Pipelines are designed and constructed to rigorous standards, to ensure that the public and the environment are protected. Over time, corrosion and outside forces can degrade a pipeline to the point that a spill or release might occur. Pipelines are comprised of cylindrical metal sections with varying wall thicknesses determined by the expected operating pressures. A defect or anomaly is a deviation from the original configuration of the pipeline. This could be a change in wall thickness due to metal loss, a deformation of the pipe wall, or a crack.
Metal loss defects
Metal loss defects occur when the wall of the pipe is thinned due to internal or external corrosion. Since the majority of pipelines are buried, external corrosion is one of the primary threats to the integrity of a pipeline. While above-ground piping can also be subject to atmospheric corrosion, external corrosion is the predominant cause of metal loss.
Some of the measures employed by pipeline operators to prevent corrosion include the use of specialized pipe coatings , cathodic protection of the pipe, additives in the product carried in the pipe that reduce water content, periodic cleaning, and pipeline inspections. However, as pipelines age and environmental conditions change, a pipeline can become susceptible to corrosion.
Pipelines are designed to withstand internal pressures well above their operating pressure. This “design pressure” is one factor that determines the wall thickness of the pipe. As metal is lost from the pipe wall, the pipe wall thickness decreases and so does the amount of pressure the pipeline can contain.
Generally, a defect caused by corrosion results in a pinhole in the pipe wall and the volume of product spilled is small. Operators employ several methods of detecting leaks and help ensure rapid response. These include aerial surveys, right-of-way maintenance, leak detection systems, and landowner vigilance.
Pipes are sometimes constructed with a welded seam that runs the length of the pipe. Corrosion can occur in this seam, and cause a crevice or groove to form that can result in pipe failure at the seam.
Internal corrosion is the result of water, sediment, or chemical contaminants accumulating in the pipe. This normally occurs on the bottom of the pipe and at low points in the pipeline where sediment and water can settle out of the product being transported. In order to prevent internal corrosion, pipeline operators routinely add de-watering agents to their product and take other steps to ensure that their product is as “clean” as possible. They also periodically clean the interior of the pipeline to remove debris and water, and minimize the possibility of corrosion.
Corrosion defects are usually discovered through hydrostatic pressure testing or in-line inspection of the pipeline.
Dents and gouges
Dents are usually caused by heavy equipment excavating in the vicinity of the pipeline. Dents can also be caused by rocks that come in contact with the pipe wall.
As mentioned, pipelines are cylindrical in shape, and when they are in good repair, pressure is applied equally on the interior wall of the pipe. Any deformation of the pipeline wall results in the pressure profile being altered at the location of the deformation, leading to the possibility of pipe failure.
Dents are usually hemispherical in shape, however, dents caused by mechanical impact such as construction equipment can create a V-groove or gouge across the pipe. These kinds of deformations are referred to as dents with stress concentrators. They pose a greater hazard to the integrity of the pipe than smooth, hemispherically-shaped dents.
A gouge is created when metal is removed from the pipe wall through mechanical means. This usually happens when the teeth of a back hoe scrape across the pipe. The sharp edges of the gouge act as stress concentrators and pose a threat to the integrity of the pipe.
When mechanical damage to the pipe occurs, the pipe coating is usually damaged as a result. This in turn can increase the likelihood that corrosion of the pipeline will occur.
Dents or deformations of the pipeline can be found through the use of in-line inspection tools that specifically detect geometry defects. Pipeline operators usually have damage prevention programs in place to help prevent mechanical damage to their pipelines. Components of damage prevention programs include:
- education of the public, and excavators, on the dangers of digging near a pipeline without prior notification;
- clearly marking the pipeline; and
- inspections of pipeline right-of-ways.
A crack is defined as a “stress-induced separation of the metal”. If you could see a magnification of a cross-section of pipe, you would see that the metal is composed of granules. The manufacturing process results in a strong bond between these granules; however, conditions can occur that result in these granules separating, causing a crack.
One cause-condition of cracks is “cyclic fatigue”. This refers to pipe stress stress that occurs as a result of fluctuating cycles in operating pressure within the pipe. Fluctuating pressures cause small changes in the pipe’s shape, and those changes in turn can weaken the pipe, over a long period of time. Imagine taking a piece of steel and bending it back and forth multiple repeatedly. Eventually the steel will crack, or even break, at the bend.
Cracks can also be induced by stress corrosion cracking, where sections of pipe are welded together.
In some instances, cracks are inadvertently “built into” a pipe during manufacture. These are usually too small to cause pipe failure, and in any case are usually detected during hydrostatic pressure testing before it is placed into operation.
In-line inspection tools also often detect cracks in the pipeline before they grow to the stage where a rupture of the pipeline is likely to occur.
Pipelines – due to their buried and often located in areas of construction activity – are susceptible to defects and anomalies. Pipeline operators employ damage prevention programs to protect their pipelines from damage, and to detect the existence of anomalies before they can result in a pipeline failure. When a pipeline operator detects the presence of a defect or anomaly, the pipeline is restored to its original design configuration through repairs.
Date of Revision: 12012011