Case Study: FRP Wrap Evaluation
Inspection of FRP repair wrap on steel piping
A major petrochemical facility located in the Persian Gulf region of Saudi Arabia used FRP overwrap to repair steel pipes that were showing signs of corrosion
Industry: Chemical Processing
Around the world, fiber reinforced polymer (FRP) wrapping is increasingly used to repair pipes and pipeline systems in a wide range of settings, from oil refineries and pipelines to municipal water and sewage works to district heating infrastructure. In the oil industry, steel pipelines are the safest, most efficient way to transport oil, natural gas and other petroleum products. However, steel is prone to corrosion in harsh environments including desert areas like the Persian Gulf, where the combination of extreme heat, humidity fluctuations, sand and salt as well as wet hydrogen sulfide (H2S) in the refinery process stream can cause cracking and leaks in steel pipes.
FRP wrap provides quick, effective repair
FRP wrap provides a fast, inexpensive and safe alternative to traditional repair methods that include removing and replacing damaged sections of pipe, encasing it with a steel sleeve, or welding a patch on to the pipe. The process of dismantling damaged sections of pipe can cause even more damage and typically requires a partial or temporary shutdown of operations.
FRP is less affected by corrosive environments and provides longer lifespan with less maintenance than other materials. Hand-wrapping layers of FRP over the damaged area allows for quick, effective temporary repairs of steel pipes that save time and money with minimal downtime, while allowing infrastructure managers to delay major repairs until their next scheduled maintenance period. However, because the work is done by hand, the quality of repairs using this method is variable and highly dependent on individual workmanship. Therefore, regular inspections are essential. Done properly, FRP wrap provides additional structural reinforcement and leak protection that will likely outlast the pipe itself.
A better way to evaluate and monitor FRP overwrap
UTComp’s ultrasound-based methods for evaluating and monitoring FRP overwraps are scientifically proven in the lab and in the field.
In general, companies that use FRP reinforcement to extend the life of their equipment want to know two things:
- Is the overwrap in good condition?
- Is the FRP properly bonded / attached to the steel?
While it’s impossible to predict the remaining lifespan of the overwrap until the material encased by the FRP begins to fail, UltraAnalytix makes it is easy to quickly and accurately measure the thickness of the FRP wrap and determine the quality of the bond.
In laboratory tests where clients supplied FRP-wrapped samples with built-in defects, UltraAnalytix precisely detected all the flaws. In real-world conditions, UltraAnalytix has been used to assess the quality of FRP overwraps on a variety of materials, usually generating accurate results in one minute. UltraAnalytix is also suitable for use with structural steel and concrete that has been reinforced with FRP.
UTComp successfully uses the same approach in work with a major chemical company in The Netherlands. For seven years, UTComp has been monitoring a steel pressure vessel wrapped in FRP. Without the overwrap, the vessel would need to be replaced; however, regular monitoring using UltraAnalytix has extended the life of the equipment indefinitely.
A key factor to consider when making FRP repairs is protecting the surface of the FRP overwrap itself. Rough surfaces make it difficult to send a clear ultrasound signal through the material and back to the transducer. To avoid this problem, UTComp recommends applying a coat of resin to the outer surface of the FRP wrap, which will protect the material and make future ultrasound monitoring easier.
Contact factor, tap testing and ultrasound
Layers of overlapping FRP provide the repaired pipe with structural reinforcement for added strength, while contact factor (CF) represents the quality of the bond between the FRP and the steel pipe as well as between layers of FRP itself. FRP thickness and CF both play a role in determining the quality of the repair, but CF is the key to leak prevention. A number of factors can cause defects that lower CF including:
- uneven application of resin
- contamination by foreign particles
- gaps or air pockets between layers of FRP
- improper curing due to temperature fluctuations and other variables that can weaken the bond.
Repairs in this case study align with the ASME PCC-2 standards set out by the American Society of Mechanical Engineers for the repair of pressure equipment and piping. However, the ASME standard does not cover the use of ultrasound to determine CF; instead, it calls for tap testing to identify areas where the FRP isn’t well bonded to the steel pipe.
The tap test, or acoustic impact test, is a conventional non-destructive option for qualitative evaluation of FRP. It involves using a small hammer or steel bar to tap the FRP surface to detect air voids. Perfectly bonded FRP generates a different sound than a delaminated FRP wrap where the layers are separating or misaligned. This low-tech method is useful but also highly subjective as it relies on the discerning ear of the person doing the test.
UTComp’s ultrasound technology, on the other hand, evaluates FRP thickness and CF with scientific precision. UltraAnalytix analyzes ultrasonic data with a proprietary algorithm that provides clients with fast, reliable, quantitative data that can be tracked over time to help make the right choices about the quality, performance and lifespan of composite assets.
