Behaviour of wrinkled thin-walled steel pipes subjected to displacement-controlled axial cyclic loads

Abstract

Thin-walled steel pipes are used for transporting oil and gas. The wall thickness-to-diameter ratio of such pipe ranges from 30 to 80. Limited research data or guideline(s) are available on how to assess the severity of wrinkle defects of such oil and gas pipelines. Failure to assess the severity of these wrinkle defects, may lead to unnecessary shutdowns of the pipeline operation leading to loss of revenue for the pipeline operators (companies) and interruption of oil and gas supply. Therefore, this study focused on investigating the structural behaviour and failure of wrinkled pipes with varying wrinkle geometries when they are subjected to axial cyclic loads representative of seasonal temperature variations and/or ground movements. Eight full-scale tests were carried out on API 5L pipes with D/t of 57 in two loading steps. In the first load step, a wrinkle (outward buckle) with an amplitude of about 4%, 6%, 8%, and 10.5% of the outside pipe diameter was formed by maintaining the internal water pressure at 30% or 60% of the yield pressure, and while applying a monotonically increasing axisymmetric axial displacement. In the second load step, cyclic axisymmetric axial displacements were applied on the wrinkled pipe until a fracture or a leak occurred. Results of this study show that a steel pipe with a wrinkle defect subjected to axial cyclic displacements, may fail due to a fracture at the crest of the wrinkle defect where the stresses and strains are concentrated. A linear relationship between the wrinkle amplitude or wrinkle severity and the number of cycles to failure was found. This study also found that a failure in the wrinkle region occurs earlier when the operating (internal) pressure increases.