Protecting pipes during pullback

Ron Raphoon, Denso North America, outlines key considerations for protecting pipeline coatings during trenchless pipeline installation.

The world’s network of pipelines is ever increasing. On a daily basis, pipe is getting welded up and put into the ground to carry a wide array of materials and often these pipes are right beneath our feet. For those of us that live in high population density locations, the likelihood that a pipeline is running under the roads we drive on, beneath the neighbourhoods we live in, and under the buildings we work in, is very high. And if there wasn’t a pipe there yesterday, there could be one there today thanks to the use of horizontal directional drilling (HDD). HDD is the process of installing a pipeline without digging a trench. HDD is used in high consequence areas, such as population dense areas, beneath structures and roads, or to span beneath waterways. It is achieved by drilling a hole deep beneath the area that the pipeline is trying to avoid disrupting, then dragging the pipeline through that hole. Figure 1 provides a general idea of what that looks like.

As you can imagine, drilling a hole in the ground isn’t as easy as it sounds. The various soil layers beneath our feet contain a wide variety of soil types, rocky layers, and random obstructions. While the drawings of an HDD are much like Figure 1 (long, smooth, curving, and uniform), the reality is that the hole can be filled with shards of

rocks, roots, sand, or any number of random objects from the ground. Beyond the items that are suspended in the hole, there are variations in the path that might move the pipe up, down, left, or right along as it is drilled. Any time the pipe moves up or down abruptly, there is the potential for the pipe to drag against the wall of the hole. Doing so can cause numerous issues, including a frac out of the hole, abrasion of the coating, bending of the steel, or shearing a coating completely off. Pipeline damage during pullback While the industry continues to grow and learn from its brief history, the truth remains that there are some damaged pipes and areas of protective coating worn down or removed that exist on pipelines that are lying beneath high consequence areas. The truly unfortunate part is that these pipes are inaccessible for repair. This is obvious when the reason for using the HDD method for installation is evaluated. Those sections of pipe were put into the ground in that way specifically because they couldn’t dig a trench there, meaning they can’t dig down to repair it if necessary.

This means that it is vital that the HDD path is designed well to avoid damage to the steel from bending. It also means that the cleanliness of the hole and the soil that the pipe is passed through should be of the highest concern. At the current time, American Figure 1. A typical example of horizontal directional drilling in a high consequence area. Petroleum Institute (API) has some best practices developed, as does the Pipeline Research Council International (PRCI), and other organisations are developing them as well. In reviewing best practice, they primarily focus on minimising the impacts of damage from the design aspect. API offers eight practices to improve the success rate of an HDD installation: of those eight best practices, seven of them directly address pipeline damage during pullback. What this tells us, is that they know the damage is occurring and that the installation itself is the highest risk of producing that damage. Anti-corrosion coatings are vulnerable to damage In the pipeline industry, one of the most common threats to the long-term performance of a pipeline is the presence of corrosion. This has been addressed for decades by applying anti-corrosion coatings to the outside of the pipe. When anti-corrosion coatings are in place, they can prevent corrosion that could wear through the steel and result in leaks or even possibly be the source of explosions. Unfortunately, most anti-corrosion coatings are very vulnerable to damage when the pipe is being pulled into an HDD hole. Obviously, if pulling in the pipe causes damage to the steel, it is also capable of damaging the coating. The coating is the first line of defense against a pipeline failure and as such Denso has developed coatings specifically designed to absorb and endure the hazards that are present from HDD installations. Some examples of coating damage that is seen in HDDs are shown in Figure 2. Abrasion In most of the images that comprise Figure 2, the drilling contractor was following best practice, yet the result of the pipeline installation was still a damaged anti-corrosion coating. Abrasion from dragging the pipe into a hole is virtually Figure 2. Examples of coating damage as a result of HDD guaranteed. As such, the vast majority of coatings that are installation. used in HDD applications have fairly decent abrasion resistance. However, abrasion alone is probably the least likely reason for complete removal of a coating resulting in exposure of the bare steel. The most common cause of coating failure comes from impact to the coating and gouging from debris or protrusions from the hole wall. Impact and shearing You might be asking yourself: “Where does impact take place in a horizontal directional drill?” The answer to that is fairly simple. Figure 3. Showing how a gouge that passes across a weld Any elevated surface that comes into contact with a hard object creates an impact. in the hole as it is dragged along, will impact on the edge of the

elevated surface. Elevated surfaces include weld seams, off set pipe ends, and coatings that have raised lips over the edge of a coatings (i.e. heat shrink sleeves or tapes). Many coatings have difficulty surviving impact and will commonly chip, break or tear when something like a rock hits the weld then drags over it. Sometimes that gouging will lead to shearing when a softer coating gets caught up and the pipe continues to be pulled (Figure 3).

