Ratcheting of corroded pipes under cyclic bending and internal pressure

Abstract

Corroded pipes for oil transportation can eventually undergo ratcheting after several years of operation. Evaluation of the defects caused by corrosion in these pipes is important when deciding either to repair the line or to continue its operation. Under normal operational conditions, these pipes are subjected to constant internal pressure and cyclic loading due to bending and/or tension. Under such loading conditions, regions in the pipes with a reduction in thickness due to corrosion could experience a phenomenon known as ratcheting. The objective of this paper is to study the effect of a loss in thickness due to the corrosion through combined numerical models and experiments. Three constitutive models capable of representing the ratcheting phenomenon were selected and assessed. Experimental tests were developed to verify the eventual occurrence of ratcheting in corroded pipes under typical operational load conditions. In parallel small-scale cyclic tests were performed to obtain the material parameters necessary to calibrate the constitutive models that were adopted to simulate the phenomenon. The results reveal the existence of loading combinations that cause ratcheting in the defect region, but for the intact pipe, would result in elastic behavior. Numerical and experimental results were compared and showed that a good prediction of the plastic strain along the cycles can only be achieved with the model that requires a more complex calibration, but the shakedown/ratcheting occurrence can be accurately predicted with a model calibrated solely with uniaxial tests.