Integrity Assessment of Pipelines With Ovality

Abstract

Ovality in a pipe results in a stress concentration and may present a pipeline integrity concern. If ovalization is generated during manufacturing or transportation and found during the girth welding process before the pipe is buried, replacement of the oval segment is usually the solution. However, ovalization is sometimes also found in buried pipelines via in-line-inspection (ILI) either before the pipeline is put into service or during regular pipeline maintenance. The mitigation and/or replacement of oval pipes after burial can be expensive. It is vital to identify what constitutes an unacceptable ovality level and to remediate only those that pose a threat of failure during the service life of the pipeline. The existing assessment approaches for oval pipes, such as that in API 579, generally require knowledge of the amount of ovality at zero pressure. However, the ovality of buried pipes usually is only available from ILI runs carried out at a non-zero internal pressure. The internal pressure tends to push the pipe back toward a circular shape, a phenomenon known as re-rounding. As a result, the ILI-reported ovality is smaller than that under zero pressure. Described in this paper is a new approach which can be used to assess the integrity of oval pipe segments based on ovality either as measured with no pressure in the pipe or as reported by ILI conducted at an elevated level of pressure. The approach considers both burst failure and fatigue damage. The accuracy of the approach was verified by finite element analysis (FEA). With the help of this new approach, a general discussion about the threat of ovalization to the integrity of a pipeline is provided. The analysis indicates that ovalization in amounts usually observed does not reduce the burst pressure in most pipelines, but fatigue damage could be a concern. The fatigue damage due to ovalization is sensitive to pipe geometry, amplitude of pressure variation, and the minimum pressure level within the pressure cycles.