Assessment of pipeline damaged by isolation plug

Jee was contacted by a client who suspected there may be an outwards dent present in one of their major pipelines because of a stuck isolation plug.

Our engineers were asked to define acceptance criteria for outwards dent defects, both on and away from girth welds. This problem is unusual as industry code guidance for assessing dents is mainly focused on the assessment of inward dents.

Our team has significant experience in performing defect assessments and adopted a two-strand approach to this interesting problem:

1. A finite element (FE) based approach was adopted, with the pipeline modelled, axial and hoop stresses applied, and the denting process simulated to understand the stress distributions around the location of the dent under varying pressures and dent sizes.
2. Dent assessment calculations were performed using the dent assessment methodology defined in API 579 for a range of pressures and dent sizes
3. The results of the FE analysis and the API 579 dent assessment calculations were brought together to develop a dent acceptance criteria.

The acceptance criteria that were developed included charts where dent size versus pipeline pressure could be plotted. The charts were divided up into three regions - red, amber, & green - to allow a quick check to be performed on the acceptability of various dent size and pressure combinations:

• Green region – dent and pressure combination acceptable - no detail engineering assessment required
• Orange region – dent and pressure combination marginal - detail engineering assessment and / or further work required
• Red region - dent and pressure combination not acceptable - proceed to pipe repair

Following an inline inspection run to accurately measure the dent size, Jee’s acceptance criteria was used to demonstrate that the dent present fell within the green region of the acceptance criteria. Therefore, the pipeline could be deemed fit for service, without any further inspection, analysis, or repair work being required. Furthermore, the client now had the added benefit of an acceptance criteria that could be readily used in the event of further outward dents being identified in the future.

For more information, visit www.jee.co.uk/integrity-management

To contact our Head of Integrity Management, Graham Wilson, email graham.wilson@jee.co.uk, or call +44 (0)1732 371 371.