Horizontal directional drilling is increasingly being used to install sections of pipe across many spans that are considered high consequence areas. While regulation currently calls for extra consideration to be given to the coatings used to protect these sections of pipe from corrosion, the systems currently being employed may be insufficient. Cathodic protection systems and casings shouldn‘t be a substitute for an intact coating system. Fiber-reinforced urethane abrasion-resistant overcoat systems can be employed to ensure the damage to the coating done by these types of installations is reduced or eliminated.
In our modern world, new pipeline installation is a constant, and nearly every one being constructed has some portion that will be installed using horizontal directional drilling (HDD). HDD has become a vital part of new pipeline construction and there are many reasons for its acceptance. When a pipeline is installed by HDD or microtunneling, it is done so because an obstruction is in the way that must remain undisturbed, such as a road, lake, building, railroad, or river. HDD has gained popularity because the pipe cannot be trenched into the ground in these areas the same way it is over stretches of open land. While HDD-installed pipe accounts for <10% of new construction, it could possibly represent the most crucial 10% that should be protected. The very reasons why these vital sections should be protected are the same reasons that trenchless technology is used—the pipes run beneath our roads, lakes, buildings, railroads, and rivers. Typically, these are classified as high consequence areas (HCAs).
What is being done to ensure these pipelines are as safe as possible in these HCAs? While investigating that question, many discussions were held with integrity engineers and pipeline operators to evaluate their methods and decision-making criteria. Nearly all the respondents, at some point, reference the guidelines provided by the U.S. Code of Federal Regulations (CFR) Title 49, Parts 192 and 195 as they relate to their field.1-2 The objective was to determine if enough was being done to ensure the primary corrosion barrier was in place after the pipe was installed. Because HDD installation methods expose pipeline coatings to the most aggressive situations a coating might experience, it is imperative that all methods are utilized to ensure the coating is protected during installation. Conducting inspections or performing repairs that would typically require unearthing the pipe are not possible. Implementing the most proven technology available ensures a coating will remain intact in these inaccessible areas where spot repairs are not reasonably feasible.
Guidance
In reviewing the CFR regarding coatings and corrosion control, Parts 192 and 195 very closely mirror one another. In general, they both say that new pipelines must have a coating used for corrosion control and it must work with the cathodic protection (CP) system. Those seem like good and reasonable statements, but the interest here is in how to ensure those coating systems are still in place once thrust through a hole. Part 192 contains the following verbiage and 195 is similar:
Ҥ192.461, External corrosion control: Protective coating.
• (c) Each external protective coating must be inspected just prior to lowering the pipe into the ditch and backfilling, and any damage detrimental to effective corrosion control must be repaired.
• (d) Each external protective coating must be protected from damage resulting from adverse ditch conditions or damage from supporting blocks.
• (e) If coated pipe is installed by boring, driving, or other similar method, precautions must be taken to minimize damage to the coating during installation.”
While this guidance is not very detailed as to what “precautions must be taken to minimize damage,” it does mandate that precautions must be taken. The U.S. Department of Transportation (DOT) says that when a decision is made to do something, the only requirement to ensure that what is done complies with the regulation is to inspect it prior to installation. There is no requirement to check or test the effectiveness of the coating system once the pipe is pulled through the excavation. So, the guidance referred to by the integrity engineers and pipeline operators does not require evidence that the corrosion barrier is still in place after pipe installation.
Industry Practice
In the last few years, conscientious pipeline operators have implemented methods of inspecting the effectiveness of their selected corrosion barriers; however, these methods are not required or necessarily standard operating procedures. Some operators will pull several joints of coated pipe through the HDD excavation first for visual inspection, but this is typically reserved for shorter pulls because of the added expense. Other operators will add lengths of pipe to be pulled out on the other side so they can inspect the coatings to evaluate the amount of damage done to the coatings, and potentially the pipe itself. Both methods are far better than simply checking the coating before the pipe is installed; however, neither is a solution for coating damage. Typically, a test pull will allow a company to address issues it sees as a threat to the coating’s integrity, but that does not prevent other possible coating damage from occurring when the final pull is made. While pulling extra joints out on the other end is good for evaluating coating damage, what is usually done if damage is present at that point is to add more CP. The damaged coating, however, remains the same. The industry states that coatings should be the first line of defense against corrosion, but the trend of leaving pipelines in the ground with a damaged coating means CP is the only line of defense in areas where repairs cannot be made!
In an interview with the president of a large HDD contractor, it was noted that most companies still do not employ these more cautious test methods. Historically, one of the ways coatings were protected from damage was to use a casing pipe. Many of the casings were even installed by HDD. While that method may have been effective in some ways, today more is known about the issues that casings can cause, and nearly all are being phased out. Several years ago, a panel was interviewed regarding casings. This method of protection is no longer a widely accepted method, and the numerous reasons why were presented in a discussion, published in Pipeline and Gas Journal.3
The Issues
The limited amount of inspection and vague guidance from the DOT lead to the question of whether simply adding more CP will suffice. In an article regarding pipeline corrosion, Moriber4 notes:
“While there are modern coatings that approach perfection, at progressively higher costs, it is often most practical to accept the inevitability of some coating flaws, where corrosion can be controlled by CP. The costs associated with CP for various ranges of coating efficiency are well known. Therefore, it is possible to select a cost-effective combination of reasonably good coating and CP, bearing in mind that generally, it is not practical to increase the investment in higher quality coating beyond the related savings in CP costs. One exception to this guideline may be the choice of a protective coating system for pipe that is to be installed by HDD. Because future access for coating rehabilitation is unlikely to be available, additional costs for abrasion-resistant (over) coatings (ARO) may be justified.”
