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Composite Coatings: Basics of Fiber-Reinforced Polymers for Pipe Repairs

From JPCL November 2016

By David A. Hunter, Hunter Engineering Consulting, Inc.
Photo 1 ©iStockphoto.com/kozmoat98
Composite coatings are a class of materials that are described as fiber-reinforced polymers (FRP) and consist of extremely strong tensile fibers saturated with a binding resin. From the original development as tank bottom lining materials designed to handle surface movement and corrosion, the applications for composite coatings have broadened and moved into the mainstream, with industry-accepted design codes written around their uses and applications.
 
 

The Background

Composites, as the name implies, are combinations of two or more materials, which act as one material. Often the combination will take advantage of each material’s individual strengths to provide a functional combination that is greater than each individual material by itself.
 
In a general sense, composite materials have been around for a very long time. One composite with which most people are familiar is reinforced concrete. Concrete is often rated in terms of its compressive strength in pounds-per-square inch (psi); a typical value might be 4,000 psi. Unreinforced concrete is a very durable material that handles stresses extremely well in compression but has very poor tensile strength. Unreinforced concrete has a tensile strength of only about 10 percent of its compressive strength, so if, for example, samples of concrete have a compressive strength of 4,000 psi, the tensile strength would only be about 400 psi. In order to overcome this limitation of the concrete material, engineers place steel “reinforcing” bars into the concrete in the location of tension in the member. The steel can pick up the tensile stresses to overcome the brittleness, thus addressing the concrete’s lack of ability to handle this by itself. When this concept was first introduced, the materials were tested exhaustively to understand how they would behave under different types of loads, weathering conditions, immersion environments, buried environments, salt exposure, chemical exposure, radiation exposure and more, until the behavior was predictable enough that design and construction standards (codes) could be written around the use of these materials for safe and reliable construction. From those standards, text books are written today that are considered the gospel of civil engineering curriculums worldwide. This is a good thing. You might be surprised to know that the first reinforced concrete house was built in France in the 1850s. All those Roman structures you see are simply unreinforced concrete, which is why the Romans used a lot of arches and not long-beam expanses for bridges because arches are in compression. Without the reinforcing steel, it simply cannot be done.
 
The key feature in adopting a construction product as a mainstream building material is the standardization of manufacturing of processes so they can be specified with confidence as design elements. As technology has progressed, new composite materials have been developed which combine properties of several materials in order to achieve a desirable material that solves industry challenges. New is a relative term, in that, in the case of composite coatings or fiber-reinforced polymers (FRP), we are talking about more than 30-year-old technology, based on resin and fiber-reinforcing materials differing in weave density, direction and make up. As with all newly developed materials, it takes time for the manufacturing processes to be honed in order to make materials consistent from batch to batch. Once batch consistency or tolerances are assured, the materials go through extensive testing to determine the dynamics of the matrix.
 
Let’s explore an example near and dear to the author’s heart — an aviation example. The author flies about 250,000 miles per year on all sorts of aircraft. The new Boeing 787 is the first plane to be designed with carbon fiber composite materials, in particular in the wings, which is where the stress is very high and cyclic in nature, which could lead to metal fatigue. I promise you, that since I am personally involved in flying on the aforementioned aircraft, I want to be 100 percent sure that the materials are properly tested and manufactured to a standard that eliminates error or as near as is statistically possible.
 
Figure 1 Fig. 1: View of a pipe section over pressure testing. Photos courtesy of the author unless otherwise noted.
Which is why I’m happy to say that Boeing has literally conducted millions of hours of simulation and testing to have a thorough understanding of the dynamics of their composite formulas.
 
This technology, when properly installed, can achieve the following.
  • Increase the load capacities of bridges, parking decks and other structures.
  • Restore maximum operating pressures of existing pipes and pipelines where wall thickness has been lost due to corrosion.
  • Maintain the operating pressures of existing pipes with corrosion wall losses of up to 80 percent (Fig. 1).
  • Provide extreme abrasion and impact resistance in horizontal directionally drilled (HDD) lines.
  • Provide reinforcement for storage tanks — pitting or general corrosion.
  • Allow low-pressure leak repairs on up to 12-inch-diameter pipes and operating up to 4-bar pressure.
  • Provide protection of pipes at support areas from abrasion caused by thermal expansion, as well as crevice corrosion and galvanic cells by isolating any dissimilar metals, if present.
  • Some interesting features include the following.
  • Installation that can usually be done while in service and in certain situations, even underwater (Figs. 2 and 3). 
  • Applications that can be installed for ISO 12944 (Paints and varnishes — Corrosion protection of steel structures by protective paint systems) service environments of C1 to C5I and C5M, as well as Im1 through Im3 environments for immersion and below-grade.
  • Newer formulations that provide operating service temperatures up to 300 F (150 C).
 

