An inline inspection of a subsea oil pipeline located in the North Sea identified a large top of line dent.
The pipeline was scheduled for decommissioning within 3 years, however conventional fatigue assessment of the dent indicated that the pipeline could become unsafe before the scheduled decommissioning date.
As the required remaining life could not be justified by conventional assessment, a review was completed to determine whether a case-specific assessment methodology could be developed to reduce the conservatism, while reliably demonstrating that the pipeline could be safely operated up until the planned decommissioning date.
This review identified that the pipeline had a unique pressure history, which included a large one-off pressure spike that had occurred following detection of the dent. This raised the possibility of a pseudo-hydrotest type assessment to justify the required remaining life.
This paper describes the non-conventional pseudo-hydrotest assessment that was undertaken for the dent to demonstrate acceptability, which used a combination of state of the art finite element analysis and fatigue crack growth assessment.