The UKOPA Dent Management Strategy
Lockey A., Palmer-Jones R., Pailor G., Goodfellow G.
Ageing Pipelines Conference; Ostend; October 2015.
Most thin-walled onshore pipelines around the world are found to contain dents. Many of these dents have depths less than 2% of the pipe diameter, and have only recently begun to be reported reliably by high resolution in-line geometry inspections. In older pipelines, dents are often found on welds of unknown toughness, and can be subject to severe pressure cycling.
Much of the existing guidance for dent management was published before dents with depth less than 2% of the pipe diameter were being reported, and did not necessarily consider them. Dents on welds are not generally allowed without detailed knowledge of weld quality, which is often not available. The existing guidance can be over-conservative, leading to unnecessary repairs. Furthermore, recent failures in Canada have demonstrated that the existing guidance can be non-conservative when a shallow dent is combined with fatigue loading or other undetected damage.
The United Kingdom Onshore Pipeline Operators Association (UKOPA) has developed a strategy to provide guidance to operators on the management of dents. The strategy allows dents to be screened and assessed based on in-line inspection data. It does not require numerous feature investigations, or detailed knowledge of weld and material properties. Weld quality is assessed using a set of indicative criteria using information which is more likely to be available to operators of older pipelines than toughness measurements. Fatigue analysis is carried out using a combined finite element analysis and SN curve methodology, which has been validated against the results of 31 full scale tests carried out for PRCI.
This paper describes the complete UKOPA dent management strategy in detail, presents the validation against full scale tests, and demonstrates the improvement in accuracy compared with existing assessment guidance.