Manufacturing Considerations

Figure 2
Fig. 2: Installation of a composite coating system.
Composite coatings use a liquid-cured polymer (the binding resin), typically a two-part epoxy or moisture-cured polyurethane (MCU), with an extremely strong fiber cloth. The fiber can be unidirectional (allowing tensile stress in one direction), bidirectional (allowing tensile stress in two directions), and can be made of fiberglass, carbon fiber or a combination of both. If a pipe repair requires only reinforcement in the hoop direction, then uniaxial fibers are sufficient. If additional strength is needed for bending, torsion or shear loads, however, then a biaxial cloth should be selected. Biaxial fabric has strength fibers oriented at 90-degree angles to each other.
 
The choice of carbon fiber versus fiberglass fiber is primarily one of strength versus cost. Carbon fiber tends to make a higher strength composite per layer, but costs more. The FRP resin can be either a pre-impregnated (the resins are pre-applied into the fiber in the plant) or a field-wetted system, denoted as wet layup. Pre-impregnated systems consist of a single-component that is cured by environmental exposure, such as moisture, or ultraviolet light, whereas field-wetted systems are typically two-part epoxies.
 
There are advantages and disadvantages to each. For example, the moisture-cured systems offer good strength and are very user-friendly during application, even including application underwater. Epoxy-based systems have higher strength characteristics, but limited working times due to finite pot lives.
It all seems fairly straightforward, but the devil is in the details. Depending on the specific fiber used and the specific formulation of resin, the tensile strength of the material will vary. Any changes in the resin formulation or any change in the fiber used, such as thickness, weave or type, will change the properties of the materials. Most importantly, simply combining the materials together does not yield the full tensile strength properties of the fiber. On average, the tensile strength of the fiber is reduced by nearly 80 percent. In addition, resin-rich combinations and resin-poor (drier) combinations can also change the performance of the materials.
 
Figure 3
Fig. 3: View of the application of an epoxy/carbon fiber system repair of a T-section.
Fiber-reinforced polymers are an interesting class of materials that call for proper specification of the material properties in order to ensure that the mechanical properties of the material meet the required tolerances. This is a process that may not be fully understood, so clearly specifying the testing and acceptance values of a composite coating material is of the utmost importance.
 
ASME International Post-Construction Code, PCC-2, has issued guidance on the use of nonmetallic and bonded repairs to piping systems in its publication, ASME PCC-2-2011, “Repair of Pressure Equipment & Piping.”1 It contains a section detailing material test requirements under Part 4, Section 3.2, Table 1, and additional mandatory testing in Appendix II. Many companies have entered the market using systems which may not meet such a standard, or they may not even be aware that such standards exist. Most importantly, under Part 4, Section 3.6, any change in a repair system, such as resin formula or new or different fiber reinforcement requires requalification testing of the system.
 
Note that these are not the only standards. For instance, ISO/TS  24817:2006 “Petroleum, petrochemical and natural gas industries — Composite repairs for pipework — Qualification and design, installation, testing and inspection”2 might be applicable for a given facility, and for concrete structures, ACI 440.8-13, “Specification for Carbon and Glass Fiber-Reinforced Polymer (FRP) Materials Made by Wet Layup for External Strengthening of Concrete and Masonry Structures.”3
 
 

Fiber-Reinforced Polymer (Nonmetallic Composite) Repairs

Figure 4
Fig. 4: Underwater application of composites. Note the prepackaged, pre-impregnated resin system.
Fiber-reinforced polymers have been in use in a variety of industries for over 30 years, including aviation, oil and gas development and distribution, concrete structural repairs and upgrades and even water and wastewater transmission and collection.
 
FRPs are a useful option for repairing steel pipes damaged by either internal or external corrosion. One of the most significant advantages of these materials is that a section of pipe can be reinforced without shutting down operations, allowing many large industrial facilities to operate for extended periods of time between scheduled maintenance shutdowns. If a pipe is found to have internal or external corrosion, mechanical damage, or requires a higher factor of safety, there is no need to cease operations to fix the problem. Often the repair or reinforcement can be performed quickly with damaged sections being reinforced in a matter of hours or days and minimal disruption to ongoing operations.
 
Although the FRP reinforcement can be installed while a pipe is pressurized, the FRP is most effective if applied to a pipe in an unstressed (depressurized) state. All pipes expand, or strain, when pressurized. By relaxing the load on the pipe the strain is reduced so that when the system is repressurized, the FRP and steel both absorb the pressurization load and the composite immediately goes into load-sharing with the existing steel. As the FRP absorbs load, the amount of load carried by the steel at the repair is reduced.
A design engineer, knowing the strength and material properties of the composite, can use the equations from PCC-2-2011 to calculate the number of layers necessary to meet a specified repair requirement. Interestingly, the repair can be designed so as to make the reinforcement stronger than the pipe itself, in pristine condition. In other words, the repair could be applied to a brand new pipe with a defect that simulates a wall loss of up to 80-percent, the pipe pressurized to failure, and the pipe would fail but not the repair, as in Figure 1. In addition, the code provides for a 20-year design life of the repair, and a 50-year design life can be interpolated.
 