The Australian Pipeliner5 explains with examples and calculations, why the typical CP methods employed for use with HDD are unsuitable. The summary says HDD-installed pipe is “usually installed in areas where pipe is impractical to repair, so they have to comply with the full design life requirements of the pipeline with no repair option. There are many circumstances in which pipe installed by HDD cannot be cathodically protected and can be subject to high rates of corrosion. Despite this, the acceptance criteria for pipe in HDD is of a lower standard than would be accepted for normal trenched construction. The existing HDD practices and coating evaluation need to be revised.”
These two excerpts indicate that simply adding more CP to protect pipe installed by HDD is not an ideal approach. Much of the guidance provided for pipeline construction is geared toward the ±90% of the pipelines that are simply lowered into a ditch and then backfilled. In these cases, the damage is very minimal, the coatings are very effective, and the CP can usually be implemented in a way that is measurable and effective.
As Moriber suggests, an ARO will normally be needed to ensure the corrosion barrier selected by the pipeline owner will still be in place when the pipe is pulled through. Currently, NACE International has a task group (TG) in place, TG 352, “Coatings Systems (External) for Pipeline Directional Drill Applications,” to write a guidance document specifically for HDD coatings. While the task group plans to provide some guidance on the use of AROs as well as some field coating suggestions, the industry currently does not have an accepted standard to utilize until this is published.
If pipelines now being installed by HDD aren’t being inspected after they are installed and do not have sufficient protection against coating damage, then there is no guarantee that CP can provide sufficient protection. The standard procedures need to be evaluated and changed to ensure that these HCAs are not being subjected to more danger than is necessary. The most common cause for repair is corrosion associated with coating failures, especially at the girth welds. Figure 1 shows some examples of pipe coating damage that occurred when the pipe was pulled through an HDD hole. Currently, there is no way of knowing where the damage was initiated on any of these. It could have occurred in the first 40 ft (12 m) of the pull and much of the pipe in the hole looks the same way, or it may have happened in the last 10 ft (3 m) of the pull. The lack of surety in this practice calls for increased attention, and diligence is needed to protect these HCAs.
Most AROs are designed to work very well over a smooth surface—pipelines have a smooth surface over most of their length— and industry testing proves that. When the same AROs are applied on girth welds, however, damage can occur due to the raised profile between each length of pipe. That coating damage then initiates a corrosion foothold. There are many reasons why the most common field-applied coatings fail in these applications, but an effective solution exists. There are abrasion-resistant sacrificial outer laminate fiber-reinforced AROs being offered in the market today that can be employed to dramatically reduce or eliminate damage over the weld areas that plague this type of pipeline installation. Figure 2 shows the use of a fiber-reinforced ARO that has been employed by many companies.
The best time to use the right materials is in the planning stages and to build it right the first time. Through application of the proper materials, damage to anticorrosion coatings and the dependence on CP can be decreased, and the safety of the HCAs at risk from HDD applications can be increased.
Conclusions
The current guidance and methodologies employed for protecting pipelines installed by HDD may be insufficient to ensure corrosion does not gain a foothold. The most common field-applied coatings in the industry continue to fail over the weld seams, and additional CP cannot always reach the areas that need to be protected. The best means of protecting these HCAs from the coating damage done by HDD is to ensure the corrosion barrier being used stays in place during the installation. This objective can be obtained by ARO systems that serve as a reinforced barrier to the common damage produced from HDD installations. Keeping the corrosion control coating in place is the best answer for protecting these HCAs in the future. One way this can be achieved is by adding a fiber-reinforced ARO over the barrier coating.
References
1 U.S. Code of Federal Regulations (CFR) Title 49, Part 192.461, “External Corrosion Control: Protective Coating” (Washington, DC: Office of the Federal Register, 2011).
2 U.S. Code of Federal Regulations (CFR) Title 49, Part 195, “Transportation of Hazardous Liquids by Pipeline” (Washington, DC: Office of the Federal Register, 2017).
3 T. Keane, “Protecting Pipelines at Crossings: Are Casings Obsolete?” Pipeline & Gas J. 236,3 (2009).
4 N. Moriber, “Pipeline Corrosion,” Trenchless Technology 11 (2008).
5 “Evaluating Pipeline Coating Condition on Horizontal Directional Drills,” The Australian Pipeliner, March 16, 2016, https://www.pipeliner.com.au/2016/03/16/evaluating-pipeline-coating-condition-on-horizontal-directional-drills/ (June 20, 2017).