Fatigue assessment of pipeline with plain dents under cyclic pressure loading using finite element method

One of the many challenges faced in fatigue assessment today is determining the stress concentration factor 'SCF' associated with the dents, which are used with appropriate SN curves to determine the fatigue life. This historically has been determined empirically or by using finite element analysis.
This paper presents finite element analysis on a parametric range of industry pipes (both offshore and onshore) to extract SCF data used for fatigue assessment. The parametric dataset focuses on the effects of pipe geometry, dent geometry, material properties and pressure cycling on the prediction of the fatigue life. This parametric dataset will eventually be used to develop an algorithm for fatigue prediction using an artificial neural network.
Two types of indenters (Dome and Bar) are used to simulate circumferential and longitudinal dents. Four different dent depths ranging from 2% d/D to 10% d/D are also simulated to investigate the effect of dent geometry. Four different pipe grades (X42, X65, X80 and X100) are analysed to investigate the effect of pipe materials on dent fatigue. Similarly, eight pipes with different diameter to thickness ratio D/t ranging from 18-96 are analyzed to investigate the effect of pipe geometry. Stresses are computed at both 50% SMYS and 72% SMYS to investigate the effect of pressure variation.
The results from this study indicate that longitudinal dents have higher stress concentrations compared to circumferential dents of similar dent depth. Results also indicate that the re-round dent depth (i.e. dent depth after pressurization) increases with increasing D/t and increasing dent depth. Similarly, the pipe material has a major effect on the fatigue life. Pipes with higher material strength have higher stress concentration compared to pipes with lower strength of similar dent depth. The stress concentration factors SCF associated with the dents are then computed.