Due to recent events, it is understandable that the midstream community is under pressure to ensure safe and reliable operations. Pipeline infrastructure has to handle rapidly expanding demands as new sources of energy come online and must be transported to users of all kinds downstream.

Pipeline systems have revealed their age in the growing number of incidents and what PHMSA (the Pipeline and Safety Hazardous Materials Administration) has called "…a record number of enforcement actions." For every new pipeline mile that is constructed, there are a hundred more miles that were built a half-century ago, and thousands laid down in the 1920s.

Infrastructure is also presenting operating and safety challenges. Corrosion-induced failure is one of the greatest threats to pipeline safety and productivity, and pipeline corrosion engineers are intimately aware of the ever-increasing battle they face to maintain their aging pipeline systems.

Historically, pipeline operators have employed an interrelated, two-pronged approach to mitigate corrosion risks—coatings and cathodic protection. The coating system acts as a barrier protecting the steel pipe from its environment and is the first line of defense against corrosion. In older pipelines, coatings applied during their initial installation had a finite service life. Different coating technologies age differently, and a variety of other factors can influence the reliability and service life of the original coating. Over time, the effectiveness of the coating can diminish significantly, and the deteriorating coating system can negatively impact the effectiveness of the cathodic-protection system.

Cathodic protection supplements the coating system by providing protective current to the holidays or defects within the coating system. Current capacity can be increased and added to over time in an effort to compensate for the reduced effectiveness of the coating system as it deteriorates. However, at some point conventional cathodic-protection augmentation ceases to effectively protect many older pipelines.

Because the two protective approaches are interrelated, remediating an aging pipeline involves considering both the deteriorating pipeline coating and the over-taxed cathodic-protection system. Initial options are to supplement the cathodic protection system with continuous linear-anode installation or to recoat the pipeline. In extreme cases, where significant corrosion has already occurred, the only option is to replace the pipeline.

Weighing the alternatives of upgrading the cathodic protection system versus incurring the cost of recoating can keep pipeline operators awake at night.

Coating systems

Since the coating provides the primary defense against corrosion, the operating and safety risks of corrosion increase exponentially as the coating system ages and deteriorates. Coating systems have been used on buried pipelines during the last hundred years and the technology remains the subject of much research and innovation. Coating manufacturers are continually searching for better coatings to meet the varied needs of industry.

At first, the coatings were simple mixtures of crude pitches and solvents. These early bitumastic/asphaltic systems evolved into engineered coal-tar-enamel coating systems that were prevalent into the 1960s. The introduction of fusion-bonded epoxies (FBE) in the 1970s quickly captured much of the pipeline market, although polyethylene, polypropylene and coal-tar-enamels are still used as well.

This background is important because one of the challenges operators have to address is properly identifying the type and vintage of the coatings along a given pipeline. In many scenarios, different sections of pipeline may have different coating systems, depending on the age of the pipeline and the standards in place at the time a particular section of pipe was installed, repaired or replaced. This process can be difficult in the case of older pipelines, where accurate records concerning repair methods and repair history of the line may not exist.

One of today's best practices is to identify whether the aging pipeline-coating system fails in a "shielding" or "non-shielding" mode. Coating systems that fail in a non-shielding mode do not inhibit the flow of current to the pipe surface, making upgrading of the cathodic protection system a viable remediation alternative to recoating.

Other coating systems, principally tape-coating systems, can fail in a manner that shields cathodic protection current and thus makes recoating the only viable option.

Modern, over-the-line survey technologies are quite effective in evaluating coating quality and finding coating holidays for non-shielding coatings. Technologies, such as pipeline current mapping (PCM), which utilize a carrier signal transmitted along the pipeline and a receiver to measure the line attenuation along the pipeline length, accurately pinpoint areas of significant coating degradation even under concrete or asphalt.

The information gathered using PCM, in conjunction with pipe-to-soil close interval surveys (CIS) and direct current voltage gradient (DCVG) testing, form the basis for identifying critical risk areas along aging pipelines. For pipelines with shielding coatings, over-the-line surveys and indirect-assessment technologies have been proven to be less than effective. For these pipelines, soil surveys and an increased frequency of direct examination (digs) are used to determine coating condition.

In-line inspection technologies using smart pigs also provide valuable data about coating quality and are especially useful for pipelines with shielding coating systems where indirect assessment is not very effective.

Cathodic protection

Pipeline-coating systems are typically augmented by the application of cathodic protection. This is mandated for hydrocarbon and hazardous-material pipelines throughout the U.S. and in most other countries. With a well-coated pipeline, cathodic protection can be economically applied to protect infrequent and small coating holidays and defects by the placement of discreet anode beds capable of distributing current over long distances.

In many cases, ground beds can be located several kilometers apart and still provide sufficient current distribution to protect the entire pipeline. With some of today's high-technology, factory-applied coatings, the coating efficiencies are exceptionally high, and the ground-bed output requirements are very low. These discreet ground-bed systems can either be deep anode ground beds or shallow ground beds located some distance off the pipeline.

Several issues must be considered when designing a cathodic-protection system. These include coating quality, soil resistivity, available locations for electrical power, ground-bed right-of-way issues, accessibility for maintenance, AC and DC stray-current interference, and a host of additional issues. What is critical for aging pipelines is regular evaluation of the cathodic-protection system's effectiveness.

