External Corrosion of Pipelines, Bulk Tanks, and Other Metal Structures in the Eagle Ford Shale
The Eagle Ford Shale region of South Texas has been booming with a huge amount of oil and gas exploration, construction of infrastructure, and massive job creation. Similar things are happening in other Texas oil shale plays such as the West Texas Sprayberry, Wolfcamp, Cline, and Haynesville. Similar booms have been happening in the Bakken of North Dakota and Montana, Niobrara/DJ Basin in Colorado, the Marcellus and Utica in Ohio, Pennsylvania and West Virginia, and other successful oil and gas plays.
Any business doing oil and gas exploration, drilling and production should protect its assets from corrosion. These include wells, pad equipment, pipelines, bulk storage and processing facilities, and other expensive infrastructure. These companies must pay attention to corrosion of the assets they own. There are at least three major issues with this new infrastructure, just related to external corrosion.
Designers and construction crews need to plan for, and then install a good-quality, well-adhered protective coating on metal that will come in contact with soil or water. The metal should be completely covered by this coating to help prevent external corrosion.
The construction and installation work must be professionally inspected to ensure that coatings are applied properly, AND are not damaged during transport, fabrication, installation, burial, and so on. Chips and scratches in coating can happen if pipe is not loaded on to trucks correctly and by poor-quality installation. If damaged, coating should be properly repaired.
In addition to coating, owners of oil and gas assets need to get good cathodic protection systems installed and maintained. These serve as a second “line of defense” against external corrosion. This protects the metal exposed by the remaining holes in coating that go undetected and unrepaired, often called “holidays.” No coating job is perfect, and all coatings degrade over time. For steel tank bottoms, no coating is typically used at all, so these structures need larger cathodic protection systems designed, installed and kept operating long-term.
Cathodic protection is done by building a low-voltage, DC electric circuit that connects to a set of anodes buried in soil, and to the structure needing protection. This causes the tank bottom, pipeline, or other subsurface structure to become a strong cathode. Anodes get consumed by corrosion over time, while cathodes are protected from corrosion for as long as this electric circuit is working properly. Cathodic protection systems, when properly designed and installed, do a great job of minimizing external corrosion.
In the Eagle Ford Shale region, as well as others, there are different properties in soil and shallow geology that cause corrosion to speed up. One is very low soil resistivity, a measure of how easy it is for Mother Nature to flow electric current across a piece of metal in the soil. The more current flows, the more steel (if the structure is made of that) corrodes over time. As a glaring example, the photo below shows a portion of a weld joint that coupled together a pipe joint and a 90-degree elbow.
Corroded weld in a section of pipeline.
This steel elbow had been installed less than three years before the corrosion pitting caused a leak. The crews who welded, then coated and buried this piece of steel did several things wrong.
Number one, the welding job was not properly done. Number two, the metal was not properly cleaned before the field-applied epoxy coating was mixed and laid on over the weld area. This meant the coating was not properly adhered to the metal, so moisture was able to get under the coating and directly to the metal. This is kind of like applying band aid to wet skin. It just will not stick properly. The third problem was with the epoxy coating. It was not cured properly before the pipe and elbow were placed in the ditch, and then backfilled. The coating was ripped, punctured and wrinkled in many areas. Finally, this pipeline portion was apparently not given cathodic protection for at least part of its service life. All of these negative factors combined to cause a leaking pipe.
Soils in contact with this steel were extremely low in resistivity, and very salty. The more chemical ions are present in soils and shallow geology, the easier it is for electric current to flow. This land was actually under shallow seas a very long time ago. Anyone who has spent a winter in the north is probably familiar with salted roadways. Automobiles pick up salt from roads, and if not kept clean, the steel frame and skin are going to rust. Dings in paint will expose metal, which then rusts from the salt contact. For this pipe, the combination of water, salt and electric current flow caused several ounces of good steel to turn into rust in one severe pit. Once that little tiny hole opened in the pipe wall, oil under pressure began flowing out of the pipe and into soils. The leak was discovered by pipeline personnel soon afterward. Not only is an oil leak bad for the environment, it could also cause a fire and severe public safety hazards.
Coatings are the best investment anyone can make to protect against external corrosion. When selected properly, and put on a clean, prepared metal surface using the right procedures, coatings provide great protection to 95 percent, 99 percent, or even more of that external metal surface. It’s just like putting a good paint job on your car or truck. Having said that, every metal structure in contact with soil or water also should ALSO have a properly designed and installed cathodic protection system. NACE International, the leading corrosion protection organization in the world, specifies that coatings and cathodic protection are both required, for a certain piece of metal infrastructure to provide full service life. Good coatings and cathodic protection represent up-front investments, rather than a costly bunch of repairs down the road.
Any company with assets in the ground should do solid research on coatings, cathodic protection consultants and contractors. Many practitioners of coatings work and cathodic protection try to use “one size fits all” approaches. They don’t take into account conditions local to the job site. They may not consider the nuances of a particular project and its geography, topography, geology, soil science, and construction plans. If a $50 million plant or pipeline is being installed and is supposed to give a 30-year service life or longer, make sure that a little more money and attention are spent at the front end of the project. Get appropriate coatings selected and applied. Get a quality installation. Make sure qualified inspectors are overseeing the work. Have a high-quality and appropriately designed cathodic protection system built and operating. Make sure there are enough test stations or other measurement points built into the system.
Looking back at the pipeline elbow photo shown earlier, think about how much money this one incident probably cost. Money had to be spent because of an oil leak and there was an environmental cleanup to be carried out. Some of the pipe had to be replaced. The pipeline operator also needed to stop the flow of oil and that caused lost revenue. All that had to be done before the repair and cleanup could even take place! Down-time is a huge cost. But it’s not necessarily the worst cost. What if there had been a fire, and injuries, or a major spill rather than a small leak?
The lesson leaned? If proper procedures had been followed when making the weld and coating had been properly applied, this may not have become a problem. Further, if an appropriate cathodic protection had been in place, a leak would have been prevented. If good inspectors had been involved to check all this work during construction, this leak may not have occurred. The pipeline operator could have continued normal operations and could have saved a lot of money.
Chapman Engineering – protecting assets and the environment since 1989.