June 2017, Vol. 244, No. 6
Features
A Review of the State-of-the-Art in Composite Repair Technology
Over the past 25 years, the gas and liquid transmission pipeline industry has integrated composite repair systems into their integrity-management programs. In the early stages of this process, the composite repair technologies were rather limited; however, over the past decade there has been a proliferation of composite repair technologies introduced to the market.
Their introduction has been accompanied by extensive research efforts funded by both operators and composite repair companies well in excess of $15 million. This has been an exciting season in the pipeline industry as we have witnessed a partnership between pipeline operators, technology companies, research organizations, and regulators to advance the composite repair state-of-the-art built on rigorous engineering-based assessment techniques involving analysis and testing.
The purpose of this article is to provide an overview of advances in composite repair technology focused primarily on the gas and liquid transmission pipeline industry in the United States. It should be noted that several UK-based organizations have developed and evaluated composite repair systems used mainly in the North Sea; a discussion on these efforts is considered to be outside the scope of this article. Further, composite repair technologies are frequently used to reinforce process piping in refineries and chemical plants; however, for the most part, these installations are employed to reinforce relatively low pressure piping systems.
Worldwide, more than 15 companies manufacture composite repair technologies for the pipeline industry. Furthermore, most of these companies offer multiple repair systems, so that the total number of systems available to operators easily exceeds 50. The benefit for industry is that a wide range of technologies are available to reinforce an array of anomalies, fittings, and features. The challenge for the pipeline industry is to ensure that the composite technologies perform as designed. The good news is that industry has risen to the challenge, as documented in several of the case studies presented in this article. Organizations such as the Pipeline Research Council International, Inc. (PRCI) have been instrumental in this process.
Each composite repair system comprises relatively few components (i.e., the load-transfer material and the composite material itself); however, each component serves a critical role to ensure that the repair system performs properly. Once the pipe has been inspected and the severity of the anomaly has been quantified, the required amount of material must be determined. If the anomaly is corrosion, prescriptive methods are provided in the ASME PCC-2 and ISO 24817 composite repair standards for determining the proper composite thickness and length. Prior to installation of the composite system, the surface of the pipe should be sandblasted to near-white metal (NACE 2) to ensure maximum adhesion of the repair to the external pipe surface.
Once the surface has been repaired, the load-transfer material (i.e., filler material) is used to fill the void in the pipe created by the anomaly, with the goal of restoring the profile of the pipe to its original circular configuration. The importance of this step is easy to appreciate when considering the surface irregularities caused by corrosion and dent-like features.
The next step is installation of the composite material. Installation crews must be careful to follow all of the manufacturer’s instructions and pay careful attention to install the appropriate number of wraps. For most repairs, there are a limited number of steps where things could go wrong; not installing enough composite material would certainly be on the list.
The final installation step prescribed by most manufacturers is sealing the ends of the repair. This is typically done with additional resin, although it is sometimes accomplished using filler material. Before the exposed ditch is backfilled, operators should verify that the materials are sufficiently cured. The author is aware of several incidents when uncured resins existed in composite repairs. While this has been a rare occurrence, the prudent operator will ensure it never occurs.
Case Studies
The sections that follow provide several situations studies illustrating how composite materials can be used to reinforce several different types of anomalies in transmission pipeline systems. As will be noted, these case studies integrate extensive testing to validate the use of the respective technologies.
Corrosion Reinforcement Subjected to Cyclic Pressures
In the early history of the composite repair industry, the primary anomaly of interest was corrosion. This remains true as even today the vast majority of repairs still involve the reinforcement of corrosion. What was neglected in the early efforts was the effect of cyclic pressure in quantifying the long-term performance of the repairs.
In response to this shortcoming, an extensive testing program was conducted to quantify the number of cycles to failure for different composite technologies used to reinforce 75% deep corrosion in 12.75-inch x 0.375-inch, Grade X42 pipe (cf. Figure 1).
Figure 1: Schematic of corrosion sample configuration.
The repaired samples were cycled from 890 to 1,780 psi (36% to 72% SMYS) until failure occurred (leakage). Listed below are cyclic testing results for eight systems:
- E-glass system: 19,411 cycles to failure
- E-glass system: 32,848 cycles to failure
- E-glass system: 129,406 cycles to failure
- E-glass system: 140,164 cycles to failure
- E-glass system: 165,127 cycles to failure
- Carbon system: 256,344 cycles to failure
- E-glass system: 259,537 cycles to failure
- Hybrid steel/E-glass system: 767,816 cycles to failure
The number of cycles to failure ranges from approximately 20,000 to 770,000 cycles, a significant variation considering that all of the above systems are composite technologies currently installed on transmission pipelines. To establish a design life, the number of cycles to failure is divided by a fatigue safety factor, typically between 5 and 10. Hence, if a fatigue safety factor of 10 is used on the experimental data, the range of design life is between 2,000 cycles and 77,000 cycles considering a stress range of 36% SMYS.
