On Strain Ageing Testing Methods for Reeled Offshore Pipelines

Abstract

Abstract The pipeline design standard DNVGL-ST-F101 requires performing the reeling simulation tests with subsequent strain ageing evaluation. The experimental methods for evaluation of strain ageing of low-carbon microalloyed steels upon reel-laying process were analyzed and compared for API X60 and X65 grades seamless line pipes with outer diameters 355.6mm and 219.1mm, respectively. The strain ageing tests were performed using the full-thickness segments of the pipes and pipe samples by small-scale reeling (SSR) and full-scale reeling (FSR) simulation, respectively. The finite element modeling is performed in order to analyze the distribution of stress and strain within both test specimens. Firstly, the difference in stress-strain behavior between the preliminarily aged small-scale full-thickness segments and full-scale pipe samples was experimentally observed. Finite element analysis and experimental data showed the non-uniform stress and strain distribution in the gauge length of the segments samples with increasing global strain. Calculations of stress triaxiality showed the significant difference in the stress state of the segments and the pipe samples which lead to different growth of yield strength of the material after strain ageing. Therefore, the present work attempts to turn attention to the interchangeability problem of the strain ageing methods using FSR and SSR simulation as the qualification tests of the reel-laid pipes and study of the pipe material behavior.