Session: 03-03-10 Feature Assessment Case Studies - Material Properties II

Paper Number: 133950

133950 - The Influence of Pipe Toughness in the Consequences of Third-Party Damage – A Comparison Between EPRG and Fracture Mechanics Methodologies 

Abstract: 

Onshore and offshore pipelines provide one of the safest modes of transportation for oil and natural gas. Offshore pipelines pass through shipping channels, those are at risk of third-party interaction particular if the pipeline are untrenched. Most prominent ship traffic related incidents are caused by anchoring in an emergency or accidental drop of the anchor and uncontrolled dragging. These incidents can cause denting and gouging of the pipe wall leading to immediate rupture of the pipeline or fatigue failure over a short period of time if not remediated. Some examples of severe pipeline anchor incidents include the recent Baltic Sea gas pipeline incident in Finland.

In December 2022 an anchor was found to have interacted with a 16-inch pipeline in the UK North Sea. The damage was not initially obvious, so the pipeline continued to operate for few months until visual inspection revealed two severe dents with peak depth of 27% OD. The pipeline was also displaced by 1.36m over a length of 30m. Gouges and cracking were expected but were not reported. The Pipeline Defect Assessment Manual (PDAM) recommends the use of European Pipeline Research Group (EPRG) methods for assessing the tolerance and resistance of pipe to external damage to calculate a best estimate of the failure stress of a dent and gouge. However, the complexity of the interaction between dent and gouge damage, with the potential for cracking in the base of a gouge limits the accuracy of this method. High levels of strains were calculated using ASME B31.8 Appendix R and Finite Element Analysis (FEA). Loss of containment was expected, but it did not occur. 

An Engineering Criticality Assessment (ECA) was carried out based on the procedures given in BS 7910 to understand how this pipeline has survived such severe interaction and whether if return to service is possible. ECA is established based upon the principles of fracture mechanics to determine the acceptability of a known flaw under specified loading conditions, in terms of its fracture resistance and fatigue life. A range of assumed crack dimensions were evaluated at the locations where the highest tensile stresses from indentation and rerounding observed from using FEA modelling. Refinement to the fracture and fatigue assessment inputs were found to have significant impact on the results. Particularly, the Charpy V impact value where initially minimum specified values were used. Significant gain in fatigue life was observed when Charpy V impact values obtained from the materials test certificate. To further enhance the assessment, correlations were made between the low material test temperature and the pipeline ambient seabed temperature. The results of this parametric study explain somehow why loss of containment did not occur but also suggest conservatism in the EPRG dent/gouge assessment method for high toughness pipe material.

This paper draws upon this case study to discuss a comparison of the EPRG dent/gouge model and BS 7910 assessment methodology and consideration when carrying out an assessment of a high material toughness pipeline to ensure the appropriate level of conservatism is applied. 

Presenting Author: Nigel Curson Penspen

Presenting Author Biography: Executive Vice President, Technical Excellence

Authors:

Vipin Anilkumar Suvarnalatha Penspen
Nigel Curson Penspen
Thomas Mcintyre Penspen
Ali Alani Penspen