Previous research on the use of composite repair systems has focused on the repair of damage to straight sections. This paper reports verification testing completed on composite repairs for pipeline bends to support development of a safe and consistent approach that ensures an equal level of performance is maintained over the entire system.
A series of finite element analyses were completed that considered the effects of bend size (diameter and wall thickness), material grade, bend radius and angle. From this work 90° , 1.5D, 12” NB, X52 bends were selected for test. The objective was to demonstrate restoration of the fatigue life of the pipe to acceptable levels rather than just considering burst pressure. Defects equal to 0.5 times the diameter axially and 0.25 times the diameter radially were machined into the intrados of the bend (the location of highest hoop stress). Wall losses of 20% and 80% were replicated. Bounding repair thicknesses were tested to confirm performance was predictable.
Aggressive cyclic pressure ranges of 125Nmm−2 and 90mm−2 were applied to the repaired defects, with the lower stress range loaded for a larger 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 number of load cycles was selected to ensure the repaired lines satisfied the slope of the S-N curve for pristine pipe, confirming the approach maintained the required margin of safety applicable to the rest of the system.
A design method is proposed that addresses weaknesses in the methods given in ISO 24817 and ASME PCC-2 Article 401. It is proposed that this be adopted by the standards for repairs to high pressure gas transmission pipelines to ensure equality of performance with undamaged sections.