Use of Carbon Composite Repair Technologies to Reinforce Crack-Like Flaws in High Pressure Pipelines

Abstract

Pipe body cracking and seam weld crack-like flaws can be encountered on virtually all pipeline systems and are traditionally repaired via a cutout, grinding, or full-encirclement metallic sleeves. Although the effectiveness of steel repair sleeves is well established, they have drawbacks when repairing out of roundness pipe and they can require large pipe excavations to find a suitable location for in-service welding. Moreover, finding a clean in-service welding landing zone can be difficult for vintage line pipe steel which have a higher probability of laminations and the potential of having seam weld defects in low frequency ERW/EFW pipe. Furthermore, safety is also a concern when welding onto an operational pipeline. An alternative to repairing planar flaws is the use of non-weldable, light weight, wet-lay carbon fiber composite system. A composite repair system has distinct advantages as it can contour to most pipe shapes, resulting in shorter repair lengths (and therefore shorter excavations), and no welding is required. Composite repair systems have historically been used to reinforce areas of external corrosion and dents; however, their use has not been fully extended to reinforce crack-like flaws. A study was conducted where three carbon-epoxy composite technologies were explored as an effective repair option for pipelines with crack-like defects. A total of 35 reinforced external axial cracks were tested in base material and the longitudinal weld in 1960’s ERW pipe samples. Each crack was initiated by fatigue from a machined notch. Pressure cycle testing from 10%–72% SMYS showed that the carbon composite repair systems tested can effectively repair axial external crack-like defects in the pipe base and seam weld. The repaired cracks in the ERW bond line ranged from 45% to 55% wall thickness deep and all survived 25,000 test cycles with less than 10% wall thickness of additional growth. The aggressive test pressure cycles correlate conservatively to 600 to 1,000 years of fatigue life for normal gas pipeline operations, which demonstrated a fatigue life extension of 3 to 5 times when compared to unreinforced defects that leaked below 10,000 cycles. The contents of this paper and associated insights are valuable to the pipeline industry in extending the use of carbon composite repair technologies to reinforcing cracks and seam weld crack-like flaws based on full-scale testing and metallurgical assessment of post-tested fatigue growth of cracks.