Logically, as the pipeline's coating ages, the frequency and size of the coating holidays increases. This results in a significant increase in the current requirements. More importantly, it creates significant issues with current distribution. Anode systems that were effective delivering current evenly over long distance when the coating system was new can become unable to deliver current over any significant length. Left unchecked, this poor distribution characteristic can result in significant corrosion along the areas where current is not sufficiently present.

Current distribution

Proper current distribution along a pipeline is evaluated by measuring the voltage difference between the pipeline and a reference electrode with a fixed potential. These measurements are typically taken at short intervals (the length of a technician's walking stride) by performing a close-interval potential survey (CIPS is the acronym used in much of the world outside the U.S.; within the U.S. it is referred to as a CIS survey).

The close-interval survey will provide a strong indication of the effectiveness of the cathodic-system's task of properly distributing current. Should the close-interval survey indicate that the CP system is not effectively distributing current, the typical response is to increase the overall output of the existing cathodic-protection system.

This strategy of turning up the cathodic-protection output generally does not alleviate the current distribution problem. Instead, it simply causes even more current to flow to those areas already receiving current. This can result in coating disbondment (excessive levels of cathodic protection causing oxygen formation between the steel substrate and the coating, further exacerbating the problem). The higher-output current also increases the ground bed's consumption rate, reducing operating life while raising operating costs significantly. All this occurs without achieving the required levels of polarization along the entire length of the pipeline to meet cathodic-protection criteria.

The next step that is often taken to fix the cathodic-protection current-distribution problem is to add additional ground beds to reduce the distance between cathodic-protection current sources. This too may prove to be an ineffective solution, as the new ground bed provides only limited additional benefit while significant areas along the pipeline remain unprotected or under-protected.

In some extreme cases, the presence of a new ground bed installed along a pipeline can have little or no effect on the pipeline a few hundred meters away.

Mitigation strategies

At some point in the effort to mitigate corrosion risks, an ever-increasing number of discreet ground beds, applying greater and greater amounts of additional current, cannot overcome the issue of poor current distribution along a pipeline with degraded coatings.

The pipeline operator is then faced with a very limited number of options: replace the pipeline, recoat the pipeline, or install a linear-anode system.

Recoating or replacing is the only viable alternative for pipeline systems utilizing shielding-type coatings such as tape-wrap systems. Recoating, when properly performed, can restore the pipeline-coating system to an as-new condition, greatly extending the service life of the recoated section. However, recoating can be very expensive and is extremely invasive. Entire sections of the pipeline must be excavated and exposed to allow access to the pipe surface being recoated.

It is crucial that a recoating contractor carefully perform the pipeline-recoating process. Proper attention must be paid to surface preparation, environmental conditions, coating-installation procedures and installation quality control. Failure to maintain control of the recoating process can result in a costly mistake, such as replacing one poor coating with another poor coating.

An economically attractive alternative to recoating is to install a linear-anode configuration in lieu of discreet systems. This option is only viable when the coating system is non-shielding, such as asphaltic and epoxy-type systems. The application of a linear-anode system typically costs a fraction of recoating and does not require the invasive exposure of the pipeline. Instead of digging up the pipeline, the linear-anode system can be installed with minimal disturbance of the surrounding area by using horizontal-directional drilling.

These systems, unlike discreet ground beds, operate at very low current densities, but do so over the entire length of the pipeline segment to be protected. They are, in effect, an infinite series of point anodes that provide an optimum current distribution. While recoating requires extensive quality-assurance and quality-control man-hours to ensure proper application of the new coating system, the linear-anode solution simply requires a proper engineering design of the system up front to ensure a successful operation. An experienced corrosion-engineering firm with proven experience in linear-anode pipeline-rehabilitation design should be consulted.

In addition to confirming that the pipeline-coating system is appropriate for the application of linear anodes, the linear-anode system design must take into consideration the issue of voltage drop and its effect on current attenuation. Voltage drop can have a significant impact on DC power distribution to the system. Ideally, rectifiers would be located no farther than one or two kilometers apart. However, practical considerations, including the availability of AC power, right-of-way issues and other factors, can force this to be extended further, which can complicate the system design and affect the installed cost.

Meanwhile, although the design can be complicated by voltage drop considerations, one of the benefits of a linear-anode system is that the power consumption is relatively low. Ground-bed resistance, as determined by Dwight's Equation, is significantly affected by anode length. This results in a very low ground-bed resistance value for linear-anode systems relative to conventional ground beds. A linear-anode system is more suitable for low-wattage power sources, such as solar arrays and thermo-electric generators (TEGs), than conventional ground beds with wattage that could require two or more times that of a linear-anode system to achieve the same current discharge. Again, a knowledgeable engineering firm should walk through these factors and considerations to evaluate a particular pipeline operator's individual circumstance.

Minimizing risk

Aging pipeline systems with deteriorating coating systems present a significant corrosion risk to pipeline operators, and pipeline operators already go to great lengths to minimize these risks while grappling with a complex set of protection circumstances. Yet, modern demands call for modern measures.

Among remediation options, adding a modern linear-anode system can effectively protect the aging pipeline from corrosion, depending on its coating type. The results of untangling and then resolving pipeline-remediation challenges in this way can give midstream operators significant confidence in their systems and improved efficiency when moving product downstream to markets.

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