted using actual tensile test curves for two pipeline steel grades, API 5L Gr B and X70. The boundary conditions assumed axial restraint with free radial displacement. The loading comprised an initial heat of the pipe's material and, afterwards, gradual increase of the pressure until burst, assumed to occur by plastic instability. Two diameter to thickness ratio and several flaw geometries were studied. Initially, the effect of the temperature was evaluated for pipes without defect. Afterwards, numerical simulations of the burst of pipe sections with volumetric flaws of various depth and length were conducted. For both the cases of pipes with and without defect, the simulations were carried out comparing the cases of heated and not heated pipes. It was found that although the thermal effect causes a large compressive axial stress in the elastic regimen, this stress is almost completely relaxed after yielding. No effect of the temperature in the burst pressure was observed in the numerical simulations.