Over the years, there has been much written regarding pipeline protective coatings. This topic not only continues to generate great interest, but also has left a great deal to be written upon. In this article, I’ll walk you through developments through the years, the centuries and even the millennia in the world of pipeline coatings.
Ancient Coatings, Early Pipelines
The use of varnishes and enamels based on beeswax, gelatin and clay are recorded as having been used as a protective coating as early as 3000 BC. Subsequently, coatings of pitch and balsam were used as a protective coating to waterproof wooden boats as early as around 1000 BC. Bamboo pipelines used to carry brine and natural gas were developed by the Chinese in the Zigong area of Sichuan Province around 1000 A.D. (and possibly much earlier). Each length of bamboo was cut in half, down its length (the world’s first half shell). The segment walls were removed, and the insides of the bamboo further hollowed out to create a smooth surface of constant interior diameter.
The two sections were then put back together and bonded with an adhesive coating made from lime and tung oil (the world’s first pipeline coating and caulking system). It was further bound together with twine inset into grooves in the outside surface of the bamboo pipe, to provide friction protection during pipeline operation and also mechanical protection to the coating. As recently as the 1950s, there were still more than 95 kilometers (60 miles) of the bamboo pipeline in operation in the Zigong area.
Commercial Oil Wells, Metal Pipes
It is widely believed that the use of “metallic” pipe for oil transportation started soon after the drilling of the first commercial oil well, by “Colonel” Edwin Drake, in Titusville, PA, in 1859. These pipes were mainly wrought iron; however, mass drilling, production and transportation subsequently led to the transition to the more robust steel piping—and the corrosion issues that come with it.
It is widely believed that the use of “metallic” pipe for oil transportation started soon after the drilling of the first commercial oil well, by “Colonel” Edwin Drake, in Titusville, PA, in 1859. Since the days of Mr. Drake, the pipeline industry has grown significantly, and subsequently we’ve seen huge developments in corrosion control technology for pipeline operations and maintenance. Today, the industry uses sophisticated controls and computer systems for corrosion management, and advanced pipe materials and corrosion-prevention techniques. However, this was not always the case. In the 1920s, the rate of pipeline construction increased sharply, as natural gas was discovered in the Great Plains, and the need for it as a heating fuel developed in large Midwestern cities. The stimulus of increased pipe production led to significant improvements in pipe manufacturing.
The early pipeline operators became aware of the fact that corrosion of pipe buried in soil could be quite aggressive, and due to this, by the 1920s, some operators began to coat the pipe as it was being laid in the ditch, in an attempt to protect it from corrosion. The idea was to place a barrier between the pipe and the corrosive conditions in the soil, hence the term “the barrier principle.” A common early coating was coal tar and asphalt, which was typically applied by pouring and hand/mitt and rag application. It was a very primitive technique; however, believe it or not, it worked!
Corrosion Engineers Organize, Technology Speeds Up
NACE International was established in 1943 by 11 corrosion engineers, primarily in response to the high levels of corrosion failures reported on pipelines. (At the time, it was called the National Association of Corrosion Engineers.) The intention was reducing risk and providing standards and education on corrosion control in pipelines. This was a major boost to pipeline coating and technology, and the timing coincided with a number of breakthroughs in protective coating technology.
Over-the-ditch application of enamels and asphalts during construction continued up to the 1950s, when the first plant-applied, extruded polyethylene mainline systems were developed. This was a major advancement in coating technology, and had a huge impact upon the industry. The pre-fabrication of fully coated spools saw a benefit and boost for production and lay; the process was recognized as an industry changer. All that was left requiring over-the-ditch coating were field joints, which would then be subject to further advancements. (We have to remember that the mainline coating protects the whole length of the steel pipe with the exception of the variable length area where the pipe is joined.)