Fiber-Reinforced Polymer Repair Installation

Figure 5
Fig. 5: View of underwater application of composites.
A typical repair consists of proper surface preparation of the pipe or substrate. Surface oils, dirt and grease should be removed using an approved solvent, followed by a high-pressure hydroblast or dry abrasive blasting to prepare the surface to an Sa 2.54 or an SSPC-SP 10/NACE No. 2, “Near White Blast Cleaning”5 finish, and then an epoxy primer should be applied to the metal. Once the FRP fabric is saturated with resin (either factory-saturated polyurethane [MCU] or field-saturated epoxy), the fabric is then wrapped around the damaged pipe area until the desired number of layers is attained (Fig. 2). One of the principal benefits of this application method is that repairs are highly customizable — the fiber can conform to irregular shapes in the pipe, such as bends, tees and elbows (Fig. 3). If more damage is discovered, additional layers can be added. 
 
The wet, uncured laminate is then wrapped with a compression film similar to plastic food wrap. This serves to exert a net compressive force on the laminate so an effective bond will form and eliminate voids or any delamination in the repair. After the repair has cured, the compression film can be removed. If the repair will be exposed to the environment, an ultraviolet (UV) protective outer layer is typically applied as well. This is usually a coating selected from a wide range of commercially available UV coating products. Novalac-based epoxies are good choices for environments with extreme temperature and chemical exposure. Novalac epoxies are highly chemical resistant and widely used as a base resin in many FRP products. They generally have outstanding resistance to sulfuric acids (H2SO4), hydrochloric acids (HCl), acetone, toluene, naphtha and gasoline, which makes them suitable for repairs on pipes that are transporting these chemicals.
 
As previously mentioned, MCU-based resins are ideally suited for undersea applications. The preparation and application are basically the same as for dry environments: the surface of the pipe is prepared using hydroblasting or grit blasting, local damages such as pits or dents are filled with a high-compression-strength fill material, and the pipe is wrapped with the desired number of layers until a specified reinforcement is attained (Figs. 4 and 5).
 
 

Buyer Beware

Particular attention should be paid to evaluation of the testing criteria carried out by the manufacturer. The majority of manufacturers of these types of materials are small companies and with the testing being expensive, instances do exist where manufacturers have provided questionable testing results. A good coatings consultant can weed through the marketing information and recommend a product for the particular application. If you plan to use engineered composite repairs, do not do so without a well-written specification and product research.
 
 

Summary

Composite FRP materials provide facility and process operators with the ability to reduce risk as well as the flexibility to maintain operability using materials with mechanical capabilities greater than the original materials. By being able to schedule maintenance proactively rather than in a reactionary way, the cost of operation in a variety of industries can be reduced.  JPCL
 
 

About the Author

Photo 1
 
David Hunter   David Hunter is the president of Hunter Engineering Consulting, Inc. He graduated from Virginia Tech with a Bachelor of Science degree in civil engineering and has more than 20 years of experience in the corrosion and coatings industry. Hunter is certified as an SSPC Protective Coatings Specialist (PCS), an SSPC Level-2 Coatings Inspector, a NACE Level-3 Coating Inspector, a NACE Offshore Corrosion Assessment Technician (OCAT), and is an instructor for each of these associations.
 
 

References

  1. ASME PCC-2-2011, “Repair of Pressure Equipment and Piping,” The American Society of Mechanical Engineers, 2011.
  2. ISO/TS 24817:2015, “Petroleum, petrochemical and natural gas industries — Composite repairs for pipework — Qualification and design, installation, testing and inspection,” International Organization for Standardization, 2006, 2015.
  3. ACI 440.8-13, “Specification for Carbon and Glass Fiber-Reinforced Polymer (FRP) Materials Made by Wet Layup for External Strengthening of Concrete and Masonry Structures,” 2014.
  4. ISO 8501-1:2007, “Preparation of steel substrates before application of paints and related products — Visual assessment of surface cleanliness — Part 1: Rust grades and preparation grades of uncoated steel substrates and of steel substrates after overall removal of previous coatings,” International Organization for Standardization 2007.
  5. SSPC-SP 10/NACE No. 2, “Near White Blast Cleaning,” (Sa 2.5), SSPC, NACE International.
 

Tagged categories: Coating Materials; Coating types; David Hunter; Features; Fiberglass reinforced plastic; Pipeline; Pipelines; Repair materials

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