|Fracking (hydraulic fracturing) has marked a new era of natural gas exploration. Through this technique, huge deposits of oil shale like the Marcellus and Utica, which extend from the Appalachians into Canada, are now producing enough gas to meet North America’s needs for the next 14 years. This boom in gas exploration has opened up new markets for pipeline coatings and joint coating materials as estimates of pipeline construction in Pennsylvania alone are in the range of 12,000 to 27,000 miles by the year 2030.This article will describe the coating materials, surface preparation requirements and application methods used to protect gas pipelines. It will also provide valuable insight into the types of polymer-based coatings that are both cost-effective and have a high level of acceptance in the gas pipeline industry.
Corrosion is the greatest danger to buried steel pipeline, and uncontrolled corrosion of the pipe wall leads to leaks, service interruptions and even explosions. Interstate pipelines are regulated by state or Federal agencies, primarily the Departments of Transportation (DOTs). At a minimum, these agencies mandate corrosion protection through the application of protective coatings and the installation of supplemental cathodic protection. However, intrastate pipelines are not regulated by the DOTs, and the condition of the coated pipe as it is buried in a trench must be carefully considered
1930 to 1950
The first steel pipelines were in the ground in the late 1800s, and owners realized then that burying steel pipe without additional corrosion protection was not an acceptable long-term strategy because corrosion quickly caused pipeline leaks. From 1930 to 1950, industrialization in the Midwest and Northeast increased demand for energy, and oil and gas pipelines that originated in the Texas oil fields and refineries fulfilled a large portion of this demand. Their reliability depended on the effectiveness of corrosion control measures. As the pipeline industry matured, so did the technology of protective coatings, and one of the early innovations was the use of a built-up system where the hot products (asphalt or coal tar) were reinforced by application of a tar-saturated felt mat that was worked into the hot matrix. The system was completed with a spiral wrap of kraft paper. Coating application typically took place over the trench. For many years, the built-up coal tar and asphalt coatings were the predominant systems used on buried pipelines. Another popular system was based on using petroleum-based wax coating reinforced with fiber mesh, and these two systems accounted for nearly all of the protective coatings applied to buried pipe from 1930 through 1950.
The first large-scale pipeline project in the U.S. was the “Big Inch” (24-inch) crude oil and the “Little Big Inch” (20-inch) petroleum products pipelines project, constructed from August 1942 through August 1944. The federal government funded the project through a quasi-public company called “War Emergency Pipelines, Inc.” At the time, the “Big Inch” was the longest and most expensive pipeline in the world. The impetus for the project was the German submarine menace that resulted in the sinking of hundreds of tankers as they attempted to transport crude oil and gasoline over a distance of 1,475 miles, from Baytown and Beaumont, Texas to Phoenixville, Pa. and Linden, N.J. The pipeline steel was seamless with a pipe-wall thickness of 3/8-inch. It was machine-wrapped over-the-ditch with a three-part coating system consisting of a hot-applied coal tar primer, fiber-reinforced coal tar tape and a reinforced fiber outer wrap.
1950 to 1970
By the mid-1950s, new hydrocarbon polymers were introduced into pipeline coating products to improve performance in the underground environment. Epoxy resins were formulated using coal tar-based pigments to make a liquid-applied pipeline coating. Coal tar epoxy exhibited superior resistance to penetration by water. Over the same period, application methods also changed. Systems like the multiple-layered coal tar tape, applied hot and over-the-trench, gave way to cold-applied, prefabricated tapes which used adhesives to bond to the pipe surface (Fig. 1). These tapes used advanced polymers like vinyl and polyethylene, with butyl rubber adhesives. They were flexible, tough and highly resistant to water penetration. The 1960s also saw the introduction of fusion-bonded epoxy (FBE), which would eventually replace many of the earlier pipeline coating systems.
The Evolution of Pipeline Coating Materials
Today, pipeline coatings have evolved from basic, single-material products like coal tar or asphalt. They now consist of mill-applied products that use epoxy primers in conjunction with overwraps of various layered polymers. When used in combination, these composite coatings have better physical durability, thermal resistance and insulation properties than the earlier coating materials. Today’s pipeline coatings utilize a variety of materials that are field-applied, liquid materials or systems pre-fabricated at a coating mill. Figure 2 displays the most commonly used pipeline coating materials.
