As a population, Australians are critically dependent on the pipelines that deliver essential services to our homes and businesses. Pipeline owners need to be vigilant in protecting their assets from corrosion. We take a look at some of the common methods for protecting these vital pieces of infrastructure.
In Australia, tens of thousands of kilometres of steel pipelines run underground right beneath our feet, carrying basic human essentials such as gas and water to our houses. In addition, there’s also a significant network of pipelines to take things like sewage and stormwater away from the community.
The length and diameter of the pipelines in our network is varied; from large diameter transmission pipelines stretching hundreds of kilometres across the country, carrying precious cargo between states, to the smaller diameter gathering and distribution pipelines transporting gas and water between delivery points.
Pipelines are an integral part of our infrastructure, and given the way we live our lives today, it is important that they are able to fulfil their function. There are several important factors to ensure that the steel pipelines we all depend on stand the test of time in a safe and cost efficient manner.
Laying the right foundations
There are a few simple steps that can be taken at the construction phase to ensure pipelines are up to the task. They include choosing the right material, using the right coating application method and fabrication technique, working with experienced installers during the build and adhering to quality assurance standards.
These are the things asset owners can do before the pipeline is in the ground – but what can be done to ensure a pipeline stays operational once it is buried?
The most common reason for pipeline failure is coating degradation or failure, which causes bare steel to become exposed and then allows it to corrode and fail. This is predominantly a problem with older inferior coating systems used before 1990; but it can also be an issue with new pipelines that were not manufactured or installed properly, or that have suffered some mechanical trauma.
The challenge is that it is difficult to determine where coating defects (and hence corrosion) will occur. It’s also hard to estimate the extent of corrosion damage in a buried pipeline. Further, with older pipelines, it is impractical to excavate and repair large amounts of coating – even if you know where the damage is.
When it comes to combatting coating degradation, utilities and asset owners generally have two options. The first option is to conduct a coating defect survey, usually using the specific technique Direct Current Voltage Gradient (DCVG), a test that can be used to locate coating defects as small as a pinhole, without any excavation. To perform this test, experienced technicians are required to walk above the length of the pipe with equipment that can pick up changes in a signal that is applied to the pipeline from corrosion test points. These test points consist of an electrical cable being welded to the pipe, with the end stored in a testing box above ground which is easily accessible. Once the DCVG survey has been completed, defects can then be excavated and repaired so the pipeline is fully protected from the soil again and will not corrode.
The second option is cathodic protection. Rather than repairing damaged sections of coating, cathodic protection supplements the coating. Corrosion is an electrochemical reaction, in which a metal loses electrons before chemically changing into rust. Cathodic protection works to supply the pipeline with electrons from an alternate sacrificial metal rather than from the metal of the pipeline itself, hence controlling rust. Cathodic protection is a legal requirement for many authorities around Australia, and AS2832.1:2015 Cathodic protection of metals Part 1: Pipes and cables is the industry guide to cathodic protection.
There are two main types of cathodic protection, both of which work to the same end: they provide a sacrificial material that corrodes in place of the pipeline. The first technique is widely used in most residential hot water systems, and is known as a sacrificial anode system. In a sacrificial anode system, a metal, usually zinc or magnesium, is electrically attached (this can be through a wire or through physical contact) to the structure to be protected, whether it be the internal surface of a hot water heater, or a large pipeline. The idea is that the sacrificial anode (the zinc or magnesium) corrodes in preference to the steel structure, and protects that structure in doing so. These systems can easily be installed on a pipeline by simply welding a connecting wire between the anode and the pipeline and leaving them in the ground together. These systems are very cheap and easy to install, and not much can go wrong with them; however, they only supply a limited amount of protection – so it is best to use these systems on pipelines that are very well coated or for ‘hotspot’ protection.
The other type of cathodic protection that can manage pipeline corrosion is impressed current cathodic protection (ICCP). These systems work using the same scientific basis as sacrificial anode corrosion protection; however, they use a power source connected to the sacrificial anodes to supply extra protection. As a result this technique can protect kilometres of pipeline with a single impressed current system. These systems are more complex to design and install compared to sacrificial systems and, by law, as well as in good practice, they require bimonthly monitoring. The advantages, however, are that they allow additional control over protection levels; last longer, with a typical design life of 30 years; and of course, protect a longer amount of the pipeline. In some situations where the coating is particularly bad, as is the case with most pipelines installed prior to 1990, ICCP is the only option to protect the pipeline from corrosion.
Before installing any system it is essential to do a ‘potential survey’, which gives a picture of the severity and nature of corrosion issues the pipeline may be facing. Armed with this information, a system can be designed to best cater to the needs of the pipeline.
Criteria for results obtained from potential surveys to classify the pipeline as protected or unprotected are provided in AS2832.1:2015. To conduct a potential survey test, corrosion test points are required, usually every 500m along the pipeline. ’Potential charts’ are obtained at each location and then compiled to give an overview of the pipeline’s corrosion activity. From this survey, recommendations can be made on what extent of cathodic protection would be required to protect the pipeline.
Select Solutions has extensive experience in the design, installation, and commissioning of corrosion protection systems, as well as corrosion condition assessment and DCVG surveys. Their cathodic protection systems are designed, monitored, and maintained as a cost effective means of preserving and controlling maintenance and replacement costs on underground and overhead metallic structures.
For more information on how Select Solutions can help you protect your pipelines, visit their website select-solutions.com.au.