In-Line Inspection Probabilistic Assessment

Benefits

Atteris has applied a probabilistic approach to assess pipeline corrosion pit growth and determine the inspection frequency. This approach provided the operator with assurance of pipeline integrity and enabled deferral of an in-line inspection.

Background

In-line inspections are commonly used to measure pipe wall loss including pit depths. Standard approaches apply a conservative forecast of future pit depths that results in high pit growth estimates and no definition of the risk, which may be acceptably low. Atteris’ approach considers the uncertainties in measured pit depths and future operating conditions; and determines the probability of exceeding the pipeline’s serviceability limit state (SLS) due to pit growth.

Work Performed by Atteris

As part of a design life-extension study, a pipeline was found to be subject to significant bottom of line corrosion (BOLC) due to carbonic acid corrosion, resulting in thousands of pits, all of which are currently within acceptance criteria.
A conservative pitting factor was derived from the nominal pig data and an initial estimate of the worst case future pit depth determined that excessive pitting was possible, which drove a near-term in-line inspection requirement. This inspection of this pipeline is challenging and expensive due to the requirement for a subsea launch.

Based on known uncertainties in the pig data, Atteris randomised each measured pit and developed a probability distribution function (PDF) for the pitting factor. Atteris also derived a PDF for corrosion inhibitor persistency. The PDFs were then combined to determine the probability of an excessive pit.

This work demonstrated that the pitting risk was tolerably low for at least another 5 years, enabling a deferral of the next in-line inspection. The approach was approved by the regulator and the operator was able to defer the A$20M inspection cost.