Field-Applied Pipeline Coatings
Liquid epoxy is a copolymer that is formed by the chemical reaction of an epoxy resin (part A) and a hardener or catalyst (part B). When the two are mixed together, a chemical reaction converts them to a hard coating. After mixing, the epoxy can be applied by brush, roller or spray.
Liquid epoxy is primarily used as a field- applied coating to cover girth welds, fittings and valves, as well as to perform field rehabilitation of short sections of pipe. It is also used as additional protection at the soil-to-air interface where buried piping transitions to atmospheric exposure (Fig. 3).
The applied thickness of liquid epoxy coatings will depend upon the solids content and the number of coats applied to the steel. Epoxy applications for pipeline work are in the range of 20-to-35 mils. Once full cure is achieved, the epoxy product becomes a thermoset coating, meaning it is insoluble in solvents and will not soften when heated. However, when heated above 250 F, the coating will decompose (Table 1).
Coal Tar Epoxy
Coal tar epoxy (CTE) is a variation of liquid epoxy where some of the mineral fillers have been replaced with semi-liquid coal tar pitch. CTE also cures through a reaction of resin and hardener (parts A and B) to form a thermoset coating that is typical of all chemically-cured epoxy materials.
As with liquid epoxy coatings, the applied thickness of coal tar epoxy coatings will depend upon the solids content and the number of coats applied to the steel. CTE applications for pipeline work are typically in the thickness range of 15-to-35 mils (Table 2). Coal tar epoxy has a relatively slow cure time. At ambient conditions (for example, 75 F), it will require five to seven days to fully cure. In some cases, force curing at 150 F can reduce the cure time to 8 hours.
Liquid polyurethane is a hydrocarbon polymer formed by a chemical reaction between a polyol (resin) and a hardener (catalyst). Polyurethane is a chemically cured polymer. It is fast setting and the types used as pipeline coatings are typically applied using plural-component spray equipment.
Polyurethane has a temperature resistance of 235 F. It has excellent flexibility and good resistance to abrasion, impact and mechanical damage. It exhibits superior resistance to water penetration and most hydrocarbon solvents. It is a 100-percent- solids material that can be rapidly applied to reach film builds in the 15-to-50-mil range (Table 3). Because of its fast-setting properties, polyurethane can be field-applied for both the rehabilitation of pipeline sections and as a coating for girth welds. The 100-percent-solids polyurethane used for buried piping is a different type of polyurethane than is used as a finish coat in non-pipeline, atmospheric service.
The minimum level of surface preparation for most liquid epoxy products cannot be achieved using hand or power tools. The pipe surface must be cleaned with air-driven abrasive to achieve SSPC-SP 10/NACE No. 2, “Near White Blast Cleaning” removal of all adherent mill scale and rust with only a maximum of 5 percent of staining remaining. Failure to reach this end condition may result in poor adhesion of the epoxy to the steel.
Coal tar epoxy also requires surface preparation with air-driven abrasive. Depending upon the specification or product requirements, the pipe surface must also be cleaned to SSPC-SP 10/NACE No. 2 or SSPC-SP 6/NACE No. 3, “Commercial Blast Cleaning” where all adherent mill scale and rust are removed with staining on no more than 33 percent of the surface area.
When coating work is performed in the field on an operating pipeline, the temperature of the gas and carrier pipe may be below the air temperature. These conditions may lead to the formation of condensation on the pipe surface. There are liquid epoxies available that can tolerate application to damp surfaces and actually displace a water layer as they are rolled onto the pipe surface. Despite the availability of these products, it is more advantageous to coat dry pipe.
Heat may be used to force cure a liquid epoxy in order to increase production rates on over-the-trench work. Induction heating can be used to heat the cleaned steel prior to coating application and to accelerate curing afterwards. Safety precautions are required any time heat is used in combination with coatings.
Mill-Applied Pipeline Coatings
Almost all of the new pipeline construction projects use pipeline sections that have been coated in a mill and shipped to the construction site. The reasons for choosing mill application over field application are production efficiency, a controlled application environment, better access for application and quality control, and the ability to apply complex coating systems that cannot be applied in the field.
