PAINTSQUARE BLOG

This is part two of a two-part post I’ve put together on the history and development of protective pipeline coatings. If you didn’t already read part one, feel free to go back and catch up: Part one covered developments up to the 1980s or so, and that’s where we’ll start in this post.

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.

Minerals Management Service – Public Domain, via Wikimedia Commons

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 blog, “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.

Coating Failures

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.

By Wilfra - Public domain, via Wikimedia Commons

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.

ABOUT THE THE BLOGGER Lee Wilson Lee Wilson, CEng, FICorr, is a NACE Level 3-certified CIP Instructor, NACE Corrosion Specialist, NACE Protective Coating Specialist and Senior Corrosion Technologist, as well as an ICorr Level 3 Painting Inspector and Level 2 Insulation Inspector. The author of the best-selling Paint Inspector’s Field Guide, Lee was named one of JPCL Top Thinkers: The Clive Hare Honors in 2012. Contact Lee. SEE ALL CONTENT FROM THIS CONTRIBUTOR