The Influence of Pipe Toughness in the Consequences of Third-Party Damage – A Comparison Between EPRG and Fracture Mechanics Methodologies

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Abstract Onshore and offshore pipelines provide one of the safest modes of transportation for oil and natural gas. Some offshore pipelines pass through areas where large vessels are active and risk third-party damage, particularly if the pipeline is not trenched. Most prominent ship traffic-related incidents are caused by anchoring in an emergency or indiscriminately. These incidents can cause bending, denting and gouging of the pipe wall leading to immediate rupture of the pipeline or fatigue failure over a short period if not remediated. Examples of severe pipeline damage incidents caused by anchor drop/drag damage include 2021 San Pedro Bay oil spill incident in USA [12] and the 2023 Baltic Sea gas pipeline incident in Finland [13]. In December 2022, an anchor was found to have damaged a 16-inch pipeline in the UK North Sea. The damage was not initially apparent, so the pipeline continued to operate for a few months until visual inspection revealed two severe dents. The pipeline was also displaced by 1.36m over a length of 30m. Gouges and cracking were expected but, initially, could not be confirmed. The Pipeline Defect Assessment Manual (PDAM) [6] recommends the use of European Pipeline Research Group (EPRG) methods [3,4] for assessing the tolerance and resistance of pipe to external damage by estimating the failure stress of a dent and gouge. However, the complexity of the interaction between a dent and a gouge, with the potential for cracking in the base of a gouge, limits the accuracy of this generalised method and demands a reasonable level of conservatism. An assessment of the dent using the methodology described in ASME B31.8 Appendix R revealed a high strain level (up to 12.7%) and a risk of material failure. To establish whether a return to service is possible, an Engineering Criticality Assessment (ECA) was carried out based on the procedures given in BS 7910 [5]. The ECA results concluded that the dent is not expected to threaten the pipeline’s integrity during its service life. This paper draws upon a case study to compare the EPRG dent/gouge model and the combination of Finite Element Analysis (FEA) and BS 7910 assessment methodology based on an arbitrary defect equal to a typical gouge and crack at the bottom of the gouge (this is an assumption specified in PDAM, based on the principle that it is hard to rule out the presence of a crack or limited dimensions). Correlations were made between the low material test temperature and the pipeline ambient temperature to obtain a less conservative CVN value for use in the EPRG assessment. However, a more accurate and comprehensive methodology to calculate fracture toughness and assess the fracture resistance as per BS 7910 proved to be a more rational approach for assessing defects in a modern high-toughness line pipe.

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