The usefulness of generating empirically based design lives is that operators can establish the service life for repaired corrosion features using actual pressure history data. Historically, industry has established composite repair design lives ranging from 20-50 years based on long-term creep rupture data. While this was important in the early days of establishing composite repair credibility, a comprehensive assessment process for establishing service life should integrate actual history data. The same approach can be used for evaluating the reinforcement of other anomalies and features such as dents, wrinkle bends, and planar defects.
Plain Dent Reinforcement
While the early research work associated with composite repair technology focused on the reinforcement of corrosion, it was not long before the industry explored the reinforcement of dents using composite materials. Stress Engineering Services, Inc. started evaluating this subject in 1994, which interestingly enough was the same year the U.S. Department of Transportation Office of Pipeline Safety (now PHMSA) granted the waiver for the use of the Clock Spring® system in reinforcing transmission pipelines. By 2005 (10 years later), numerous composite systems had demonstrated the capability to reinforce dents so that PRCI initiated the MATR-3-5 program to further explore this subject in 2009.
Figure 2 is a schematic of the test sample configuration used to evaluate each technology in that program. The samples incorporated 12.75-inch x 0.188-inch, Grade X42 pipe material. After denting and installation of the composite materials, the samples were pressure cycled from 100 to 890 psi (8% to 72% SMYS) for 250,000 cycles (runout) or failure. If a failure occurred in a given repair, that section of the sample was cut out and the remaining sections were welded together for continued testing. Including the reinforced and unreinforced samples, a total of 62 dent samples were tested (Product J was a retest and only included plain dent reinforcement).
Figure 2: Schematic of dent sample configuration.
Figure 3 plots the number of cycles to failure for all of the tested samples. As shown, the average cycles to failure for the unreinforced samples was less than 10,000 cycles, while most of the reinforced systems reached the 250,000-cycle runout condition.
Figure 3: Pressure cycle fatigue results for dent study.
Figure 4 plots stress concentration factors (SCFs) computed for each tested dent based on strain gage measurements, where the SCF is the ratio of stress measured in the dent to stress measured in the base pipe. The average SCF for the unreinforced dents was on the order of 4.0, while the SCF for reinforced dents that achieved the 250,000-cycle runout condition was 1.0. The use of SCFs is beneficial for operators who are evaluating the use of composite materials for reinforcing pipelines and seeking to quantify design life based on the relative severity of a given dent; this approach is especially useful for operators of liquid lines.
Figure 4: Dent study stress concentration factors.
For the most part, the greater concern in the reinforcement of anomalies is not static (burst) pressures; rather, the greater reinforcement challenge posed to composite repair technologies is cyclic pressure. This research demonstrates that properly designed and installed composite systems can reinforce dents, including those interacting with girth and seam welds. Operators should verify that no crack-like features are present in the dented regions.
Conclusions
This article has provided a basic overview of today’s composite technology used to repair and reinforce high-pressure transmission pipelines. In addition to the reinforcement of corrosion and dents, numerous other pipeline anomalies and features have been reinforced with composites including vintage girth welds, wrinkle bends, branch connections, field bends and elbows, and planar defects in low-frequency ERW pipe. To support these efforts, numerous research programs have been conducted to validate the use of engineering-based composite repair technologies.
In looking toward the future, there is no doubt that composite repair technologies will play an increasingly important role in the future of integrity management programs of pipeline operators around the world.
Author: Chris Alexander, Ph.D is a Principal at Stress Engineering Services Inc. in Houston where he leads the Midstream Pipeline Practice. For the past 24 years he has worked with technology companies worldwide as well as all major U.S. oil and gas pipeline operators. He has used advanced engineering methods to solve challenging problems, including the assessment oswf damaged pipelines and evaluating innovative reinforcing technologies. He has assembled over 15 Joint Industry Programs that have played an important role in providing technology solutions for operators. He has published over 120 technical papers and articles, is author of the business parable Be the Beans, and has spoken at conferences around the world on a wide range of subjects. He has B.S., M.S., and Ph.D. degrees in Mechanical Engineering from Texas A&M University
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