Coating systems applied at the mill include multi-layer polyethylene, multi-layer polypropylene, single and multi-layer FBE, liquid epoxy, polyurethane, and coating systems encased in a weight coat of concrete.
These materials may be selected to maximize properties such as adhesion to the steel, mechanical resistance and resistance to water penetration.
Methods to apply pipeline coatings in a mill are highly specialized processes that include application by crosshead extrusion, side extrusion, electrostatic spray and plural-component spray.
Crosshead extrusion is a process whereby a polymer is squeezed onto the steel pipe as it is passed through a metal die. Both the die clearance and speed of the pipe through the die determine the thickness of the coating layer. Becausee many molten polymers do not adhere well to bare steel, the coating is extruded over an elastomeric adhesive to achieve bonding. Coatings applied by crosshead extrusion are a continuous layer and have no seams. This application process is used for pipe in the diameter range of 2-to-20 inches.
Side extrusion is for coating pipe in the range of 4-to-145 inches in diameter. In this process, cleaned and adhesive-primed pipe travels spirally through the extruder where several layers of molten polymer are applied in flat sheet form and squeezed against the pipe by a silicone rubber roller. The roller improves interlayer adhesion and eliminates air entrapment between the layers. As the spiral seams are fused together, side extrusion provides a uniform coating with virtually no distinction or separation between layers.
FBE is typically applied at a pipe mill. The coating process involves cleaning the steel pipe surface with abrasive grit in a centrifugal blasting cabinet to a cleanliness level of SSPC-SP 10/NACE No. 2. Induction or oven heating is used to heat the cleaned pipe to the 356-to-482 F range before it is sent through a fluidized bed of suspended epoxy particles. The particles melt and fuse on contact with the heated pipe. Curing (cross-linking) of the epoxy occurs within several minutes and is followed by a water quench. The FBE coating thickness is controlled by the speed of the pipe’s movement through the fluidized powder-coating bed.
FBE can be mill-applied as a one- or two-layer pipeline coating. Single-layer FBE is applied in the range of 12-to-16 mils. A dual-layer process can be used where two consecutive FBE layers are applied to the heated pipe. The inner layer, at 12-to-16 mils dry film thickness (DFT) provides corrosion protection to the steel pipe. While it is still soft, a second or outer layer at 30-to-36 mils DFT is applied as an additional barrier coat and provides abrasion resistance (Table 4).
Polyethylene Pipeline Coatings
Polyethylene (PE) is the starting point for many pipeline coatings. PE is a thermoplastic polymer. Unlike the thermoset epoxy coatings previously described, a polyethylene will soften with heat and has a melting point in the range of 221-to-266 F. High-density polyethylene (HDPE) is typically used for extruded pipeline coatings.
HDPE is a mill-applied coating whereby the molten polymer is extruded onto the steel pipe as it is passed through a metal die. Manufactured as a two-layer product, it consists of 20-to-30 mils of polyethylene extruded over 10 mils of rubberized asphalt adhesive.
The manufacture of high-density polypropylene (HDPP) is quite similar to that of HDPE. It is a thermoplastic polymer that will soften with heat. Depending upon the degree of cross branching, polypropylene has a melting point in the range of 320-to-340 F, with better heat and impact resistance than HDPE.
For pipeline application, HDPP is used as an extruded pipeline coating. In the 1960s, application of HDPP pipeline coating was as a seamless 40-to-50-mil product when extruded over an adhesive primer. HDPP has better high-temperature resistance than any other polymer in use as a pipeline coating. It has superior mechanical resistance to penetration, impact and abrasion. HDPP is the material of choice when resistance to mechanical damage is an important design requirement. The water penetration resistance of HDPP is slightly better than HDPE (Table 5).
Composite Pipeline Coatings
Over the past 20 years, technological advances in material fabrication have resulted in a new class of pipeline coatings with enhanced physical endurance, thermal resistance, and resistance to water penetration and cathodic disbondment. The systems are called composites. They represent combinations of existing polymer materials, formerly used in a single-layer configuration, that have been combined to produce improved multi-layer pipeline coating systems.
A three-layer-polyethylene (3LPE) system is a multi-layer coating composed of an FBE base coat, a copolymer adhesive and an outer layer of side-extruded HDPE.
