Integrity Assessment of Pipelines With Ovality

25/06/2019
    Publication: 2018 12th International Pipeline Conference. Volume 2: Pipeline Safety Management Systems; Project Management, Design, Construction, and Environmental Issues; Strain Based Design; Risk and Reliability; Northern Offshore and Production Pipelines Date: September 24–28, 2018
    A new approach for assessing an oval pipeline integrity segment is developed in the paper. The existing approaches require ovality measured in an empty pipe. The new approach can be used with ILI-reported ovality by considering the ovality reduction due to internal pressure during ILI. The approach evaluates the threats of ovalization due to both burst and fatigue and was verified by FEA.
     
    The integrity threats of ovalization are discussed based on the new approach with following observations.
    • Ovalization within the range of deformation typically observed on pipelines does not reduce the burst pressure independent of interaction with other conditions.
    • Ovalization could possible aggravate the threat of fatigue. This concern is less for gas service than liquids service. For liquid lines with aggressive pressure cycles, the fatigue life of oval pipes can be estimated following the assessment approach developed in Section 2 of the paper. The enhanced stresses due to ovality should be combined with stress concentrating effects associated with a longitudinal seam.
    • The ovality is reduced by rerounding even at a low internal pressure level in pipes having large 𝐷/𝑡 ratios. Consequently, large ovality is not often reported by ILI running under moderate or high internal pressures.
    • The additional amplitude of stress change within a pressure cycle due to ovalization decreases significantly when the minimum pressure is nonzero and only affects limited areas in the pipe cross section. Therefore, few failures have been observed due to ovalization.
    • Other concerns related to an oval pipe were also discussed, including the ovalization near a girth weld, the safe passage of ILI tools, and the potential for large ovality to be an indication of significant bending acting on the pipe cross section.
     
    To sum up, ovalization is not a major threat to pipeline integrity. However, for liquid lines with severe pressure cycles, the fatigue life of the detected oval sections should be analyzed following the approaches provided in the paper. The conclusions do not consider interactions with other conditions that could be present in some pipelines including pitting corrosion, environmental cracking, seam defects, or mechanical damage. The potential for adverse interaction may be a subject for future research.
     
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