Gouging and bending Another common reason for coating failure is from gouging. One of the ways gouging can become an issue is along the top of the pipe when it is dragged across a curve in the hole, as seen in Figure 4. This dragging will not only cause abrasion on the surface but, as it wears away the soil, hard objects such as cobble can be exposed and can dig into the coating as the pipe is dragged along. These gouges can occur all along the length of the pipe, exposing long lengths of steel or small spots here and there. Exposure like this leads to the potential for pitting corrosion, which can be far worse than general corrosion if not properly protected. The bending of the pipe around these curves can be a source of damage as well. Many pipeline coatings are very rigid – this is typically a good quality when addressing abrasion – however rigid coatings are inflexible, especially as they increase in thickness. Unfortunately, a common practice in HDD is the increase in coating thickness, adding the possibility that a coating will crack or disbond while following the curvature of a bore hole.

Protecting anticorrosion coatings All of these sources of coating damage need to be mitigated to ensure that the anti-corrosion coating selected to protect the pipeline remains intact once the pipeline is installed. The previously mentioned best practices from PRCI and API both suggest steps to minimise the source of these damages, however DensoTM has taken the precautions one step further. By understanding the causes of coating damage which continue to occur even when following best practice, Denso was able to design a protective outerwrap which addresses each cause of damage. The product, aptly named Denso Bore-Wrap™, is an outerwrap that can be used to protect all the major pipeline coating systems used in trenchless boring operations. Each component of Bore-Wrap was selected specifically to address the elements of abrasion, gouging, impact, shearing, and bending.

Denso Bore-Wrap is made up of several key components, each with benefits selected for their value in this application. The base of the Bore-Wrap is its under layer of woven roving (WR) fibreglass. This layer of fibreglass is composed of woven layers that run both circumferentially and longitudinally along the pipe. The woven structure provides some resistance to gouging due to the continual change in direction, but the most important factor with the WR is the fibres that wrap the pipe circumferentially. This continual stranding around the pipe offers a level of compression strength, which will resist shearing even when applied over softer coating systems. The WR also has significant thickness which will increase the level of abrasion resistance. On top of the WR layer is a layer of continuous stranded matting (CSM) which is a long randomised mat of fibreglass that is stitched to the sub layer of WR. The CSM has omni-directional fabric so that no one angle is prone to separation and there are no lines that a gouge can naturally follow. Having so many random angles offers an incredible resistance to gouging, so much so that, when tested, the addition of this layer reduced the overall gouge depth to almost half that of five different fibre architectures used in other coating systems in the market.

Bringing all of the components together, Bore-Wrap uses a moisture-cured urethane (MCU) which, as its name indicates, cures with water. The curing of the MCU with water creates a CO

Figure 4. Turns in an HDD can force the pipe to wear along the walls of the hole.

Figure 5. Denso Bore-wrap™ used to encapsulate the entire length of pipe during HDD installation.

Figure 6. Denso Bore-wrap™ used to protect field joints during HDD installation.

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reaction, generating a porous structure of resin holding the fibres together while simultaneously offering incredible flexibility. By having a porous structure, Bore-Wrap is able to offer a higher level of flexibility than fusion bonded epoxy (FBE), which is the most common underlying anti-corrosion layer on the pipelines. The MCU cures and bonds each layer together into one large homogeneous outerwrap. The porosity of the structure also offers two invaluable benefits. Firstly, it is very impact resistant, in orders of magnitude higher than most pipeline coatings. Secondly, it allows more moisture to be absorbed and it has very low electrical resistivity. This allows for Bore-Wrap to act similarly to the soil surrounding the pipe and thus it will not shield cathodic protection from doing its job.

All of these claims sound great in theory and much consideration was put into the elements, including how they interact and how they are applied. While Bore-Wrap is easy to apply and simple to understand, its performance is all that matters to the pipeline owner or the contractor tasked with dragging a pipe into a bore hole. Since the development of Bore-Wrap, it has been used on pipelines of small diameter and large diameter. Bore-Wrap has been applied over FBE coating, liquid epoxy, and wrapped over PE coated pipe. Denso has offered this product across the globe and it has been applied to protect vulnerable field joints, elevated weld seams, and also to encapsulate the entire lengths of pipe that were installed by HDD.

Conclusion Denso Bore-Wrap has even been used and applied over the top of other products labelled as abrasion resistant outerwrap (ARO) products. While those products are already abrasion resistant, abrasion isn’t the most detrimental source of damage by any means. Denso Bore-Wrap is an ARO, but it is also an impact resistant outerwrap, a gouge resistant outerwrap, and a shear resistant outerwrap. Perhaps the industry needs to re-evaluate its need for an ARO and instead recommend a shear, abrasion, impact, and gouge (SAIG) resistant outerwrap. Bore-Wrap is a solution that benefits society, not just the pipeline operator. Bore-Wrap is also an excellent tool for a pipeline contractor to ensure they are offering the best protection for their customer and saving themselves from the difficulty and expense that comes from a re-pull of the pipe should the coating fail the inspection after getting dragged in a hole beneath our homes, businesses, waterways, and roads. Until the industry does begin the practice of applying SAIG outer wraps, Denso Bore-Wrap will continue to offer all of these protections, keeping the pipeline beneath our feet protected as it is installed.