FBE Arrives on the Scene
With the jump from liquid-based epoxy coatings (coal tars and asphalts) to polyolefin materials (polyethylene or polypropylene), the late '50s and '60s also saw the widespread use of the now-infamous FBE, or fusion bonded epoxies. These powder coatings were used either as standalone systems or as part of multi-layer system—which really gave birth to the three-layer PP and three-layer PE systems. We began to see a combination of systems: The 1960s really saw the birth of mainline coating systems. As early as the 1920s, operators began to coat pipelines; the idea was to place a barrier between the pipe and the corrosive conditions in the soil, hence the term “the barrier principle.”
The benefits of FBE were apparent to all within the industry—the corrosion-protection properties of this material were excellent, with exceptional adhesion properties to the spool. (As I have stated on numerous occasions: If a coating doesn’t stick to the substrate it is intended to protect, it is of little use as a method of corrosion control.) FBE also provided excellent flexibility properties, an answer to the failures of many previous materials due to handling and bending of the pipe spools during pipelay and subsequent cracking of the system. However, single application of FBE was only used in basic (or, should I say, “standard”) applications where environmental conditions were not as challenging or corrosive, and the terrain was not so problematic. To compensate for this, the industry then developed the dual-layer FBE (2L FBE) system. This introduced the application of Abrasion Resistant Overcoat (ARO) and offered much greater handling and abrasion-resistance properties as compared with single-layer FBEs.
Polyethylene Coatings Emerge
Whereas the early coatings had been mostly coal-tar enamel or asphalt enamel, the 1960s and 1970s saw the increasing use of polyethylene tape coatings—two-layer extruded polyethylene coatings consisting of an adhesive base coat and a polyethylene topcoat—in addition to FBE. In the 1980s, as I’ll discuss in detail next week, three-layer systems comprised of a polyolefin (polyethylene or polypropylene) topcoat, a copolymer adhesive intermedia layer, and a fusion-bonded epoxy primer, were developed and widely utilized outside North America and the U.K.
In addition, liquid coatings such as epoxies and polyurethanes, as well as other types of coatings, were developed and used by some pipelines as a lining or a coating in order to enhance the flow characteristics of the natural gas or oil transported through the pipelines, to improve resistance against corrosion or mechanical damages associated with installations and operations of the pipelines, or to provide field rehabilitation capability. Various field joint or girth weld coating technologies were also developed to optimize performance with the mainline coating and provide field-application friendliness.
As we are well aware, durability and integrity are key to pipeline performance, and with climate extremes and increasingly harsh operating conditions, pipe coatings have had to develop in order to meet industry challenges. Pipeline coatings are critical to the long-term performance of a pipeline and subsequently the selection of products for corrosion control should not be taken lightly.
In the early years of pipeline construction, bitumen and coal tar applied over the ditch were considered as the original “field-applied” coatings for the entire pipeline, not just the field joints. These were excellent coatings to meet the requirements for corrosion protection back then; however, we have to consider that the majority of pipelines were small bore, with far less demanding operating conditions, as opposed to more challenging contemporary operating requirements.
I’ll look at developments in three-layer polyolefin coating systems and other technologies from the 1980s through the present. I’ll also discuss some of the factors that go into specifying protective coatings for pipelines, and what an inspector needs to know in order to evaluate them.
The 1980s really did see the birth of the three-layer systems comprised of a polyolefin (polyethylene or polypropylene) topcoat, a copolymer adhesive intermediate layer, and a fusion-bonded epoxy primer.
The three-layer polyethylene coating (3LPE) systems were first developed as pipe construction moved into more challenging and corrosive environments. The addition of the polyethylene topcoat provided a number of advantages to the system. Mechanical damage properties were greatly increased, proving invaluable during storage and transportation. Moisture penetration was greatly reduced and chemical resistance was greatly increased, ensuring the lifespan of the FBE primer coat. These systems were a major advancement in pipeline coating technology, providing excellent corrosion control in extremely harsh and adverse weather conditions.
