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  Title Finding and assessing the severity of interacting threats using ILI
  Author(s) Jane Dawson and Geoff Hurd  
  Abstract HISTORICAL PIPELINE FAILURES illustrate that there are often several mitigating factors that have led to the release. For example, mitigating factors can be the presence of one or more defect types, multiple loadings acting on the pipeline and/or the parent pipe or weld material deficiencies.

Threat identification is a key component to managing the integrity of pipelines. Various methods are described in both regulations and industry literature and have been used successfully to prevent, detect and mitigate the pipeline threats individually but not necessarily as threats in combination. Pipelines can, and often are, susceptible to the colocation of multiple threats such as internal and/or external corrosion, seam weld and/or girth weld defects, environmental cracking, laminations, gouging, dents, wrinkles, ground movement loads and other axial loads etc.

These different types of damage and loadings may occur in isolation or can occur coincidental at the same location in the pipeline; the result of which is more damaging than either of the individual threats themselves. An example of threat interaction is where additional axial bending loads from ground movement are present and the conditions for stress corrosion cracking (SCC) exist resulting in the formation of circumferentially orientated SCC occurring. Another example would be ground movement exacerbating construction-related threats such as vintage girth welds or wrinkle bends. The occurrence of these threats individually may not be significant, however, where multiple threats occur together the resulting condition can be more severe.

This paper looks at how to manage certain interacting threats via the identification of the locations where multiple threats exist and through to the evaluation and prioritization of the interacting threats in terms of severity. Focused examples are given on identifying circumferential cracking threats such as transverse SCC or where imperfect girth welds with reduced load bearing capacity are affected by the presence of axial tensile loads from ground movement. ILI and other supporting information is used to identify the individual threat indicators and the locations where multiple threat indicators interact enabling the threat interactions to be evaluated and the appropriate mitigation identified.

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