Why is a cathodic protection (CP) system important in maintaining the integrity of a pipeline?

A pipeline is protected from corrosion primarily by application of an anti-corrosion coating such as yellow jacket or dual-layer fusion-bonded epoxy. A CP system is vital in inhibiting the corrosion at the locations where there are imperfections or damage caused by age or non-notified third-party damage.

How does CP work?

CP is an electro-chemical process where the ‘energy’ of the pipe steel surface is lowered to a point at which corrosion is no longer thermodynamically favourable.

The electro-chemical reactions for CP are demonstrated by both the anodic and cathodic reactions respectively:

Fe --> Fe2+ + 2e-

2H+ + 2e- --> H2

In order to prevent the dissolution of elemental iron it must receive two electrons from a source, namely the CP unit. This in turn results in the formation of hydrogen gas at the surface of the steel commonly known as the passivation layer.

What are the main advantages of CP systems over other forms of anti-corrosion treatment?

Employment of CP systems over other forms of anti-corrosion treatments has many advantages including:
• Low capital and operational cost per kilometre;
• Protection against extensive range of environmental conditions;
• Typically simple installation set-up; and,
• Extremely effective at arresting the corrosion rate.

Does the design of a CP system differ according to various types, or specifications, of pipelines? If so, how?

Technically the principles of CP are the same regardless of the pipe, coating or fluid being transported. However, in reality, the design of a CP system differs only due to the following conditions:
• The regulations to which the asset owner must satisfy;
• The code to which the CP is designed in accordance with;
• The environment in which the pipeline is operating within; and,
• The operations and maintenance philosophy of the asset owner.

There are also slight nuances in the pipeline design that will affect the CP design that must be considered including length and diameter of the pipeline in addition to the type of backfill material, but these are not as critical as the above mentioned conditions.

What types of CP systems are available? How do these differ?

Typically there are two types of CP systems:
• Impressed – This CP system works by applying a small current (typically milliamps per kilometre) to the pipeline via units known as transformer-rectifiers. These units convert AC electricity into DC and use this electricity to lower the ‘energy’ of the pipeline. This system enables an asset owner to protect several kilometres of pipeline, provided the AC power remains connected.
• Sacrificial – This CP system essentially performs a similar function via the electrical connection made between the pipeline and the buried anodes, namely zinc or magnesium. This system differs in that the DC electricity generated is due to the galvanic difference between the pipeline and the anodes. This system is also limited in protection range but is relatively maintenance free, however the anodes have a finite life and will need to be replaced.

How is a CP system implemented into a pipeline?

Each Australian state and asset owner may implement their CP system slightly differently, but they are all required to undertake implementation in accordance with the standard AS2832.1 Cathodic Protection – Pipes and Cables.

Typically, once the initial site investigations and design have been completed according to the client’s requirements:
• The pipeline is fitted with test points during construction to enable future testing of the protection levels on the pipeline and therefore the performance of the system;
• Designers may choose to install sacrificial anodes along the way to provide temporary CP during the construction phase, until the impressed unit(s) is installed, commissioned and operational;
• The anode bed is installed, along with the CP unit;
• Once these are installed, the pipeline operator is generally allowed to operate the impressed unit for no more than 24 hours to enable data logging of the protection levels on the pipeline;
• The impressed unit is also allowed to operate short term during regulatory testing to prove if the CP system causes any electrolysis effects on other nearby foreign assets;
• Provided there are no construction defects and all tests are successful, an application for approval to operate the CP system is submitted to the regulating authority. Once approval is granted the sacrificial anodes are disconnected and the impressed unit is turned on.

What are the implications for a pipeline that does not have an effective CP system in place?

Any pipeline built to AS2885 is required to be protected via CP. An ineffective CP system, if left unchecked, can and will allow the pipeline to degrade thereby affecting the integrity of the asset.

In a day and age where the cost of installing and/or repairing a pipeline is significantly greater than that of operating a CP system, it is financially unsound to allow a pipeline to deteriorate from an ineffective CP system.

What limits are there to the usage of CP?

There are few limitations to the usage of CP, but all can be overcome through careful design, construction and operation:
• Excessive negative potentials;
• May cause pipeline coating disbondment due to the excess hydrogen evolution;
• In some instances hydrogen cracks have also developed within pipelines due to the excess hydrogen evolution;
• Combined with the incorrect coating selected for the operating conditions, it may contribute to the development of stress corrosion cracks;
• Interference (stray currents) from the anode bed can affect neighbouring foreign utilities if the location of the anode bed is too close them; and,
• Disbonded coating or other foreign objects located next to the pipeline can cause a shielding effect. This would mean that the pipeline could not be protected locally at that point of shielding.

Can you explain the final design of Jemena’s Colongra Pipeline CP system?

The final design for Colongra was a mixture between an impressed unit, utilised for corrosion protection of the pipeline, and a modified sacrificial system which was installed in order to mitigate against the electrical hazards associated with running metallic-coated pipes within the vicinity of high-voltage powerlines.

These sacrificial anodes sole function are to act as an 'earth' for the AC hazards induced onto the pipelines. They were laid parallel to the pipeline and could span distances of up to a couple of hundred metres.

Are there any specific attributes to the CP system on the Colongra Pipeline due to its unique design?

No. Any specific attributes of the CP design are purely driven by the need to mitigate the effects caused by the surrounding environment, namely AC hazards. These hazards are the effects resulting from supplying gas-to-electricity generation plants.