The focus of research on the use of composite repair systems to date has been on the repair of damage on straight pipe sections.  However, pipeline systems also comprise other components such as bends and tees and there has been very little work to demonstrate whether or not the more complex loading in these components can be accommodated by the repair systems.  Experience in use of composite repairs on bends to date has primarily been on low pressure systems in chemical plants and refineries where the design concept is markedly different to that utilised for high pressure pipe systems.  This paper reports work completed in the UK by DNV GL and TEAM, on behalf of the main gas pipeline operators (Cadent, SGN, National Grid, Northern Gas Networks and Wales & West Utilities) to determine how to specify composite repairs for pipeline bends in a safe and controlled manner that will ensure a consistent approach and equal level of performance is maintained over the entire system.

Bends were selected that were considered bounding examples of those used within the UK gas transmission and distribution network.  A series of finite element analyses were completed to consider the effects of bend size (diameter and wall thickness), material grade and bend radius and angle.  From this work 90° bends of 1.5 x diameter radius were selected for test, manufactured from 12” diameter, X52 material. 

Rather than just consider restoration of design pressure the main interest was whether or not the repairs would restore fatigue life under cyclic pressurisation to acceptable levels.  Defects were machined into the intrados of the bend of dimensions equal to 0.5 times the diameter axially and 0.25 times the diameter radially.  Wall losses of 20% and 80% were replicated.  This meant that both thick and thin repairs were tested to identify whether the change in stiffness, due to repair thickness, at the repair location would have an influence on the life achieved. 

The pressure cycles were selected to be aggressive with the hoop stress range in the steel pipe being either 125N/mm^2^ or 90N/mm^2^ with the lower stress range loaded for a higher number of cycles.  A successful test was defined as one which exceeded the target number of cycles by a factor of 10 AND for which there was no visual sign of repair degradation.  The results enabled the slope of the S-N curve for the repaired samples to be verified as matching that of an undamaged steel pipeline and confirm that the repair approach would maintain the required margin of safety within the system. 

The work demonstrated a design approach for repairs to be proposed that differs slightly from that in ISO 24817 and ASME PCC-2 Article 401 in that it is based on restoring the full strength of the pipe spool rather than just restoring to a design pressure.  It is shown that this method is necessary and it is proposed that this be adopted by the standards when considering repairs on high pressure gas transmission pipeline systems to ensure equality of performance of the repaired sections with undamaged sections. 


Repair and Reinforcement of Blunt Defects on Pipeline Bends Using Composite Materials

Division / Sponsors: 
Pipeline Systems Division

Paper Proceedings: https://asmedigitalcollection.asme.org/IPC/IPC2020/volume/84447

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