In September 2020, Petromaster technicians conducted visual inspections and collected ultrasonic data to evaluate recent FRP repairs to two pieces of equipment: a steel kerosene line and a steel gasoline line.
During visual inspection of the gasoline line, technicians observed minor surface cracking patterns on the FRP wrap, as well as some tearing and separation in the layers of laminate. There were no abnormalities observed during visual inspection of the kerosene line.
Because the steel pipes were not perforated and have not failed to date, data analysis didn’t follow standard FRP mechanical integrity reporting with Percent of Design Stiffness (PDS) and remaining service life. Instead, the team focused on the unique factors associated with bonding FRP to steel: FRP thickness and contact factor (CF). The FRP wrap itself will not experience any strain from the piping or its contents unless the steel pipe begins to fail; at that point, the repair will allow the pipe to continue operation until the end life of the FRP wrap. Subsequent UltraAnalytix inspections will be used to estimate the remaining service life of the FRP.
For the purposes of brevity, this case study reports on data collection and analysis for the 20-inch kerosene line.
Inspection of a 20-inch NP-19 kerosene line
The steel 20-inch kerosine pipe had been repaired in 2019 with FRP wrap measuring approximately 1 meter (39 inches) in length that was wrapped by hand over the damaged area and bonded to the steel pipe surface.
No abnormalities were observed during the visual inspection.
After visual inspection of the finished repair, UltraAnalytix data was collected from a total of 550 readings in a 19×29 grid pattern. The inspection plan is illustrated in Table 1 below, provided by Petromaster. Each cell corresponds to one ultrasonic reading.
Key UltraAnalytix findings:
- Average Thickness: 11.6 mm
- CF Factor: 0.75
These readings were then sent to UTComp for data analysis. Thickness and contact factor for each reading was calculated from the data provided.
Thickness was calculated using sonic velocity statistics and transit time. The sonic velocity of FRP composites is dictated primarily by the polymer used and is affected by characteristics such as state of curing, crosslinking, void volume along the signal path and impurities or discontinuities. It is common for the sonic velocity of FRP to vary for adjacent readings and along the signal path of a single reading. These variations make using a calibration standard unreliable for conventional ultrasonic velocity determination. This report uses a model based on the statistical distribution of empirical data to determine sonic velocity and thickness. Figure 1 below illustrates how thickness is determined from ultrasonic readings.
Table 2 summarizes thickness results for the kerosene pipe. The confidence interval used is 90%.
Contact Factor (CF) is calculated by evaluating the reflected ultrasonic signal. In the calculation, it was assumed that the FRP overwrap would meet average workmanship and formation standards. The calculation includes two variables: ultrasonic transmission through the FRP overwrap, and reflection from the interface between the FRP and the surface of the pipe.
CF is expected to range from approximately -0.5 to 1.5, with 1.5 representing ideal FRP formation and bonding to 100% of the steel substrate. CF does not relate significantly to the tensile strength of the overwrap.
- CF values less than 0.6 represent a combination of poor FRP formation and incomplete bonding to the substrate
- CF values less than 0.5 are not acceptable
It’s important to note that the correlation between FRP thickness and CF is low. Areas of lower thickness indicates fewer layers of FRP. Thickness of the FRP varies due to the hand layup method which involves overlapping layers of glass roving or mat by 75 mm or more to ensure all inner edges are covered and saturated with resin. Thickness provides tensile strength to the FRP, but greater thickness does not correlate with greater leak protection. The quality, integrity and durability of the bond between the initial layer of FRP and the steel is the key to ensuring leak prevention. This highlights the importance of properly preparing the steel surface and following correct lamination techniques at the beginning stage of the repair.
Locations where the CF is less than 0.5 — and especially where the CF is less than 0.3 — may indicate a substantial risk that the FRP is not well bonded at those locations. However, if the low CF locations are surrounded by areas with CF > 0.6, then potential leaks will be contained. Few readings with CF below 0.5 were detected.
Conclusion and Recommendations
After analyzing the data, the engineering team concluded that the FRP repairs are suitable for continued service until the next recommended UltraAnalytix inspection in 2023.
Evaluation with UltraAnalytix is recommended after all significant process malfunctions or errors or spills and leaks into the environment. UTComp recommends that a competent FRP engineer be engaged for all recommended engineering activities including replacement, review, evaluation, repair design and repair inspection.
The surface of the FRP cladding should be painted with polyurethane paint for UV protection to extend the life of the FRP repair and prevent degradation of the resin over time.