The tough outer layer of HDPE protects the coating system during transportation and installation. Lowering-in damage is reduced and protection against abrasive soil conditions is maximized. HDPE has good water penetration resistance. The FBE primer develops an excellent bond to the steel and has superior resistance to cathodic disbondment. The coating system can endure operating temperatures up to 185 F but is available only as a mill-applied system (Table 6).
A three-layer polypropylene (3LPP) system is a multi-layer coating composed of an FBE primer, a copolymer adhesive and an outer layer of side-extruded HDPP — similar to the 3LPE previously described, but with an HDPP outer layer in place of the HDPE.
The FBE primer component of the coating system provides excellent adhesion to steel and imparts long-term corrosion resistance. FBE also has superior resistance to cathodic disbondment. The tough outer layer of HDPP protects the coating system pipelines during transportation and installation. Costly repairs from lowering-in damage are reduced, while in-ground protection against shear forces, chemicals and abrasive soil conditions are minimized (Table 7).
This mill-applied coating system can endure operating temperatures up to 185 F. Because it can withstand rough handling, 3LPP is a preferred system for offshore pipeline projects.
Abrasion-Resistant Coating Systems
Abrasion-resistant pipeline coating systems are manufactured to provide superior resistance to damage caused by rock-filled backfill and directional boring operations. Following are descriptions of the more common systems.
Polymer concrete is a mixture of concrete aggregate and an epoxy binder. In a popular configuration it is used to protect the FBE on steel pipe when it is subjected to severe handling conditions during installation. Polymer concrete is a highly abrasion- and impact-resistant coating that provides a smooth surface to allow the pipeline to be pulled under the crossing in a slick-bore operation with much less drag resistance than conventional concrete. It can also be used as a rock shield. Polymer concrete can be sprayed onto the FBE-coated pipe in a thickness range of 20-to-125 mils.
Abrasion Resistant Overcoating (ARO) provides physical protection to FBE pipeline coatings. ARO is also an FBE product, but is an extremely hard and mechanically strong overcoating designed to protect the FBE basecoat from damage during pipeline directional drilling and boring. ARO also offers strong abrasion protection to coated pipe and pipe pilings installed in river crossings and rough terrain.
ARO is compatible with all FBE coatings and chemically forms a high-adhesive bond at the layered interface. The tough outer coating also retains a high degree of flexibility that exceeds specification limits of steel for field bending. One typical ARO system consists of an initial primer layer of 8-to-16 mils of epoxy covered with 20-to-35 mils of a higher-density epoxy. Liquid, 100-percent-solids epoxy ARO coatings can be spray-applied in the mill or in the field at a thickness of up to 60 mils.
Anti-corrosion polymer pipeline coating systems can be augmented with an exterior layer of reinforced concrete of from 1-to-9 inches for buoyancy control. In this arrangement, the added exterior layer of concrete provides both negative buoyancy to sink the pipeline and mechanical protection for pipelines in deep-water marine and wet environments, such as tidal swamps, wetlands and rivers.
Nearly 80 years have gone by since the first rudimentary coating systems, utilizing coal tar and asphalt, were applied to underground pipelines to control corrosion. As time passed, the pipeline-coating industry has matured to the point where reliable coating materials are now available that can be applied at high-production rates in a coating mill with an equally high level of quality control. Depending upon the service requirements, coating materials like epoxy, polyethylene and polypropylene will be the building blocks for most of the pipeline coating demands brought about by the hydraulic fracturing process.
About the Author
E. Bud Senkowski, P.E. is a senior consultant with KTA-Tator, Inc., where he has been employed for over 21 years. He is a registered professional engineer in several states, an ANSI-Certified Nuclear Coatings Inspector, a NACE-certified Level 3 Coating Inspector, and an SSPC-Certified Coatings Specifier. Bud has over 45 years of coating engineering, inspection, training and project management experience. He has an extensive background in the engineering, application and inspection of coatings and linings used in nuclear and fossil-based service water and chemical storage facilities, and is the primary instructor for KTA’s Nuclear Inspection training course. Bud holds an MBA from Drexel University and a Bachelor of Science degree in fuel engineering from Pennsylvania State University. JPCL
As seen in JPCL 2015
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