Introduction to field-applied pipeline coatings
Pipeline networks are crucial to keeping the world economy running.
Many take these key assets for granted until something goes wrong. Even temporary pipeline shutdowns for minor repairs can send disruptive economic ripples outward.
Field application of coatings over weld joints is on the front line of pipeline protection. Applying these coatings is one of the final steps before pipelines are put in service. They provide a critical layer of protection that, when properly specified and applied, can last the lifetime of the asset.
Pipeline failure risk factors
Both internal and external factors put pipelines at risk of failure. If those factors aren’t addressed and pipelines fail, potential consequences include significant, costly product loss or an environmental disaster that ruins habitats and threatens populations of plants, animals, and people.
Internal failure risk factors
Pipelines carry all manner of fluid or gas, and most of it is corrosive. If pipeline interiors aren’t properly coated, the steel will corrode, significantly increasing failure risk. And the worst part is, it’s much harder to inspect a pipeline’s interior than its exterior. Even the smallest problems can turn into disasters if they go unnoticed.
Sometimes, pipeline contents are abrasive. Coatings are formulated to resist against abrasion, but that resistance is compromised if the coating is not properly applied.
External failure risk factors
Corrosion risk is a given in ambient environments; that risk is amplified in maritime settings where assets are constantly impacted by salt water. Protective coatings can stop—or at least significantly impede—exterior pipeline corrosion.
Another consideration is abrasion risk. Pipelines are most often built underground to stay out of sight and out of the way, but the tradeoff is that subsurface soil and sediment can gradually weaken pipelines. That’s because pipelines expand and contract due to the movement and temperature of the materials flowing through them. The movement causes the pipeline to rub against its exterior environment, so abrasion protection is necessary.
Learn more about corrosion and how to keep it at bay by visiting the world’s largest library on the topic, the AMPP Store.
Pipeline surface prep, coating types, and application
Pipeline surface preparation is always specified by the asset owner with advice from a coating manufacturer. For most pipeline coating projects, one of these standards are specified:
- SSPC – SP 5 (WAB)/NACE WAB-1, "White Metal Wet Abrasive Blast Cleaning" covers the requirements for white metal blast cleaning of unpainted or painted steel surfaces by the use of abrasives. Steel surfaces must be completely clean and free of all staining to comply with SP 5.
- SSPC – SP 10 (WAB)/NACE No. 2 (WAB), "Near-White Metal Wet Abrasive Blast Cleaning" covers the requirements for near-white blast cleaning of unpainted or painted steel surfaces by the use of abrasives. To comply with SP 10, steel surfaces can contain staining on no more than 5 percent of each unit area of surface.
Fusion bond epoxy (FBE) powder systems are most commonly specified for pipeline coating projects. Powder coatings are used for pipeline projects because they’re formulated to be stronger and tougher than traditional solvent-based coatings. In some cases, an abrasion-resistant overcoat (ARO) is also applied. Many different ARO coatings exist, with many formulated to provide cathodic protection in addition to abrasion resistance.
Most of the surface area of pipeline sections is coated in shop settings where the environment is more controllable, and quality is more easily maintained. However, two inches at each end of a pipeline section are left uncoated. That’s because pipeline sections must be welded together in the field, and coating weld joints is only successful after welding.
Overlap requirements are critical here. Prior to coating, the final inch of coated surface area must be sanded to promote better adhesion of the overlapping joint coat. Failure to meet overlap requirements is the most common culprit when pipeline coatings fail, and corrosion occurs.
Automatic spray machines are most often used to coat pipelines in both shop and field settings. In cases where rough terrain or close quarters prevents the use of automatic units in the field, manual spray coating is viable. However, automatic machines are usually faster and provide more consistent coverage for field application.
Manual application via rollers or brushes is almost never recommended because these tools degrade quickly under the strain of heavy-duty pipeline coatings.
Coordination with pipeline welders
While welding and coating of pipelines are separate, successful pipeline coating depends on weld quality. Variables like evenly shaped root beads, sharp edges, section misalignment and section undercut all affect how well—or how poorly—a pipeline joint coating will perform.
These variables are accounted for in the strict standards welders must meet, but it’s important that project managers verify weld quality and communicate with coating contractors if it falls short.