This led to the three-layer polypropylene (3LPP) system, which is applied in a specialized coating facility or coating mill. The application process is very similar to that for 3LPE systems, and the characteristics and properties are also very similar. The main differences between the systems were primarily operating temperatures of the pipes: Typically, most mainline 3LPE coatings are rated up to 85 degrees Celsius (185 degrees Fahrenheit) whereas 3LPP can go as high as 110 degrees Celsius (230 Fahrenheit) for onshore applications.
Both technologies have pros and cons. 3LPP is more resistant to high temperatures and is tougher; however, 3LPE is more flexible and more damage-resistant, especially at cold temperatures of installation. 3LPE is also a bit more field-friendly, especially as it pertains to the field joint method selection.
More and More Layers
Technology has advanced even more in the new millennium, as in the 2010s, we saw the introduction of multi-layer polyurethane and polypropylene systems, such as a five- to seven-layer PP system, albeit for offshore pipe coating systems.
Over the past decade, many new coatings and application technologies for both onshore and offshore pipeline applications have been developed. As the need to transport hydrocarbons expands into rougher terrains and deeper environments, the use of syntactic foams for thermal insulation has also been added to the system. We are now seeing five- or seven-layer PP syntactic systems that can work up to 150 degrees Celsius (302 Fahrenheit) in operating temperature and to depths up to 3,000 meters.
These systems are typically composed of the following:
Layer 1: Our good old friend, the fusion bonded epoxy primer (for adhesion and anti-corrosion protection).
Layer 2: A copolymer adhesive applied between the FBE and the third layer; its sole purpose is to promote adhesion.
Layer 3: The extruded solid polypropylene coating; this ensures the integrity of the anti-corrosion system (i.e., the FBE and adhesive).
Layer 4: A syntactic polypropylene foam; this provides the system with thermal conductivity properties. (See Article, “Deep Dive into Subsea Coatings.”)
Layer 5: A solid polypropylene outer layer; provides further mechanical protection, abrasion resistance and UV protection.
Depending on the project water depth and thermal insulation requirements, many variations of these systems can be used, and more combined layers of polypropylene foam and solid can also be added, with some deep-water pipeline projects even seeing a nine-layer coating system. So a combination of the new and old is what is now being used for high-tech coating performance. However, the anti-corrosion (i.e., the FBE coating) and the adhesive copolymer essentially remain the same.
Over the past two decades, several incidents of pipeline coating failures have been reported with massive disbondment of 3L polyolefin mainline coatings and cutback disbondment of multi-layer polypropylene systems. Adding to the issue is a lack of consistency in coating quality and the performance of the pipeline coating systems from one applicator to another. Disbondment of 3L polyolefin coatings can cause shielding to cathodic protection current and further expose the pipeline to environmentally induced cracking. Developing or selecting the right pipeline coating systems for use depends upon a huge number of factors.
The observed 3L polyolefin coatings have raised concerns in the industry worldwide about the long-term performance of these coatings, resulting in several industry initiatives to determine the failure mechanisms and corrective/preventive measures.
High Performaance Powder Coatings
Among these measures was the development of high performance composite coatings (HPCC), later called high performance powder coatings (HPPC). A technical paper presented at the 2005 China International oil and gas pipeline technology conference and expo, co-authored by industry expert Dr. Shiwei William Guan, summed up the technology benefits of the HPCC or HPPC coating rather well:
“The High Performance Composite Coating system (HPCC) is a single-layer, all powder coated, multicomponent coating system consisting of a FBE base coat, a medium density polyethylene outer coat and a tie layer containing a chemically modified polyethylene adhesive. All materials of the three components of the composite coating is applied using an electrostatic powder coating process. The tie layer is a blend of adhesive and FBE with a gradation of FBE concentration. Thus, there is no sharp and well-defined interface between the tie layer and either of the FBE base coat or the polyethylene outer coat. The adhesive and polyethylene are similar to each other and intermingle easily to disperse any interface. The coats are therefore strongly interlocked and behave as a single-layer coating system without the risk of delamination. Delamination has been a performance issue with some three-layer polyethylene coatings, especially under cyclic conditions. Being a single-layer coating and thinner, the HPCC will have less internal stress development when subjected to large temperature changes.”
