How to Assess Pipeline Anomalies and Effectively Prioritise Pipeline-Integrity Digs
By: Ian Smith, Senior Consulting Engineer, Quest Integrity
As seen in the December 2014 issue of Pipelines International. View the PDF version.
Quest Integrity used its InVista™ technology to provide both a metal-loss and geometry inspection of the pipeline, and various dents and ovalities were identified and quantified. Based upon their size, 16 of these geometric anomalies did not meet common depth-based acceptance criteria, and therefore required investigation and remediation.
The client wanted to perform an advanced assessment to investigate the lifecycle of the anomalies and establish a risk-based programme, since using depth alone is not an effective criterion for prioritising anomalies.
Quest Integrity's InVista ILI inspection identified 52 geometry anomalies including dents, double dents, and ovalities. Of these, 16 had depths greater than the conventional depth criterion of 6 per cent and would have required field investigation and remediation.
Finite-element analysis (FEA) models were created for the anomalies. Using a proprietary methodology which combines all of the component stresses, the stress-concentration factors (SCFs) were calculated for each location. The SCF measures the amount of stress magnification due to the presence of the anomaly. The magnitudes of the SCFs are driven by the sharpness of a dent or the abruptness of an ovality.
Using the operational pressure data that was provided by the client, pressure-cycling histograms were created at each anomaly location using the hydraulic modelling capabilities of Pacifica™, a software program that automatically performs pressure-cycle fatigue analysis. This method accounts for the decrease in magnitude of the pressure cycles the further downstream the anomaly is from the pump station.
The remaining life of each anomaly was then estimated by multiplying the pressure histogram by the SCF, and by using an S-N fatigue-life curve to determining the number of allowed cycles. A safety factor of two was applied when converting allowed cycles into remaining life. An allowance for the damage that occurs during dent formation and subsequent re-rounding was included in the life prediction.
Only three anomalies had a calculated remaining life of less than 100 years. The client scheduled digs for these three locations and has the advanced engineering assessment documentation to support the remainder of the geometry anomalies.
TWO KEY BENEFITS OF USING ADVANCED TECHNOLOGY TO ASSESS PIPELINE ANOMALIES