Assessing mechanical damage in offshore pipelines

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Assessing mechanical damage in offshore pipelines - Two case studies

MacDonald, K.A. / Cosham, A. / Alexander, C.R. / Hopkins, P.

The most common causes of damage and failures in onshore and offshore, oil and gas transmission pipelines in Western Europe and North America are external interference (mechanical damage) and corrosion. Assessment methods are needed to determine the severity of such defects when they are detected in pipelines. Mechanical damage in the form of dents and gouges is recognised as a severe and common form of mechanical damage to pipelines. In general terms, dents and gouges reduce both the static and cyclic strength of a pipeline. The severity of a dent depends on a number of factors, including: the size and shape of the dent; whether it affects the curvature of a girth or seam weld; and whether it contains other defects, such as a gouge or a crack. An understanding of the issues that influence the severity of mechanical damage is required to ensure that the appropriate action is taken on finding such damage, and that all of the necessary information is gathered for conducting an assessment. For example, what inspection method(s) are appropriate, and when is it better to repair such damage, rather than assess it? This paper discusses some of the issues that may arise during the assessment of mechanical damage, such as: when is a feature really a dent; when does a superficial scrape become classified as a gouge; and the limitations of existing assessment methods. These issues are illustrated with two recent case studies of damage to offshore gas transmissions pipelines.