The development of the HPCC or HPPC coating is just one example of how the pipeline coating industry has addressed the new challenges with new product and technology innovations. Over the past decade, many other new coatings and application technologies for both onshore and offshore pipeline applications have also been developed. Examples include the interpenetrating polymer network (PNC) coating system, 100% solids novolac coating for high temperature application, Heat Shrinkable Sleeve (HSS) Automatic Field-Application System, and multi-layer polystyrene alloy thermal insulation coating for unlimited water depth.
Factors in Selection
Developing or selecting the right pipeline coating systems for use depends upon a huge number of factors, and many questions need to be asked prior to choosing a protective coating material. For example: What is the diameter of the pipeline? What is the steel material to be coated? What is the operating temperature? What materials is the pipeline exporting? Is the system to be buried or immersed? What is the corrosivity of the soil? What is the environment type? What is the anti-corrosion and thermal performance requirement? Is cathodic protection to be used in conjunction with the system? And so on and so forth.
Answering these questions and others like them then leads to the selection of materials. In today’s industry, it is a far cry from the over-the-ditch coatings applied in the 1920s.
Additionally, I have to reiterate that there are so many design factors that manufacturers have to take into consideration for materials; for example:
Thermal conductivity dry (ASTM C518);
Thermal conductivity wet (ASTM C518);
Heat capacity (ASTM E1269);
Thermal diffusivity dry (ASTM E1461);
DSC specific gravity (ASTM D792);
Compressive strength (ASTM D575);
Tensile strength (ASTM D412);
Tensile elongation (ASTM D412);
Poisson’s ratio (ASTM E132); and
Mechanical testing (ASTM D638).
These are just a few properties that a material is tested upon; there are of course more. Materials have to go through endless rigorous inspection and testing compared to the 1940s, and inspectors are required to know and understand the inspection requirements, techniques, test methods, associated standards, and acceptance and rejection criteria for PQT, etc.
The Inspector's View
As we can see, there have been major developments since the first transportation of products way back in 1000 A.D., with huge advancements in coating technology. The search for oil and gas and the endless challenges that must be undertaken are ensuring rapid evolution in coating technology and applications. There have been major advancements in corrosion control, adhesive bonding, abrasion resistance, impact resistance and thermodynamics, but what effect does this have upon the inspector?
Any developments in materials must lead to development with regard to inspection of these systems, taking into consideration the technologies involved.
At a minimum, inspectors should be able to:
Understand the differences between different pipe coating materials and their generic types;
Understand pipe coating specifications and standards;
Have an understanding of application methods;
Identify common failures and defects;
Understand ITP and test methods;
Be familiar with PQT (pre-qualification test) requirements;
Be familiar with product curing times in order to conduct inspection during application and curing phases;
Be competent with inspection equipment; and
Know how to document and report findings identifying coated areas and status of completion. (This would require an understanding of pipe coating equipment equipment).
Over the decades, the oil and gas pipeline industry has reduced the risk of operations by way of advances in pipe manufacturing technology and changes in pipeline construction practices. The selection of pipeline coatings over the years has followed the development of corrosion protection materials and application technologies.
From hot bituminous coatings hastily applied over the ditch in the early years, to epoxy and polymer based materials applied in highly sophisticated coating plants that operate today, the technology has come a long way since 1000 A.D.
I wonder what the future holds?
I would like to thank Dr Shiwei William Guan for technical correspondence.
By Lee Wilson Director of CORRTECHLTD www.corrtechltd.com "world leaders in corrosion control and mitigation"